Post on 11-Sep-2021
HAL Id in2p3-00971281httphalin2p3frin2p3-00971281
Submitted on 2 Apr 2014
HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents whether they are pub-lished or not The documents may come fromteaching and research institutions in France orabroad or from public or private research centers
Lrsquoarchive ouverte pluridisciplinaire HAL estdestineacutee au deacutepocirct et agrave la diffusion de documentsscientifiques de niveau recherche publieacutes ou noneacutemanant des eacutetablissements drsquoenseignement et derecherche franccedilais ou eacutetrangers des laboratoirespublics ou priveacutes
ThomX Technical Design ReportA Variola J Haissinski A Loulergue F Zomer (eds )
To cite this versionA Variola J Haissinski A Loulergue F Zomer (eds ) ThomX Technical Design Report 2014 164p in2p3-00971281
lsquo
1
ThomX
LAL ndashpub etc etc
EDITORS Alessandro Variola
Jacques Haissinski
Alexandre Loulergue
Fabian Zomer
[THOMX TECHNICAL DESIGN REPORT] [Tapez le reacutesumeacute du document ici Il srsquoagit geacuteneacuteralement drsquoune courte synthegravese du document Tapez le
reacutesumeacute du document ici Il srsquoagit geacuteneacuteralement drsquoune courte synthegravese du document]
lsquo
2
INTRODUCTION
1 The ThomX project
The fast performance evolution of laser and particle accelerator systems has opened the way to compact
radiation sources based on Compton backscattering (CBS) [11] While CBS-based sources do not
compete with the more classical synchrotron sources as far as the photon total flux and beam brightness
are concerned the CBS scheme has several attractive characteristics it provides the most efficient
photon energy boost so that hard X rays can be produced with a comparatively low energy electron
beam resulting in very compact and low cost devices This is extremely important in view of the
integration of such a radiation source in a lsquonon-laboratoryrsquo environment like a hospital or a museum
Another attractive feature of CBS sources resides in the energy versus emission angle correlation This
correlation makes it possible to use a simple setup with a diaphragm to obtain a quasi-monochromatic
beam with a bandwidth on the order of one percent Moreover the CBS scheme allows the tuning of the
photon energy range in several ways by varying the laser wavelength the beam energy or the collision
angle Since the laser polarization is conserved in the electron-photon collision a CBS-based machine
may provide polarized photons
The ThomX project is taking advantage of the preeminent French technology in accelerator and laser
fields The goal is to design and build a demonstrator with cutting edge performances compared to
similar projects either in operation or planned A flux between 1011
-1013
γs in the hard X-ray range is
expected and the photon energy tunability will provide a Compton edge that can be set between 50 and
90 keV
This project is a direct outcome of the sustained effort made by several laboratories to achieve a high
amplification of laser pulses by stacking them in a passive optical resonator These studies resulted in a
record amplification efficiency in the picosecond regime obtained in the PLIC experiment [12] They
are presently pursued within the MightyLaser program at the ATF accelerator facility at KEK (Tsukuba
Japan) where a vacuum-compatible four-mirror cavity is installed [13]
The ThomX collaboration gathers experts in accelerator physics coming from SOLEIL and LAL
experts in laser systems from CELIA (Bordeaux) in optical resonators from LAL and in X-ray line
instrumentation from the Neel Institute in Grenoble The ThomX community includes representatives
from the medicine field and from the cultural heritage field The ESRF and INSERM (Grenoble)
together with the CNRS C2RMF will design and perform a series of demonstration experiments to
check the assets of such a radiation source Furthermore the ThomX overall characteristics give to this
machine a strong industrial potential the latter will be investigated by an industrial partner THALES
Electron Devices which will play an active role in the ThomX collaboration
As a first step in this program a CDR has been written [14] It describes the basics of the Compton
backscattering effect and gives a first estimate of the ThomX performances based on simulations The
main technological solutions are also described in this document The machine lattice and the entire
machine layout are provided together with the design of the optical resonator and its integration in the
collisions region The main items which need RampD were identified A global integration plan and a
machine cost evaluation were also given
The present TDR provides a detailed description of all the technical sub-systems together with the
machine integration scheme It also provides the results of complete beam dynamics simulations which
take into account the ring characteristics and all the effects resulting from the collisions between the
lsquo
3
electrons stored in the ring and the photons stored in the optical cavity These simulations lead to
precise predictions concerning the X-ray yield The last part of the TDR deals with the project structure
and its management Finally the updated cost estimate is presented
2 Scheme and options
X-ray sources based on Compton backscattering can be designed with various configurations
depending on the main performance goals (average flux peak flux brilliance monochromaticityhellip)
Once this choice is made one may envisage either a high or a low electron-photon collision frequency
In the first case the electron beam is recirculated in a storage ring or in an ERL in CW mode and
continuously interacts with the laser Another option is to operate directly a high duty cycle or CW
super-conducting Linac Correlatively the laser system has to provide a high average power in the CW
locked mode [15] In this case to amplify the laser pulse energy one may envisage the use of a passive
optical resonator with a high gain [16] If a high peak instantaneous flux in single bunch or multi-
bunch pulsed mode is required the laser system will be a high power laser with a low repetition
frequency Recirculation of the laser beam is possible for a few passages by means of passive or
regenerative cavities [17] As far as the accelerator technology is concerned the warm Linac
technology can be made to work in burst mode [18] This allows the production of short low emittance
bunches to increase the source brilliance Because of the Compton energy-angle correlation this
scheme can also provide an excellent performance as far as monochromaticity is concerned by using a
diaphragm which selects a fraction of the X-ray beam
The ThomX source is designed to maximize the average X-ray flux Another goal of this project is to
provide a compact reliable and tunable source which can be operated in hospitals or in museums in a
user-friendly way
These constraints imposed the choice of a high collision rate scheme and of the warm RF technology
Thus the ThomX accelerator system is based on a 50 Hz warm S band (2998 MHz) linac whose energy
is tunable up to 70 MeV an injection line and a compact electron storage ring whose RF cavity will be
warm and will operate at 500 MHz The ring revolution frequency is 178 MHz The laser is a fully
integrated fibre laser which provides a high average energy Its power is amplified by stacking the
pulses in a high gain four-mirror Fabry-Peacuterot resonator with a 30000 finesse The laser and the optical
cavity repetition frequency is twice the ring revolution frequency ensuring an electron-photon collision
every e- bunch revolution
The basic operating scheme of ThomX is illustrated in Fig 1 A 1 nC electron bunch is produced by an
RF gun and then accelerated up to the ring injection energy by an S band section A transport line
ensures the bunch transfer from the linac exit to the injection section of the storage ring After injection
the electron bunch is recirculated for 20 ms in the ring The ring optics and the RF peak value are such
that the beam size is very small (in all directions) at the interaction point (IP)
In parallel a high power fibre laser produces light pulses of ~ 14divide28 microJ energy at ~ 12 eV These
pulses are stacked in a four mirror optical resonator at twice the ring revolution frequency The
expected energy gain per pulse is 103 divide10
4 corresponding to a beam power of ~ 70divide700 kW in the
optical cavity
The synchronization system allows a near head-on collision (the crossing angle is two degrees) between
the electrons and the laser pulse every revolution of the electron bunch These collisions produce hard
X rays which are backward emitted in a cone whose angular opening is ~1γ (as a result of a relativistic
effect)
lsquo
4
Fig1 Schematic diagram of the ThomX source
At the end of a storage period of 20 ms the electron bunch is extracted from the ring and dumped in a
dedicated beam dump to avoid the background noise that would be produced by uncontrolled particle
losses At the same time a new lsquofresh beamrsquo is injected in the machine
3 General layout of the machine
As previously mentioned the ThomX design takes into account a strong constraint coming from the
goal of a small size machine Nevertheless this design allows for the possible integration of two
collision regions [19] Figure 2 shows the general layout of the machine The linac includes an electron
RF photogun (1) followed by a section dedicated to diagnostics (2) the acceleration structure (3) and a
quadrupole triplet (4) to allow for a three-gradient measurement in the direct line dump (5) where a
diagnostic chamber will be located The injection line consists of four 45 degree dipoles to steer the
beam four quadrupoles and two chambers (6 7) for energy spread measurements and beam
characterization before injection The injection is performed by a septum (8) and a fast kicker (9)
Another kicker (10) is inserted in the same line to provide together with the septum a fast extraction
Once extracted the beam is transported towards a dedicated beam dump (11)
lsquo
5
Figure 2 ThomX layout
The ring design ensures a four-fold symmetry based on a double bend achromat (DBA) optics It
includes 8 dipoles 24 quadrupoles and 12 sextupoles for chromatic corrections The bunch storage
time is too short (20 ms) and the beam energy is too low for synchrotron damping to play a role in
regard to the Compton back-scattering recoil effect nor in regard to collective effects such as CSR IBS
and ion instabilities For this reason an lsquoELETTRArsquo type RF cavity (12) will be used not to restore
energy but to bunch the electrons and to perform a longitudinal feedback Transverse position
diagnostics and feedback will be provided by a strip-line and by BPMs A synchrotron radiation
chamber will be used for transverse and longitudinal optical diagnostics The required ultra-vacuum
will be maintained by ion pumps distributed along the ring and by two dedicated pumps in the Fabry-
Peacuterot resonator
In the ring lattice two of the achromatic lines have been shortened by removing all the quadrupoles (see
Fig 3) so that the Fabry-Peacuterot cavity (13) can be easily integrated between two dipoles By minimizing
the beta functions it was possible to locate the collision region (14) in such a short section
Furthermore this design provides the possibility of implementing a second interaction region in the
future
lsquo
6
Fig 3 Layout of the interaction region and of the optical cavity integration
4 Machine parameters
The ThomX machine is composed of four main systems the injector the storage ring the laser and the
Fabry-Peacuterot resonator As mentioned previously we foresee some flexibility in the machine operation
so that the X-ray energy can be varied For this all four systems are dimensioned to withstand an
operating energy up to 70 MeV Nevertheless to characterize the machine a baseline configuration has
been chosen at 50 MeV In Tables 1 2 3 and 4 the parameter values which are given below correspond
to this baseline configuration
Table 1
Injector
Charge 1 nC
Laser wavelength and pulse energy 266 nm 100 microJ
Gun Q and Rs 14400 49 MWm
Gun accelerating gradient 80 MVm 5 MW
Normalized rms emittance 44 π mm mrad
Energy spread 04
Bunch length 43
lsquo
7
Table 2
Table 3
Laser and FP cavity
Laser wavelength 1030 nm
Laser and FP cavity Frep 3568 MHz
Laser power 50 ndash 100 W
Laser pulse energy 14 ndash 28 microJ
Fabry-Perot pulse energy 28 mJ
Fabry-Perot pulse length (rms) 5 ps
FP cavity finesseGain 3000-30000 1000- 10000
FP waist 70 microm
Power circulating in the FP cavity ~ 007 ndash 07 MW
Ring
Energy 50 MeV (70 MeV possible)
Circumference 168 m
Crossing-Angle (full) 2 degrees
βxy IP 01 m
εxy just after injection 5 10-8
m
Bunch length just after injection (rms) 4 ps
Bunch length at the end of a 20 ms storage
cycle
50 ps (rms)
Beam current 1784 mA
RF frequency 500 MHz
Transverselongitudinal damping time 1 s 05 s
RF Voltage 300 kV
Revolution frequency 1784 MHz
σx IP (just after injection) 70 microm
Tune xy 317 174
Momentum compaction factor αc 00136
InitialFinal relative energy spread (with IBS
and Compton back-scattering)
0406
lsquo
8
Table 4
Source
Photon energy cut-off 46 keV (50 MeV) 90 keV ( 70 MeV)
Total Flux 1011
-1013
photons
Bandwidth (with diaphragm) 1 - 10
Divergence 10 mrad (1γ) without diaphragm 50 MeV
5 Summary of the TDR main technical points
After the publication of the CDR much work has been put in simulations design and technical choices
The goal of this TDR is to provide a reference document in view of the machine construction as well as
a more precise planning and a more precise costing
The first chapter deals with the machine performances and the tolerances to be met so that the collisions
between the electron bunch and the laser pulse do take place in the conditions specified in the CDR
Then the main systems are described and our technical choices are presented This last section is
subdivided as follows
1 Linac
2 Beam transfer and injection section ring
3 Laser and Fabry-Peacuterot cavity
4 X-ray user line
A special attention is given to the integration of each system and to the global integration of the ThomX
machine The lsquoOptical systemrsquo chapter includes a short report on recent results obtained in the laser and
Fabry-Peacuterot cavity RampD program both in the laboratory and on a beam line at KEK-ATF (Japan)
Special chapters are dedicated to diagnostics the control of the main systems and their synchronization
safety and radioprotection The last chapter presents the project management
lsquo
9
Chapter 1 THE THOMX INTERACTION REGION
11 Electron beam and laser pulse characteristics in the interaction region
The ThomX injection frequency has been fixed at 50 Hz (20 ms period) by taking into account the
Compton scattering recoil effect which continuously deteriorates the beam longitudinal emittance This
effect is all the more important as synchrotron damping is strongly suppressed since the beam energy is
low Because of this lack of damping the Compton scattering recoil effect and eventual beam
instabilities must be controlled by longitudinal and transverse feedback systems
We stress the fact that in ThomX the beam does not reach a permanent state during a storage cycle its
properties are continuously varying To describe the Compton interaction dynamics we define a set of
nominal beam characteristics which are those prevailing just after the injection of a fresh e- bunch
Simulations have been carried out to characterize the X-rays emitted during a single crossing of the
electron bunch and the photon pulse (stored in the optical cavity) as a function of the collision
geometric parameters
These nominal collision parameters are as follows
Parameter value unit
Crossing angle 2 degree
Electrons energy 50 MeV
Electrons energy spread 03
Number of electrons 625 109
Fabry Perot pulse energy 28 mJ
Repetition rate 1784 MHz
Laser pulse transverse dimension (rms) 35 microm rms
Electron bunch transverse dimension (rms) 70 microm rms
Laser pulse duration (rms) 5 ps rms
Electron bunch length at injection 4 ps rms
Table 11 Nominal collision parameters
To extend these results to other geometric configurations one has to take into account the parametric
dependence of the Compton collision rate The X-ray flux is the product of the Compton cross-section
and the luminosity It is proportional to the electron bunch intensity and to the laser pulse power The
geometry dependence is given by the luminosity formula which involves the geometric 6D size and the
crossing angle Assuming Gaussian bunches and neglecting the hourglass effect the ring luminosity is
given by the following expression
Eq 11
L = NeNγ f cosφ
2π
1
σ ye
2 +σ yγ2 σ xγ
2 +σ xe
2( ) cos2φ + σ ze
2 +σ zγ2( ) sin2φ
lsquo
10
where σeγxyz represent respectively the two transverse dimensions and the longitudinal one of the
electron bunch and of the laser pulse f is the collision frequency Ne and Nγ are the number of
electrons per bunch and photons per pulse Φ is half the crossing angle
The X-ray energy cut-off is higher for head-on collisions and for photons which are exactly back-
scattered In this case the photon energy gain is 4 γ2
This calculation provides a useful reference to assess the efficiency of other configurations but it
remains an approximation since exact results must take into account the beam evolution during a
storage cycle and should be obtained from a full revolution by revolution simulation
12 Expected flux and its characteristics
In nominal conditions the expected flux is 78 1012
X-rayss whether one uses Eq 11 or the monte-
carlo simulation code CAIN Figure 11a shows the energy spectrum of the scattered photons and its
cut-off at 45 keV As shown in Fig 12b most photons are emitted very close to the interaction region
axis within a small cone with a 2 mrad opening angle They may be selected by using a small
diaphragm The energy spectrum of the X-rays emitted in a cone of 225 mrad opening angle is shown
in Fig 12a with a FWHM energy spread of 25 keV
Figure 11 Results of simulations performed with CAIN using the parameters listed above a)
Energy spectrum b) Transverse size of the source c) Angular emission distribution d) Horizontal (x-
xrsquo) phase space of the scattered X-rays
lsquo
11
The spot size at the source is dominated by the electron bunch of 7 10-5
m as illustrated in Fig 11b
The dimansions are assumed to be the same in the horizontal and vertical plane The total divergence
of the source is around 15 mrad given by the electron beam energy (ie the 1γ parameter) with an rms
size of 4 mrad (Fig 11c) The emittance of the photon source shown in Fig 11d is 03 mm mrad The
energy-angle correlation and the collimation effect are illustrated in Fig 12 As expected the high-
energy X-ray flux is peaked with a 1γ opening angle The diaphragm which is assumed for this
simulation selects emission angles and thus photon energies resulting in a ~ 10 monochromatic
photon beam
Figure 12 Results of simulations performed with CAIN using the parameters listed above
Left panel Energy spectrum of photons emitted in a cone with an opening angle of 225 mrad
Right panel Number of photonsmradsup2s versus energy and angle (the color bar indicates the photon
number)
13 Position stability
To optimize the X-ray flux a precise transverse alignment must be acieved in the interaction region It
can be performed either by steering the electron beam or by adjusting the optical table position These
procedures will be monitored by means of a double BPM which is integrated in the Fabry-Peacuterot cavity
vacuum chamber exactly at the interaction point The resolution of this technique is a few microns A
simulation has been performed to calculate the flux reduction resulting from a possible mismatch
between the electron bunch orbit and the laser pulse trajectory Figure 13 shows the number of X-rays
produced per bunch crossing and per keV as a function of a transverse displacement of the optical
cavity axis with respect to the electron orbit from zero to 140 microns
lsquo
12
Fig13 Number of back-scattered photons and their energy as a function of a transverse alignment
error This figure clearly shows that the losses entailed by a displacement comparable to the BPM
sensitivity are negligible The color bar indicates the number of X-rays produced at each crossing
14 Timing stability
A precise synchronization between the electron bunch and the laser pulse at the interaction point is
another important requirement A timing mismatch introduces a longitudinal shift between the two
bunches at the IP This can be due to an error in the location of the Fabry-Perot cavity with respect to
the e- storage ring or to a jitter in the synchronization system Fig 14 shows the variation of the X-ray
flux as a function of a longitudinal displacement
The flux is not so much reduced as long as the jitter is less than 10 ps This number fixes the
synchronization tolerance and thus the performance of the longitudinal feedback
Fig 14 Flux and energy spectrum of back-scattered photons as a function of the relative
synchronization of the beams (longitudinal displacement)
lsquo
13
15 Energy stability
The nominal ThomX injection energy is 50 MeV Phase rotation in the RF source or phase mismatch
will generate a jitter in the injection energy Such a jitter entails synchrotron oscillations which will be
damped by the longitudinal feedback However before this feedback has stabilized the longitudinal
oscillations the collision conditions will not be nominal This situation has been simulated the results
are illustrated in Fig15 One can see that it is in the higher part of the X-ray energy spectrum that the
loss is more important
Fig 15 Flux and spectrum of the back-scattered photons as a function of the e- energy
lsquo
14
Chapter 2 INJECTOR
The most important components of the linac injector are the electron gun and the acceleration section
which brings the beam to the desired energy Other components are the laser used for the e- gun the
magnetic elements used to transport the beam the diagnostics the vacuum system the waveguides and
the radio-frequency (RF) source Each component is detailed in the following First of all the injector
specifications are given since they lead to the choice of some of the linac components
21 Injector specifications
The project goal namely to produce a high flux of X-rays of energy 50 keV leads to the following
specifications for the linac
Energy 50 MeV
Charge 1 nC
Number of bunches per RF pulse 1
Emittance (rms normalised) lt5 mm mrad
Energy spread rms lt1
Bunch length rms lt5 ps
Average current 50 nA
Repetition frequency 50 Hz
Table 21 Nominal linac parameters
First of all one notices that the low average current 50 nA does not justify the use of cold technology
(ie of a superconductor) for the acceleration structure But there is still a large choice of possibilities
for an RF structure operating at room temperature For economic reasons and because of the long
experience accumulated in the field we have chosen an RF structure operating at 3 GHz This
frequency is a European standard
Furthermore the above specifications led us to choose a photo-injector for the electron source since a
low emittance beam is required Again to minimize risks we chose to build the same gun as the one we
constructed for the CLIC lsquoTest Facility 3rsquo at CERN [21] This gun has been running successfully for 4
years To increase the current per bunch we will use a metallic magnesium photocathode which can
deliver more than 1 nC with a laser pulse energy of a few tens of a microJ at a wavelength of 260 nm The
gun is made of 2 12 copper cells magnetically coupled to the waveguide To get 1 nC with an
emittance lower than 5 mmmrad an accelerating field of 80 MVm is required which means an RF
power of 5 MW in a 3 micros pulse The electron beam energy output by the gun is around 5 MeV
We will use an industrial laser to extract the electrons from the cathode by photoelectric effect The
main specifications of this laser are the following ones Wavelength 260 nm energy per pulse 100 microJ
pulse duration around 5 ps It must be synchronized with the RF frequency and triggered at 50 Hz This
laser will be installed in an optical hutch located outside the concrete shielding which will enclose the
accelerator Therefore its beam must be transported with dielectric mirrors and lenses over a distance of
about 20 meters
Concerning the acceleration section SOLEIL a partner of the project will lend us a lsquoLILrsquo section This
section was made by LAL for the pre-injector of LEP at CERN It has been kept as a spare for the
SOLEIL linac The ThomX project will provide an other section in case SOLEIL needs one The
section length is about 46 m To reach a final energy of 50 MeV the energy gain in the section must be
lsquo
15
45 MeV The latter value implies an electrical field of 125 MVm and an RF power of about 10 MW
However the accelerator should reach higher energies in order to produce X-rays beyond 50 keV Since
the maximum targeted X-ray energy is 90 keV the linac design should allow a beam energy of 70
MeV corresponding to an RF power of 20 MW Thus taking into account the RF system efficiency the
specification for the total RF power is set at 35 MW
This specification lead us to choose a klystron producing 35 MW in a 45 micros pulse To feed the
klystron a modulator is needed that provides high voltage pulses of typically 240 kV through a high
voltage transformer soaking in an oil tank The two technologies which can be used for this component
are discussed in detail below
For the electron beam steering two coils and two magnetic correctors are required The coils are placed
around the gun Opposite sign currents circulate in these coils (-310 A 336 A) so that the magnetic field
vanishes at the cathode location Its amplitude is 023 T roughly 7 cm downstream from the cathode
Concerning the magnetic horizontalvertical correctors once a good mechanical alignment is achieved
experience has shown that beam trajectory errors are not larger than a few mm between the gun and the
acceleration section Hence the correctors must produce a beam deflection of up to 10 mrad For the
model operating at LAL [22] two bipolar power supplies of 10 A per corrector are adequate
For the diagnostics three different instruments will be installed between the RF gun and the
acceleration section An Integated Current Transformer (ICT) will measure the charge per bunch
Beyond this device a beam position monitor (BPM) will measure the transverse positions (x y) of the
electron beam We will use a BPM based on striplines which was designed for SOLEIL The third item
comprises a vacuum chamber equipped with an arm which can be translated by compressed air in order
to introduce a YAG screen to intercept the beam The YAG crystal fluoresces when ionized by the e-
beam The emitted light shows the beam transverse shape A short optical transport line and a CCD
camera allow the measurement of the transverse dimensions LAL designed and constructed the
diagnostics chamber It has already been successfully tested in the PHIL facility A drawing showing
the linac layout is shown in Fig 21
lsquo
16
Figure 21 Linac layout In the upper figure the different components of the LINAC are illustrated
starting form the gun and subsequently passing trough the stripline the ICT for current measurements
the laser introduction chamber the diagnostics station the steerers and the accelerating section
22 Technical description of the linac components
221 Photo-injector
It is a copy of a gun built by LAL for the CLIC Test Facility 3 of CERN the so-called lsquoprobe beam
gunrsquo[22] Simulations have shown that the performances of this gun fulfill the ThomX specifications
We are thus benefiting from all the technical drawings which already exist and all the fabrication
processes are mastered by LAL from the machining to the brazing This RF gun is comprised of 2 12
cells The output cell is magnetically coupled and the other end is short-circuited The main parameters
are given in Table 22
Frequency 299855 MHz
Rs 50 MΩm
Q 14500
β 1
E 80 MVm
PRF 5 MW
Table 22 RF characteristics of the photo-injector at 30degC in vacuum
The photo-cathode will be made of magnesium It has the big advantage of producing the specified
charge without requiring an in-situ photo-cathode preparation chamber which is a heavy and expensive
lsquo
17
installation Preliminary tests performed at LAL have demonstrated the possibility to obtain good
quantum efficiency with such cathodes
The main difficulty expected in operating this device is the laser cleaning of the photocathode surface
when polluted by an exposure to air To perform this cleaning a laser intensity of 160 microJmm2 is
required together with an RF electric field of at least 50 MVm The magnesium disk holder is not
designed yet it could be press fitted screwed or brazed
222 Solenoids
The gun is equipped with two solenoids one is the focusing coil while the other is the bucking coil
which cancels the magnetic field at the photo-cathode location They have been modeled by usning
POISSON A schematic is shown in Fig 22
Figure 22 Schematic of the gun with its coils
Dimensions
For both solenoids the inner diameter is 160 mm and the outer one is 500 mm The copper part is 30
mm long for the bucking coil and 56 mm for the focusing coil
The conductor used for these magnets is a standard one with a 6x6 mm2 square shape It is cooled by
water circulating in a 4 mm diameter channel One can easily calculate the current the resistance the
dissipated power and the temperature rise assuming that the cooling water is supplied at 20degC and 8
bars Values are provided for each lsquopancakersquo since the conductors are wound by columns of two The
bucking coil Bbc has two pancakes and the focusing coil Bfoc four
Bbc Bfoc units
Number of coils 84 168 MHz
Number of pancakes 2 4
Current -274 326 A
P 23 33 kW
R 306 31 mΩ
Flow 164 164 lmn
ΔT +20 +28 degC
Table 23 Solenoid electric and cooling parameters lsquoNumber of coilsrsquo refers to one pancake
Two power supplies are required one for Bbc (400A 30 V) and one for Bfoc (400A 60 V)
lsquo
18
223 Laser
The laser extracts a bunch of electrons from the photo-cathode surface by photo-electric effect Table
24 specifies its characteristics
Parameter value unit
l 260‐270 nm
Emax 100 plusmn 05 microJ
Beam transverse quality M2 M2 lt 11
Gaussian Pulse width 5 ps
Transverse profile square
Pointing stability 10 of beam diameter
Oscillator frequency 74963750 externally triggered with jitter lt 1 ps MHz
Amplifier repletion rate 50 externally triggered Hz
Table 24 Laser specifications for the RF gun photocathode
The laser architecture is shown in Figure 23
Figure 23 Architecture of the laser proposed by industry
An oscillator provides a signal at the specified frequency It can be either a fiber or a TiSaphire laser
Then the pulse is stretched amplified (with one or two amplifiers) recompressed and converted in the
UV range
The laser will be installed on an optical table in a hutch whose temperature and hygrometry will be
regulated
224 Laser injection chamber
The laser injection chamber is located at a distance of 8011 mm downstream of the photo-cathode It
contains a metallic aluminum mirror placed 30 mm away from the accelerator axis It provides an 80
reflectivity The angle of incidence of the laser beam on the photocathode is 214deg This mirror is fixed
To adjust the laser beam position on the photo-cathode and its incident angle we will use a motorized
mirror with a remote control placed on an optical table just beside the laser injection chamber
The size of this optical table will be 1x1 m2 On it there will be
Oscillator Stretcher Preamplifier Amplifier Compressor
nJ microJ
mJ
Frequency
shifter
Clock
Syncro
Line syncro
Shaper
lsquo
19
- 2 dielectric mirrors
- 2 beam splitters
- 1 UV photo-diode to monitor the laser intensity
- 1 CCD camera to observe the laser spot on a screen (in paper or lanex) facing the photo-cathode
- An optical beam expander to adjust the laser spot size on the photo-cathode
This is the minimum number of optical items that will be put on the table some changes may be made
depending on the very final geometry of the accelerator and laser transport lines The set-up design can
be finalized during the last stage of the accelerator construction since it involves standard components
only
225 Beam steering coils
The e- beam trajectory will be adjusted by sets of two steering coils which produce dipolar magnetic
fields over short distances along the X and the Y axes One such device will be located at the gun exit
and the other about 800 mm downstream Only very small position and angle errors must be corrected
Figure 24 shows the LAL design for such a coil Experience with the PHIL photo-injector at LAL [22]
showed that an integral of Bmiddotdl of 52 G m is sufficient The latter figure has been validated by
calculations made by a private company With a current of 5 A a Bmiddotdl of 58 G m was obtained that
results in a 100 mrad deviation of a 5 MeV beam Thus a 100 mm lateral displacement of the beam can
be produced at the entrance of the acceleration section
Figure 24 Schematic of the beam steering coils Dimensions are in mm
All together the steering coils need 4 bipolar DC power supplies Their maximum current is 10 A
lsquo
20
23 Acceleration section
The acceleration section is shown in Fig 25 Its characteristics are given in Table 25
Figure 25 Picture of the SOLEIL LIL acceleration section
Length 45 m
Number of cells mode 135 2π3
Frequency 299855 at 30degC MHz
Rs 63divide74 MΩ
Q 14800
Attenuation 74 dB
Energy gain 15 MW 60 MeV
τ 135 micros
Ez max 182 MVm
Table 25 Characteristics of the acceleration section
In a second stage of the ThomX project a call of tender will be issued for a new acceleration section
that should be shorter in order to make the machine more compact
lsquo
21
231 The RF source
Figure 26 shows the RF source schematic
Figure 26 Schematic of the RF source
The RF timing and its synchronization are described in chapter 5 A clock signal is sent to the linac
which uses it to trigger a 3 GHz synthesizer Its output is fed to a preamplifier which brings its power to
350 W The preamplifier is triggered at 50 Hz Then the 3 GHz signal is amplified by a klystron whose
gain can reach 53 dB to produce a 35 MW RF wave as required by ThomX This power is distributed
via a waveguide network which is described below
To drive the klystron one needs a modulator delivering a pulsed high voltage (HV) whose
specifications are given in Table 26 The klystron cathode must be heated by a power supply that
provides a current rising slowly from 0 to 25 A and a voltage rising from 0 to 30 A in 20 minutes A
precision of 1 on the current value is required The klystron is generally equipped with focusing coils
and their power supply whose standard parameters are 40 A 120 V
Repetition rate 50 Hz
Plateau length 99 gt 3 micros
Plateau ripples lt plusmn 05
Pulse duration at 50 6 micros
Rise time 10 divide90 lt 05 micros
Fall time 90 divide10 lt 1 micros
Pulse to pulse stability lt 05
Voltage range 0divide300 kV
Peak current 285 kV 300 Hz
Table 26 Modulator specifications
The classical and well-proven modulator technology relies on the high voltage (40 kV) loading of a
delay line made of inductances and capacitances The delay line is switched by a thyratron A new
technology is presently emerging based on solid-state amplifiers Each amplifier produces a 100 V
lsquo
22
pulse Thus to produce a 40 kV pulse roughly 400 amplifiers are put in a rack and their output pulses
are added The performances of both technologies are similar
232 RF network
The RF power is transported and distributed to the electron gun and the acceleration section by a
waveguide network shown in Fig 27
Figure 27 Schematic of the RF waveguide network
The WR 284 waveguide network is pressurized at two bars with SF6 between ceramic windows at the
klystron output and at the acceleration structure entrance A 5 dB bidirectional coupler splits the power
in two channels In the branch which runs toward the electron gun there is a 10 dB attenuator and a
circulator to protect the klystron from the RF power reflected by the electron gun In the branch running
toward the acceleration structure there is a 10 dB attenuator and a phase shifter to adjust the relative
phase between the gun and the acceleration section Various RF loads (five in total) ensure the system
safety by absorbing of RF power of the reflected waves
lsquo
23
The network also includes three couplers to measure the incident and the reflected RF power One is
placed at the klystron power exit one is placed at the gun entrance and the last one is placed at the
acceleration section entrance The RF gun is equipped with a loop that measures the stored RF energy
All of these loops used for measurements are weakly coupled at -70 dB The signals are measured with
crystals or C boxes
24 Beam dynamics
241 Nominal operation
The beam dynamics was simulated with the PAMELA tracking code for the ThomX nominal
parameters ie a bunch charge of 1 nC at 50 MeV The 50 MeV can be obtained with an electric field
of 80 MVm in the RF gun and 125 MVm in the acceleration section It is clear that a different set of
field values can lead to the total of 50 MeV In particular one may wish to operate the gun at a higher
field in order to improve the beam quality We will operate the linac with this possible improvement in
view
The beam will be accelerated on crest Thus the coil magnetic fields are the only parameters savailable
to minimize the transveres and the longitudinal emittances Simulations have shown that for precise
magnetic field values (see Table 27) the e- beam emittances are optimized as illustrated in Fig 28
20
15
10
05
00
σxy
(m
m)
6005004003002001000
z (cm)
20
15
10
5
0
em
it (
mm
mra
d)
6005004003002001000
z (cm)
20
15
10
05
00
ΔE
E (
)
6005004003002001000
z (cm)
46
44
42
40
38
36
34
32
30
σz (p
s)
6005004003002001000z (cm)
lsquo
24
Figure 28 Beam characteristics (rms values) as a function of the distance along the accelerator (z
coordinate) Upper left panel Transverse size upper right panel normalized emittance lower left
panel Energy spread lower right panel Bunch length
The beam parameter values at the linac end are given in Table 27
Beam parameters value unit
Energy 50 MeV
Beam size 17 mm
Normalised Emittance 44 π mm mrad
Energy spread 04
Bunch length 43 ps
Table 27 Beam parameter rms values at the linac end
The accelerator parameters which should ensure the above beam characteristics are given in Table 28
RF structure parameters value unit
Ez in the gun 80 MVm
PRF in the gun 5 MW
Ez in the section 142 MVm
PRF in the section 14 MW
Solenoid parameters value unit
Bz max 2500 G
Ibc -274 A
Ifoc 326 A
Laser parameters value unit
Energy impinging on a Mg photocathode 10 microJ
Square transverse profile diameter 2 mm
Variation of the intensity from axis to border 14
Table 28 Summary of the linac parameters
242 Alternative operation modes
Concerning the injector other operation modes can be envisaged One at least will occur namely an
operation at 70 MeV in order to extend the X-ray energy range up to 90 keV Such an option has
attractive applications In this mode the electron beam performances should remain exactly the same as
the ones prevailing at 50 MeV (cf Table 27) For the accelerator parameters the only change is the RF
power required for the acceleration section it must reach 195 MW instead of 14 MW at 50 MeV
The operator can also be tempted to change the RF phase The one used in the simulations gives the
minimum emittance A phase shift of -10deg of the RF gun with respect to the laser could lead to an
energy spread reduction by a factor 2 at the expense of an emittance degradation Whatever the goals
the linac operator must be aware of the high sensitivity of the beam parameters to the RF phase This is
illustrated in Fig 29 An error of 5deg in the acceleration section phase results in almost a doubling of the
energy spread
lsquo
25
Figure 29 Energy spread at the linac end as a function of the RF phase in the acceleration section all
other parameters keeping their nominal values
The RF electrical field in the gun also plays an important role in the beam performances The results of
simulations of the emittance variation as a function of the gun electric field are presented in Fig 210 If
this field reaches 120 MVm the emittance is reduced by 20 with respect to the 80 MVm case
However the possibility to operate the gun at such a high gradient can only be assessed experimentally
Figure 210 Injector beam emittance as a function of the electric field in the electron gun (Gaussian
laser beam 1 nC e- bunches)
The injector performances as a function of the bunch charge are summarized in Table 29 Three modes
will be available in order to reduce the effect of collective instabilities at injection
Parameters Mode 1 Mode 2 Mode 3 Unit
Q 1 05 01 nC
σxy 165 15 08 mm
εxy 47 33 11 π mmmrad
σz 44 4 33 ps
σEE 04 03 02
Table 29 Linac beam performances (rms values) as a function of the bunch electric charge
As one can see in the above Table reducing the bunch charge by a factor 10 leads to an emittance
reduction by a factor 4 Although the X-ray flux is reduced the source brightness ie the ratio of the
flux to the divergence is improved by a factor 16 As a consequence a higher number of X-rays can be
20
18
16
14
12
10
08
06
04
02
δΕ
Ε (
)
151050-5-10
φ (deg)
10
9
8
7
6
5
ε (πmmmrad)
1401201008060
E0 (MVm)
lsquo
26
transported to a sample (number per unit area) Whatever the eventual benefits to the X-ray production
the advantage for the accelerator to operate at a lower charge per bunch is clear the reliability of the
accelerator is much enhanced the laser energy requirement is reduced and it becomes also possible to
use a copper photo-cathode instead of a magnesium one easier to handle But the main advantage lies
in the ring operation Indeed a 1 nC bunch whose length is only 4 ps induce a strong wakefield
stemming from the interaction between the beam and the vacuum pipe Such a wakefield perturbs the
beam and may cause oscillations of the bunch centroid possibly instabilities and thus a reduction of the
X-ray production rate
25 Photo-injector and acceleration section vacuum
Simulations of the photo-injector vacuum have been performed assuming a pressure of 3 10-8
Pa at the
cathode location and requiring a vacuum time constant (VS) that ensures an easy gun conditioning This
constraint must be respected to maintain a 10-7
Pa pressure at the acceleration section entrance where the
system will be baked to obtain an outgassing rate τ = 5x10-10 Pa m s-1
(N2)
Pressure in the line of injection
The use of a semi-conductor cathode (Cs2Te) puts severe constraints on the vacuum and requires a preparation
and a transfer chamber To avoid these complications some tests have been made on a metallic cathode (Mg)
The first attempt ran into difficulties (it took 12 h to reach a gradient of ~ 60 MV m-1
) we observed pressure
rises and sparks In presence of only the RF power we observed hydrogen CO and CH4 outgassing (as the
main components) A slight reduction in the quantity of water and oxygen was noticed due to chemical re-
composition When the laser beam was introduced to extract electrons a pressure increase by a factor of ten
was observed The 24th
25th
and the 26th
peaks also significantly increased for the HxCy compounds but no
Mg was measured
lsquo
27
Gas analyzes during the conditioning of cathode Mg
On the basis of simulations and experimental results we identified the need for the following items
‐ 2 ion pumps 40 l s-1
(upstream of the cathode amp injector) with their baking systems and power
supplies
‐ 2 ion pumps 55 l s-1
(for the RF network) with their baking systems and power supplies
‐ 1 ion pump 150 l s-1
(for the laser introduction chamber) with its baking system and power supply
‐ 1 ion pump 300 l s-1
(for the acceleration section) with its baking systems and power supply
‐ 1 gate valve manually actuated DN 100 (injector- upstream) and a baking valve
‐ 1 gate valve electro-pneumatically actuated DN 63 (injector - downstream) and a baking valve
‐ 1 gate valve electro-pneumatically actuated DN 40 (injector- downstream) and a baking valve
‐ 3 Pirani gauges (before the injector injector laser introduction acceleration section)
‐ 3 cold cathode gauges (after the injector injector laser introduction acceleration section)
‐ 2 power supply gauges
‐ 1 residual gas analyzer (RGA)
‐ 1 turbo pump (DN 63 ~ 80 l s-1
)
‐ 1 rotary vane pump (~ 12 m3 h
-1)
‐ 4 angle valves manually actuated for pre-pumping DN 63
‐ 2 T with RF grids
lsquo
28
Chapter 3 TRANSFER LINE and RING
31 Transfer line
The transfer line (TL) has to transport the beam from the linac to the ring and to match the optics of
these two devices In addition a set of beam diagnostics needed for an efficient injection in the ring are
localized along the line as sketched in Fig 31 The transfer line main characteristics are summarized in
Table 31
Figure 31 Layout of the injection and extraction transfer lines
A particular attention is paid to ensure a versatile optics that allows the matching of different linac
beams to different ring optics Simulations have been carried out using the BETA code [31] The
transfer line optics is optimized in order to accommodate some emittance degradation which may arise
from collective effects
Screen station + dump
Linac emit + pulse length diags
Screen + Scraper
Linac energy diags
Screen station
TL emit diags
Extraction line dump
lsquo
29
Figure 32 Transfer line optics including the ring The vertical line indicates the ring entrance point
The nominal transfer line optics is shown in Fig 32 from the linac to the ring together with the ring The
nominal input bunch Twiss parameters are x = z = 34 x = z = 42 for Q = 1 nC bunch length = 4
ps (rms) and an optimized normalized emittance of about 5 πmmmrad The strong bend produced by the four
dipoles of the transfer line increases the risk of emittance degradation under collective effects but simulations
show that the emittance increase is only of about 20 including coherent synchrotron radiation (CSR [2]) and
3D space charge effects (see Fig 33)
Figure 33 Emittance increase due to chromatic (cont lines) CSR (dashed) and space charge (dot dashed)
effects for 1 nC and 4 ps (rms) bunches at 50 MeV along the TL and the first revolution in the ring
The magnetic elements are identical to those of the ring
lsquo
30
Parameter value unit
Length 14 m
Nominal energy 50 MeV
Beta max x z 70 40 m
Maximum dispersion 1 m
Dipole numberfamily 4 1
Quadrupole number 7
Biplane corrector number 5
Table 31 Transfer line parameters and components
Besides ensuring the matching between the linac and the ring the transfer line must provide the measurement
of the beam emittances the beam energy and the energy spread Care has been taken to locate the diagnostic
stations in a way that ensures accurate measurements of these important parameters
The emittance measurements will be done by means of the lsquothree-gradientrsquo scheme (The pepper-pots
alternative is also foreseen) For instance with an adequate refocusing that produces a double waist (vertical
and horizontal) at the screen it is possible to scan both planes at the same time by varying only one
quadrupole gradient (see Fig 34) Other scan variations are also possible The following figures show the
optics retuning as well as the beam spot size variation at the screens of both stations the one at the linac end
and the one downstream of the U-turn
Figure 34 Optics retuning and beam spot size scan as observed on the screen (simulation)
The electron energy and its spread are measured in between the dipoles by a dedicated optics Note that it is
also possible to retrieve the bunch length by measuring the energy spread as a function of the linac phase
(three-phase scheme)
lsquo
31
32 Extraction line
To dump the residual beam at the end of the storage time (typically from 20 to 50 ms) an extraction line
composed of one small dipole and two quadrupoles is also foreseen This line will allow the measurement of
the beam properties after it has circulated in the ring The injectionextraction devices are symmetrically
implemented in the ring injection section
33 Storage ring
331 Optics and lattice
Thom-X must produce a high X-ray flux of 1011
divide1013
phs in a stable way The flux depends on the
characteristics of the electron bunch and of the laser pulse Assuming a 1 nC electron bunch charge at
50 MeV and a 20 to 30 mJ laser pulse ( 124 microm) the optics constraints at the interaction point (IP)
are as follows
- an rms size of the electron beam less than or equal to 70 microm in the transverse plane requiring
low beta functions of 01 m with an rms geometric emittance of 50 nmrad injected from the
linac (5 mmmrad normalized)
- a vanishing dispersion at the interaction point in order to avoid a beam size increase resulting
from an e- energy spread increase as well as to avoid potential synchro-betatron coupling
resonances
- a bunch length less than 20-30 ps to ensure a good overlap with the laser pulse (the crossing
angle will be 2deg) and long enough to reduce collective effects stemming from the pipe
impedance CSR bunch internal scattering etc
Besides these specific IP constraints there are two general layout constraints namely
- a ring circumference as short as possible with enough straight sections to accommodate the
interaction region the RF the injection and possible other equipment as RF and transverse
feedback systems
- the need for some flexibility as far as the optics and the working point are concerned
The ring layout and its optics are based on a two-fold symmetric lattice with four achromatic arcs and
four straight sections two short ones (02 m) and two long ones (16 m) This layout preserves the ring
compactness and provides an adequate interaction region in one of the two straight sections between
two adjacent dipoles (cf Fig 35) The main ring characteristics are summarized in Tables 32 and 33
lsquo
32
Figure 35 Ring layout
The laser pulse frequency is the harmonic 2 of the e- bunch revolution frequency in the ring ie 357
MHz This leaves open the possibility of storing two bunches in the ring in order to increase the
Compton flux but at present this operation mode is not taken into account because of injection
constraints The optical cavity mirrors are located outside the ring vacuum chamber thus enabling a
mirror substitution without affecting the ring In addition the extraction of the X-rays can be performed
closer to their source than in the case where the optical cavity is inserted in a long straight section
The two long free straight sections accommodate the injectionextraction devices for one and the RF
and feedbacks for the other
The ring is composed of 8 dipoles 24 quadrupoles and 12 sextupoles Its circumference is 168 m long
the revolution frequency is 1785 MHz The RF frequency is set at 500 MHz ie at the 28th
harmonic
of the ring revolution frequency The nominal optical functions have been optimized with the BETA
code [31] They are plotted in Fig 36 The dispersion vanishes in the long and in the short straight
sections (RF injection and interaction regions) The beta functions decrease down to 01 m at the IP to
provide a 70 microm focus assuming that the injected beam has an emittance of 50 nmrad (5 mmmrad
normalized) in both the vertical and the horizontal planes
RF and feedback section
Injection extraction section
IP 1
Interaction Point (IP)
lsquo
33
Ring parameters value unit
Circumference 1680 m
Nominal energy 50 MeV
Betatron tunes nx nz 317 174
Beta max xz 11 11 m
Dispersion max 08 m
Beta xz dispersion IP 01 01 0 m
Momentum Compaction Factor 0014
RF frequency 500 MHz
RF harmonic 28
RF voltage 300 kV
Synchrotron frequency 360 kHz
Period Revolution frequency 56 1784 nsMHz
Natural chromaticities -32 -82
Dipole nb family 8 1
Quadrupole nb families 24 6
Sextupole nb families 12 3
Corrector nb families 12 12
Table 32 Ring parameters
Max field 70 MeV Bore length Gap Diameter
Dipole 07 T 2765 mm
(R=352 mm)
40 mm
Quadrupole 5 Tm 150 mm 41 mm
Sextupole 15 Tm2 60 mm 42 mm
Correctors 510-4
Tm
Table 33 Main magnet characteristics
Taking into account the constraints imposed by the optical functions the chromaticity and the dynamic
aperture the operating point was chosen with tunes νx=317 and νz=174 There is still enough
flexibility to vary these tunes by plusmn 02 without spoiling the main ring performances The momentum
compaction factor can also be changed over a large range in order to scan the best working point
regarding the beam dynamics and the X-ray flux
Figure 36 Nominal beta functions and dispersion over one super-period
lsquo
34
The RF acceptance for the nominal momentum compaction factor (MCF) and a larger one including the
non-linear MCF and the vacuum pipe limit are plotted in Fig 37
Figure 37 RF acceptance Left panel nominal lattice Right panel for a MCF twice as large as the
nominal one VRF= 300 kV
RF acceptance nominal r56 = - 02 m r56 = - 04 m
Linear (RF bucket) 10 10
Non linear -32 +18 - 68 +4
Pipe limit 25 25
Table 34 RF acceptance for two different values of the momentum compaction factor
The two following plots (Fig 38) show (i) the vacuum chamber occupation by the beam at injection
when the emittance is 50 nmrad and the energy dispersion is 03 and (ii) at the end of the storage
cycle when the emittance reaches 300 nmrad and the energy dispersion reaches 06 (see section
438) The orbit components (misalignment errors) are not included since they are negligible once
corrected The choice of the pipe aperture (40 mm in H by 28 mm in V full aperture) takes into account
several constraints the beam pipe occupation the magnet bore radius the vacuum and the coherent
synchrotron shielding [32]
b a
lsquo
35
Figure 38 Transverse occupation (3 rms) of the vacuum chamber at injection (left) and at the end of
the storage cycle (right)
332 Non-linear dynamics
The natural horizontal and vertical chromaticities are respectively -32 and -68 Chromaticity
corrections are achieved by means of 12 sextupoles (3 families) located in the dispersive sections
Without performing a specific optimization the dynamic aperture is large enough about 50 times the
rms transverse size of the injected beam (see Fig 39) the quadrupole fringe fields being taken into
account Furthermore this dynamic aperture remains about the same for off-momentum particles (up to
plusmn3) compared to particles having the nominal energy The fringe field of the dipole magnets
introduces high order non-linear components Since the Thom-X dipoles are short with a large gap
their magnetic design (see section 34) has been carefully worked out in order to maintain the dynamic
aperture
Pole number 6 8 10 12 18 20 28 30
Inner dipoles +10 0 +3
Outer dipoles -16 -1 -6
Quadrupoles +10 +5 -5
Sextupoles +10 -10
Table 34 List of multipole tolerances for each kind of magnets expressed in 10-4
relative field variation
at 18 mm radial position
Figures 39 and 310 show the change of the dynamic aperture with and without considering the
multipole (the fringe field of the quadrupoles is also taken into account in these plots) These
Dynamical Apertures (DA) have been calculated with two different codes BETA and TRACYIII They
lsquo
36
both give very similar results In addition the TRACYIII code dedicated to long symplectic tracking
performs Frequency Map Analysis (FMA) giving more detailed information concerning the particle
diffusion process
Figure 39 On-momentum DA and FMA for the nominal ThomX lattice Beta code left and TracyIII
code (right) sextupoles only (top) and including magnet multipole tolerances (bottom)
lsquo
37
Figure 310 Off-momentum DA and FMA for the nominal ThomX parameters Beta code left and
TracyIII code (right) sextupoles only (top) and including magnet multipole tolerances (bottom)
333 Collective effects feedbacks
At low energy and thus low beam rigidity the electron bunch is sensitive to electromagnetic self-
perturbations that act revolution after revolution in the ring Several instability sources can be
identified
‐ Interaction with surrounding metallic items such as the resistive walls (RW) the RF cavity and
various discontinuities such as bellows BPMs and flanges HOMs etc
‐ Space charge forces
‐ Coherent synchrotron radiation (CSR)
These interactions affect the bunch energy and the bunch longitudinal time-profile leading to
distortions emittance growth bunch oscillations or even above some threshold instabilities and beam
losses Similar interactions may also affect the transverse beam dynamics but to a lower extent The
two relevant parameters for the instability threshold are the bunch charge and its length The strength of
the interaction is proportional to the bunch charge and varies inversely with the bunch length
The two plots in Fig 311 show the longitudinal wakefield over one revolution at 50 MeV generated by
a 1 nC Gaussian bunch with an rms length of 12 and 6 mm (4 and 20 ps) Five different contributions
are compared in this figure
‐ the shielded CSR following the Murphy and Skrinsky [32] formulation with a beam pipe full height
of 40 mm a dipole field of 05 T and a curvature of 0352 m
‐ the longitudinal space charge (LSC)
‐ the resistive wall (RW) following the K Banes and M Sands formulation [33] with a full beam
pipe height of 40 mm and an electrical conductivity of 14 106
‐ the vacuum vessel broadband component including 12 BPM 12 flanges and 12 bellows based on
the synchrotron SOLEIL type This potential has been calculated by means of the HFSS finite
elements code [34]
‐ the 500 MHz ELETTRA type cavity broadband component This potential has been calculated by
using the HFSS finite element code
They exhibit a very strong CSR wake field (up to few per mil) with the shorter bunch case
Figu
re
311
Lon
gitu
dina
l
wak
e
field
over
one
lsquo
38
turn generated by a 1 nC Gaussian bunch at 50 MeV for a bunch length of 4 ps (left panel) and 20 ps
(right panel)
The ThomX low energy operation (50 MeV) practically suppresses the natural damping effect of the
synchrotron radiation and complicates the bunch stabilization Each kind of perturbation grows
irreversibly implying that feedback systems in the three planes are essential to cope with the induced
oscillations of the bunch center-of-mass In addition to these collective effects the unavoidable
injection jitters have also to be taken into account To calculate these effects 6D tracking simulations
that include the longitudinal impedance as well as the large mismatch between the linac and the ring
optics in the longitudinal phase space have been performed (see Fig 312)
Large longitudinal oscillations
without feedback at injection
induced by collective effects
Longitudinal oscillations damped by the
RF phase feedback system operating at
F~360 kHz
Damping lt 50 micros
Figure 312 Tracking results (50 MeV
1 nC bunch) over the first 3000
revolutions just after injection without
and with the RF phase feedback system (VRF = 300 kV)
Large oscillations start very quickly after injection when the bunch is still short (4 ps) and are of the
same order of magnitude as the ones induced by the energy-phase jitter that affect the linac injection
This effect is the one to address in the design of the longitudinal feedback As far as the transverse plane
is concerned the growth times characterizing various instability sources are listed in Table 35 The
revolution-by-revolution feedback strength indicated for each instability source is the one needed to
suppress the oscillations
Source Type Growth time Revolution-by-
revolution kicker
strength
Beam pipe
geometry
TMCI
Head-Tail
-
160 micros
gt10 nrad
Resistive Wall
600 micros gt 2 nrad
Ions lt 100 micros gt20 nrad
Table 35 Transverse instability growth time and feedback kicker strength for a 1 nC bunch at 50 MeV
lsquo
39
In contrast with what prevails in the longitudinal phase space the main transverse oscillation source is
the injection position jitter Since the non-linear sextupole components are large this jitter may produce
a large emittance increase as a consequence of the phase space painting process (see Fig 313)
Figure 313 Sketch of the transverse phase space painting process
Simulations show that a very fast increase of the transverse emittance can result from steering errors at
injection in about 500 revolutions or 25 micros To overcome this effect a fast transverse feedback is
needed it has to damp the oscillation in about 100 revolutions or ~5 micros The strength of the revolution-
by-revolution feedback has to be on the order of 2 microrad This strength is far larger than the ones
estimated to cope with the transverse instabilities For this transverse feedback a dedicated strip-line
acting in both planes is foreseen in the RF section (see Fig 314)
Figure 314 Simulated transverse emittance increase induced by a 05 mm injection jitter with and
without feedback
After N
turns
lsquo
40
Phase space Specifications Device
Horizontal amp vertical Fr Frequency = 178 MHz (F0)
Strength = 2 microrad
Dedicated strip-line for
both planes
Longitudinal Frequency = 360 kHz (Fs)
Strength = 105 rads
Main RF phase loop
Table 36 Summary of the feedback specifications
Other collective effects such as the intra-beam scattering (IBS) which is important at low energy or the
Compton recoil effect play an important role during the 20 ms storage time Their dynamics differ from
an instability dynamics but they also increase the electron energy spread and the bunch emittance mdash
and thus reduce the X-ray flux
To summarize three different contributions to the e- bunch dynamics must be dealt with
‐ at injection jitters which may affect either the injected bunch energy or its trajectory or the precise
timing of its injection in the ring generate oscillations which may be large and which must be
quickly damped by feedbacks in all three dimensions (x y z)
‐ during a transition period corresponding to the first ~1000 revolutions strong collective effects
enhanced by a possible synchronization error between the linac and the ring may lead to a complex
turbulent regime in the longitudinal plane prior to ring matching (elliptical phase space density
profil)
‐ during the 20 ms storage period IBS and CBS produce a progressive increase of the bunch size in
all three directions This evolution cannot be stopped by a feedback system In contrast RF HOMs
and other perturbations may generate instabilities that should be controlled by feedbacks
To take into account collective effects a dedicated code has been developed including a collection of
wake field as well as non-linear dynamics It is based on a 6D multi-particle symplectic tracking
restricted to perfect hard edge drift quadrupoles and dipoles of finite length Other higher order
elements as sextupoles are treated in the thin length approximation The RF cavity is simply treated as
a longitudinal cosine kick The mapping through elements is given by the exact Hamiltonian solution
[34] for the drift sections and the dipoles (including their small curvature and dipole edge effects) The
quadrupole mapping is restricted to small radial angles It has been compared to various codes
(TRACYIII ELEGANT BETA and MADX) by performing simulations of the SOLEIL and the
ThomX rings with special care given to the dipole edge effects [35]
The transverse beam size at IP and the bunch length are plotted in Fig 315 over the first 10 000
revolutions or 056 ms with a 1 nC bunch charge This study shows that a small positive chromaticity in
the horizontal plane is necessary to stabilize the horizontal emittance During the very first revolutions
the tune dependent emittance behavior has not been fully investigated nor understood It appears that
this behavior follows from the bunch length mismatch between the linac and the ring The fact that the
linac produces very short bunches of 4-5 ps (rms) leads to a complex turbulent dynamics (see Fig
316) The effect of this mismatch disappears progressively over the first 1000 revolutions then the
bunch length reaches 20-25 ps (rms)
lsquo
41
Figure 315 Red RMS beam size and bunch length at IP Magenta bunch centroid position over the
first 10 000 revolutions (056 ms) for a 1 nC bunch The beam energy is 50 MeV
Figure 316 Turbulent dynamics in the longitudinal phase space that us taking place just at injection
These plots show the phase space density after 100 and 3000 turns
lsquo
42
334 CBS and IBS effects
The electron beam dynamics that prevails in a ring where Compton back-scattering is taking place is
similar to the one resulting from synchrotron radiation (SR) in a synchrotron light source CBS gives
rise to random excitation in the transverse and the longitudinal planes In the transverse plane the
emittance turns out to remain close to the one at injection [37] In contrast the CBS excitation strongly
affects the dynamics in the longitudinal phase space It leads to a substantial energy spread increase
over 50 ms storage time (see Fig 317a) resulting in a significant bunch lengthening
Based on the lattice parameters the calculation of the Touschek lifetime leads to a value on the order of
10 s this effect can thus be neglected
Figure 317 a) Energy spread increase and b) Radial (x xrsquo) relative emittance increase during a 100 ms
storage time Blue curves CBS effect only red curves IBS effect only black curves (almost on top of
the blue ones) CBS and IBS combined effect The IBS calculation is based on the Mtingwa formalism
[38]
In contrast the multiple Touschek effect (lsquoIBSrsquo) plays a significant role in the radial phase space (x xrsquo)
as shown in Fig 317b After 20 ms this horizontal emittance is increased by a factor ~ 4 from 50 to
200 nmrad and the relative energy spread by ~20 from 03 to 036 as can be seen in Fig 317a
Figure 317 shows that the Compton back-scattering process plays a role by far more important than the
IBS does It also shows that because of the large emittance increase produced by the CBS the bunch
density decreases by a large amount making the IBS much less effective (see Fig 317b where the
curve showing the combined effects lies below the curve illustrating the IBS effect by itself)
When the electron beam energy is increased up to 70 MeV the IBS effect on the horizontal emittance
becomes less important During a 20 ms storage cycle this effect is reduced by a factor 2 On the other
hand the energy spread degradation increases by more than a factor two compared to the 50 MeV case
This change is due to the higher energy of the back-scattered photons In contrast if the electron beam
energy is decreased down to 25 MeV the energy spread is reduced by a factor two while the horizontal
emittance increases by a factor seven compared to the 50 MeV case
Assuming an optimal overlap between the e- bunch and the laser pulse in the interaction region and
neglecting the e- bunch size variation across this region (the lsquohourglassrsquo effect) as well as the variation
of the laser beam envelope near its waist (characterized by the Rayleigh length) the X-ray flux is given
by the following formula (see Eq 11)
The nominal storage time is 20 ms only
lsquo
43
where σth is the Thomson cross section Nx is the number of photons per laser pulse f is the e- bunch
revolution frequency ϕ is half the collision angle σxyze σxyzγ are respectively the three dimensional
bunch rms sizes for the electron (e subscript) and for the laser pulse (γ subscript) In fact because of the
increase of the e- bunch size during the storage time the flux is reduced by roughly a factor two at the
end of a cycle as shown in Fig 318
Figure 318 X-ray flux as a function of the storage time These results take into account the energy
spread increase and the emittance degradation due to IBS only (blue curve) and to the combined CBS
and IBS effects (black curve)
To calculate more precisely the X-ray flux produced by Thom-X the code CAIN [39] (cf sect 12 above)
has been included in the previous 6D tracking code including the non linear effects of the lattice
magnets as well as collective effects The X-rays characteristics are computed at each ring revolution
The intra-beam scattering effect is calculated also at each e- bunch revolution based on the Bjorken and
Mtingwa formalism [38] To reduce the computing time the code extrapolates the IBS effect whenever
there is only a little change from one revolution to the next (this method reduces the computing time by
a factor 4) This code runs on linux OS Using the computer farm of the IN2P3 computing center in
Lyon it takes about 15 days to simulate a full Thom-X cycle (20 ms ie 400 000 revolutions) when the
e- bunch is modeled by 500 000 macro-particles
The way the total X-ray flux decreases during the first 10 000 revolutions is shown in Fig 319 This
decrease is due to the e-
beam quality degradation mainly due to the CBS effect Also shown in this
figure is the X-ray flux evolution as a function of the storage time for three different lower cut-offs in
the X-ray energy spectrum Figure 320 shows a typical angular distribution of the X-ray emission
Figure 321 plots the bunch longitudinal phase space for the first revolutions The macro-particles in
green are those which have been hit by photons at the last crossing while those in blue have been hit
before the others are in red
( ) ( ) φσ+σ+φσ+σσ+σ
φfNNσ=N
2
γzez
2
γxex
2
γyey
γeth
x22222
sincos
2πcos
lsquo
44
Cross comparison and benchmarking of this code against other codes developed for ThomX (6D
tracking without Compton effect or Compton + IBS without tracking) are in progress and indicate a
good agreement between the codes
Figure 319 Left panel Number of X-rays produced as a function of the X-ray energy at the end of the first
10 000 first revolutions (in red) and during the last 10 000 revolutions before extraction (in blue) Right
panel Number of X-rays produced summed over 2500 revolutions as a function of the revolution number
Figure 320 X-ray intensity as a function of the emission angle in mrad (horizontal axis) and their
energy in eV (vertical axis) The color bar indicates the number of X-rays
lsquo
45
Figure 321 Upper nine panels Evolution of the e- beam longitudinal phase-space after injection The macro-
particle positions are plotted along the horizontal axis while their energy deviations are plotted along the
vertical one Green electrons which have collided with photons at the last crossing blue electrons which
have collided before red electrons which have not yet undergone any collision These plots reveal a turbulent
dynamics Lower nine panels Time evolution of the beam longitudinal phase space as a result of a turbulent
regime from the initial lsquobanana shapersquo at injection to a Gaussian distribution after a few ms (colors indicate
the electron number-density red corresponds to the most populated region)
lsquo
46
Figure 322 Evolution of the X-ray energy spectrum when the e- beam energy is increased from 475 to 505
MeV Left panel The X-ray energy spectrum is plotted vertically for seven e- beam energies Colors
characterize the X-ray intensity Right panel Each curve shows the total number of X-rays above some lower
energy cut-off Green cut-off set at 20 KeV blue cut-off at 30 KeV red cut-off at 40 KeV
335 Scattering on the residual gas
Coulomb scattering of the electrons on the residual gas molecules induces an increase of the circulating
beam emittance The amplitude of this effect is proportional to the average of the beta functions and to
the gas partial pressure Moreover it varies as the square of the atomic number of the residual gas
atoms and is inversely proportional to the electron energy With a low beam energy of 50 MeV it is
therefore essential to keep a low pressure to minimize the time-averaged emittance Fig 323 shows the
growth of the transverse emittance during the 20 ms storage time in the nitrogen case and in the
hydrogen one Assuming that one deals with nitrogen at a pressure of 8 10-7
Pa the emittance increases
by a factor of two (see Fig 323) while the degradation is negligible in the dihydrogen case We
conclude that a pressure lower than 3 10-7
Pa must be maintained to keep the emittance growth at an
acceptable level
Figure 323 Emittance increase due to Coulomb scattering assuming that the residual gas is composed
of a) 100 nitrogen b) 100 dihydrogen
lsquo
47
336 Ion effects
The beam-ion instability [310 39 312] originates in the ionization of the residual gas molecules Ions
can subsequently be trapped by the electron beam electrostatic potential We recall that the on-axis
potential is inversely proportional to the beam size
When the trapped ion number density reaches some threshold beam instability is triggered Besides this
effect trapped ions damage the beam quality by shifting the tunes increassing the emittance and
lowering the beam lifetime since the local pressure is increased
For the Thom-X ring the critical mass above which ions are trapped is on the order of one in units of
the proton mass Thus all ions whatever their mass can be trapped in our case
Once trapped ions oscillate transversally in the potential well Their oscillation frequency depends on
the electron beam size and on the ion species (see Fig 324 1c) Because of the e- beam bunching
when the ion oscillation frequency becomes larger than the beam betatron frequency a lsquofast ion
instabilityrsquo may develop the e- bunch is submitted to the electric field of the oscillating ions eventually
resulting in an exponential growth of the oscillations of all particles (electrons and ions) This instability
arises above some value of the neutralization factor which depends on the lattice
The neutralization factor and the growth time of either the vertical or the horizontal oscillations are
shown in Fig 324 as a function of the horizontal beam size for several ion species For a horizontal
beam size averaged over the ring of 05 to 1 mm (which is about what is expected) the growth time is
very short (less than one micros) but for this would happen for a neutralization factor much larger than one
obviously this is excluded To keep under control the other ion harmful effects mentioned above one
may prevent ion accumulation over many storage cycles by introducing gaps between short series of
injections or by clearing the ions by an electrostatic potential that cancels out the potential well
produced by the e- beam A simple solution consists in polarizing the beam position monitors (BPMs)
thus avoiding the insertion of electrodes along the ring This scheme should be effective since ions will
drift longitudinally as the kicks they receive at each e- bunch revolution has a longitudinal component
The amplitude of these kicks along the z direction is shown in Fig 325 where the BPMs are
represented by circles They are placed near accumulation points where the longitudinal kick vanishes
ie where ions will converge A difficulty may arise at s =125 m This point is opposite to the IP in the
lattice at this point the e- beam has small transverse dimensions and a vacuum pump is placed Indeed
with a small beam size the neutralization factor threshold is very low (10-3
) and the growth time
constant is on the order of one micros This potential difficulty can be overcome by adding an electrode in
the short section between the two dipoles to clear the ions at this accumulation point
Ion clearing with polarized BPMs or gaps introduced in the injection sequence should keep other ion
effects at an acceptable level (more work is needed to quantify this statement)
Defined as the total number of ions divided by the total number of electrons
lsquo
48
Figure 324 a) Threshold neutralization factor b) growth time and c) ion oscillation frequency for the
following fractional becirctatron tunes 1) 064 and 2) 017 for different ion species The dashed lines show
the e- bunch betatron frequency σx is the mean beam size
Figure 325 Longitudinal kick given to on-axis ions at each e- bunch revolution
337 Electron beam injection and extraction
Originally the linac beam was injected in the ring by using only a septum followed by a fast kicker
Given the high repetition rate (50 Hz) radiation safety issues and a possible noise in the X-ray
experimentation area lead to set up an e- beam extraction line The symmetric septum magnet located in
lsquo
49
the middle of one of the long straight sections is used in a double pass mode for injection and
extraction It is flanked by two identical kickers one downstream for the injection and one upstream for
the extraction (Fig 326)
Figure 326 Sketch (top panel) implementation (middle panel) and simulation (lower panel) of the
injectionextraction process
The main characteristics of the septum and the kicker magnets are listed in Table 37 Since in both
processes the beam runs through quadrupoles some margins with respect to the nominal deflection
values must be provided to cope with the fact that quadrupole fields are not uniform
Kicker Septum
Deflection 15 mrad max (11 nominal) 170 mrad (40 mTm 70 MeV)
Pulse duration lt 56 ns half sine 130 micros full sine
Effective length 200 mm
250 mm
Overall length 450 mm 500 mm
Table 37 Main characteristics of the pulsed devices
Septum center
x =14 mm
xp=9 mrad + 170 mrad
Kicker center
x =0 mm
xp=0+11 mrad
2 fast kickers
Inj Ext
1 slow septum
lsquo
50
338 Orbit corrections
A set of 12 beam position monitors (BPMs) and 12 correctors in both planes are implemented along the
lattice in order to control and correct the orbit An additional BPM located at the IP is fixed on the
table that supports the laser cavity The main error sources are listed below Uncorrected and corrected
orbits are illustrated in Fig 327
rms value dBB dL dX dZ dS tilt
Dipoles 10-4 01 mm 02 mm 01 mm 02 mrad
Quadrupoles - - 02 mm - -
Dipoles 10-4 01 mm 02 mm 01 mm 02 mrad
Quadrupoles - - 02 mm - -
Table 38 Main magnet misalignment errors
Uncorrected orbit with typical magnet alignment errors
rms value xmax (mm) zmax (mm)
Max 7 3
IP 05 03
rms value xʹmax (mrad) zʹmax (mrad)
Max 12 4
IP 6 3
lsquo
51
Corrected orbit
rms value xmax (microm) zmax (microm)
Max 200 140
IP 200 30
rms value xʹmax (microrad) zʹmax (microrad)
Max 600 300
IP 350 60
Figure 327 Uncorrected and corrected closed orbits Red lines horizontal projection blue lines
vertical projection
Quadrupoles and sextupoles are fixed on girders with a maximum error of 50 microm The magnetic field
center error relative to the magnet yoke being of the same order the expected total alignment errors are
~100 microm in each plane The girders supporting all the elements will be aligned as solid blocks Since
lsquo
52
dipole misalignments have a large effect on the orbit dedicated additional support tools will allow the
readjusting of the dipole positions relative to the girder
One needs a maximum corrector strength of 05 mrad when this strength is computed by assuming just
one sigma alignment errors to cope with alignment errors which would be twice as large corrector
strengths of 1 mrad are thus necessary
Figure 328 Orbit bumps (red horizontal blue vertical) that can be produced at the IP
In addition to this standard orbit correction procedure a dedicated orbit scan at the IP will be very
useful to ensure an optimal overlap between the e- bunch and the laser pulse in the interaction region
(see Fig 328) To provide a scan range of up to 1 mm or 1 mrad the corrector strength must be further
increased by 1 mrad Thus the corrector maximum strength will be 2 mrad
The orbit correction will be performed by a slow feedback operating around 01 The correctors are in
the sextupoles magnets (combined magets) The orbit feedback correction will also control the RF
frequency in order to keep the orbit length at the ring design value A specific synchronization scheme
that ensures the proper injection timing and that maximizes the electron-photon collision rate is
described in chapter 5
Position amplitude scan at IP location up to 1 mm
Angle amplitude scan at the IP
location up to 1 mrad
lsquo
53
34 Magnets
Table 39 describes the magnets that will equip the transfer line the ring and the extraction line
Magnet type Dipoles Dipoles for inj Quad Sext Corr
Number 4+8+2 2 7+24+2 12 4+12+1
PS number 3 2 33 3 34
Strength 0180 T m 5 Tm 40 Tmsup2 05 mTm
Length 2765 mm 150 mm 60 mm 30 mm
Table 39 ThomX magnet parameters
341 Dipole magnetic design
The dipole bending angle is 45deg A magnetic field of 05 T is needed at 50 MeV Since their radius of
curvature is very small (0352 m) special attention has been paid to the field profile between the magnet poles
and to the pole edge shape where large multipole components are present Lateral shims and pole edge
specific profiles have been optimized with the TOSCA3D code to minimize the impact of the multipoles on
the beam dynamics
Characteristics unit value
Gap mm 40
Field at centre T 063
Overall dimension mm3 350 x 400 x 400
Current A 263
Voltage V 177
Pole width mm 100
Pole height mm 100
Yoke length mm 2765
NI Aturns 13000
Coil area mmsup2 67x40
Current density Ammsup2 116
Number of turns turns 48
Transverse size of the conductor mmsup2 6x6
Cooling channel mm 4
Water pressure drop bar 7
Temperature increase degC 47
Flow rate (per dipole) lmn 86
Electric power (per dipole) W 3100
Table 310 ThomX dipole parameters
lsquo
54
Figure 329 Drawing of a ThomX dipole
342 Quadrupole magnetic design
The quadrupole design (see Fig 330) was also performed with the TOSCA software The magnetic field
is produced by four coils carrying a DC current The conductor is wrapped around four poles which
concentrate and define the quadrupole magnetic field map
Figure 330 Quadrupole magnetic design
A quadrupole can be manufactured in four identical parts each one comprising one pole and one quarter
of the yoke or the yoke and the four poles can be manufactured separately The quadrupole will be
equipped with special shims which will be adjusted during the magnetic measurement to ensure a good
alignment of the magnetic axis and low harmonic components The ThomX quadrupoles are air-cooled
to reduce the infrastructure cost This is possible since the maximum current density is 15 Ammsup2
Figure 331 shows the magnetic field intensity versus the radial distance from the quadrupole axis The
maximum gradient is 5 Tm Figure 332 shows the magnetic field intensity at a distance of -20 mm
from the magnet axis versus the longitudinal coordinate z The maximum magnetic field is 01 T
lsquo
55
Figure 331 Magnetic field (in T) versus the distance (in mm to the magnet axis
Figure 332 Magnetic field intensity (in T) versus z (the longitudinal coordinate in mm)
lsquo
56
The main parameters of the ThomX quadrupoles are given in Table 311 Each quadrupole is powered
by a 10 A 32 V DC power supply thus the released heat power remains below 32 W A dedicated
magnetic bench equipped with a rotating coil will be used for the quadrupole magnetic measurements
This bench has been developed for the magnetic measurements of the SOLEIL storage ring
quadrupoles
Parameters unit value
Bore mm 41
Gradient Tm 5
Current density Ammsup2 15
Overall dimensions cm3 19x19x18
Current A 10
Voltage V 35
NI Aturns 810
Pole width mm 20
Pole height mm 50
Coil dimensions mmsup2 15x36
Number of turns turns 81
Wire dimensions mmsup2 5x125
Yoke length mm 150
Yoke thickness mm 20
Chamfer mm 5 (not optimised yet)
Table 311 Main parameters of the ThomX quadrupoles
343 Sextupole magnetic design
The ThomX sextupole design (see Fig 333) was also performed with the TOSCA software The magnetic
field is produced by six coils that carry a DC current The coils are wrapped around six iron poles which
concentrate the field and define the sextupole field map These magnets are combined function and are also
equipped with additional coils to produce the corrector dipole fields
Figure 333 Sextupole magnetic design
lsquo
57
Each sextupole yoke is manufactured as a single piece and the six poles are screwed on this yoke Then a coil
is wound around each pole During the design phase a chamfer was calculated with TOSCA to guaranty a low
harmonic content The sextupoles will be air-cooled to reduce the infrastructure cost The maximum current
density is 15 Ammsup2
The magnetic field intensity versus the distance to the magnet axis is shown in Fig 334 The maximum
sextupolar strength is 40 Tmsup2
Figure 334 The ThomX sextupole magnetic field versus the distance to the magnet axis
Fig 335 shows the variation of the magnetic field at a distance of -20 mm from the sextupole axis versus the
longitudinal coordinate z The maximum magnetic field is 0016 T
Figure 335 ThomX sextupole magnetic field versus z (the longitudinal coordinate)
Table 312 gives the main parameters of the ThomX sextupoles They will be powered by a 10 A 05 V DC
power supply
lsquo
58
Parameters unit value
Bore diameter mm 42
Sextupolar Strenght Tm2 40
Field 20 mm T 0016
Current density Amm2 25
Overall dimension cm3 19x19x8
Current A 10
Tension V 05
AmpereTurn At 100
Pole width mm 20
Pole height mm 40
Coil dimension mm2 6x40
Number of turns Tours 10
Conductor dimension mm2 2x2
Yoke length mm 60
Yoke thickness mm 20
Chamfer mm 15
Table 312 Main characteristics of the ThomX sextupoles
A dedicated bench equipped with a rotating coil will be used for the magnetic measurements of the sextupoles
This bench has been developed for the magnetic measurements of the SOLEIL storage ring sextupoles
344 Corrector magnetic design
Simple window frame correctors based on an already existing SOLEIL design will be used for the orbit
corrections of the two transfer line
Figure 336 Magnetic design of the window frame correctors
lsquo
59
345 Injection dipole
The two injectionextraction dipoles compensate the septum deflection with a 170 mrad deflection (cf sect 337
see Fig 326) They are also air-cooled Their design and their technical characteristics are illustrated in Fig
336 and Table 313
Figure 336 Magnetic design of the injection dipole
Parameters unit value
Gap mm 40
Field at the magnet centre T 0224
Current A
10
Voltage V 6
Pole width mm 60
Pole heigth mm 70
Yoke length mm 150
AI (per coil) At 3600
Coil cross section area mmsup2 60x60
Current density Ammsup2 09
Number of turns tours 360
Conductor size mmsup2 10 x 1
Pressure drop of the cooling water bar NC
Temperature increase degC NC
Power (per dipole) W 120
Table 313 Parameters of the injection and extraction dipoles
lsquo
60
35 Power supplies
In order to provide a high operation flexibility ThomX will be equipped with the following power supplies
ndash One power supply for the ring dipoles (the latter will be connected in series) one for the TL dipoles
and one for the DL
ndash Individual power supplies for all the other magnets (quadrupoles sextupoles and correctors)
All the power supplies will have a 16 bit-resolution and a plusmn20 ppm reproducibilitystability over 24 hours The
technology is a switched-mode one in order to achieve good efficiencies (gt 85)
Magnet Nb PS Nb mag current
(A)
voltage
(V)
type load
R
(mOhm)
L
(mH)
Dip ring 1 8 300 170 Unipol 565 48
Dip TL 1 4 300 85 Unipol 280 24
Dip DL 1 2 300 45 Unipol 150 12
Dip inj 2 1 12 18 Unipol 1330 180
Quad 33 1 12 6 Unipol 500 10
Sext 12 1 12 3 Unipol 250 08
Corr ring 24 1 plusmn12 plusmn3 Bipol 250 08
Corr TL DL 10 1 plusmn12 plusmn3 Bipol 250 08
Table 314 Overview of the ThomX power supply equipment
351 Pulsed magnets
3511 Layout
As discussed above the e- beam injection in the storage ring and its extraction will take place in the
same straight section (see Fig 337)
Figure 337 Injectionextraction section layout
Eddy current septum magnet
(under vacuum) Fast Kicker Fast Kicker
lsquo
61
The first injection step involves a thin septum magnet It bends the incoming bunch trajectory to bring it
inside the ring vacuum chamber close to the stored beam orbit Then a fast kicker compensates for the
residual trajectory angle and brings the injected bunch on-axis
Extraction is realized by another fast kicker followed by the same septum magnet as the one used for
injection This kicker deflects the bunch trajectory so that it enters into the septum channel in turn the
thin septum bends the bunch trajectory which enters then into the extraction channel that brings the
bunch into a beam dump
In this configuration the same thin septum magnet is used for both the bunch injection and the bunch
extraction This requires a symmetrical design with two flanges one dedicated to the injection and the
other to the extraction The magnetic gap is wide enough to accommodate the two bent trajectories
When operating at a 50 Hz injection rate the septum current pulse consists of a slow full sine whose
same positive half sine will be used successively for the lsquoold bunchrsquo extraction and the for the lsquonew
bunchrsquo injection with a suitable delay between the two steps
The two fast kicker magnets are identical their length has been reduced as much as possible because of
the very limited space available The current pulse in the kickers will have a half-sine shape One must
achieve a rise time (from zero current to peak current) and a decay time (from peak current to zero)
each less than a ring revolution period Thus the current pulse can have a maximum duration of two ring
revolution periods Consequently their charging voltage reaches high values even though most of the
deflection is performed by the septum magnet
The design of these three devices ensures that they can operate with a beam energy of 50 MeV
(nominal) or 70 MeV (maximum) and at a 50 Hz repetition rate
3512 Slow septum magnet and its pulsed power supply
The main constraints on the septum design are as follows This septum must produce a large trajectory
deflection (160 mrad nominal peak value 170 mrad maximum peak value) to bring the injected beam
very close to the ring straight section axis with no significant disturbance of the stored beam The
magnet mechanics should preserve the ring vacuum chamber geometry to keep its impedance as low as
possible Furthermore its stray field must remain very low within the vacuum chamber where the stored
beam circulates as the septum excitation is done by a full sine pulse whose width (130 micros) is very long
compared to the ring revolution time (56 ns)
An eddy current septum magnet installed in vacuum will fulfil these constraints The effective septum
blade thickness can be as small as 3 mm thus ensuring the closeness of the stored beam orbit and the
desired bunch trajectory at the output of the septum yoke An internal chamber connected to the magnet
yoke extends the ring vacuum chamber and preserves the impedance budget The injected bunch
deflected by the septum magnet field penetrates through a small hole in this internal chamber Such a
septum magnet is very asymmetric in the transverse plane it is driven by a copper loop that wraps the
steel yoke
In order to contain the magnetic flux the whole yoke is enclosed in a copper box which carries the eddy
current On the side facing the stored beam orbit the copper box thickness is reduced so that it
constitutes the thin septum blade As a result of the electric shielding provided by the eddy current
circulating in the copper septum such a structure is very effective to avoid a large magnetic stray field
outside the magnet gap The fact that the excitation pulse is a full sine reduces the duration and the
lsquo
62
amplitude of the stray field (the slow delayed stray field tends to be the sum of the two opposite half
current arcs this feature brings about a reduction of the stray field)
An additional magnetic shielding is required to strongly reduce the residual strayfield This shield is
provided by a 05 mm thick sheet of high permeability mumetal that will completely surround and
enclose the internal chamber (thus shielding the stored beam orbit from the strayfield)
Such a design that combines an eddy current shield a full sine pulse excitation and the use of a high
permeability magnetic material reduces the stray field to a very low level (a few 10 ppm of the main
pulsed field) [313]
Assuming a septum opening of H x V = 30 x 12 mm2 and an active length of 250 mm (the length was
chosen as short as possible because of constraints coming from the septum implementation in the
straight section) the septum magnet inductance is estimated to be 16 microH Connexions and cables
increase this inductance up to 23 microH
This design leads to the following parameters for the septum magnet and its power supply (values for a
70 MeV ring operation)
Septum deflection field integral field peak current charging voltage
170 mrad 40 mTm 160 mT 1500 A 226 V
Table 315 Septum main parameters
The pulsed power supply will be based on the lsquoclassicalrsquo resonant discharge circuit scheme composed
of a storage energy capacitor (190 microF) a fast high-current thyristor switch and a transmission line to
the magnet comprising a few low impedance coaxial cables
A 300V-20A power supply is foreseen It will be connected to the storage energy capacitor through a
current-limiting power resistor
3513 Fast kicker magnet and its pulsed power supply
The two fast kicker magnets must be identical Because of the very limited space available their length
has been reduced as much as possible The final numbers are 200 mm and 450 mm for the active length
and for the overall length respectively They will be located outside the vacuum vessel with their
lsquowindow framersquo rectangular ferrite yokes surrounding the ceramic vacuum chamber (see Fig 338)
Figure 338 Front view of the kickers
The two ceramic sections of the vacuum chamber will have a flange-to-flange length of 450 mm Their
aperture will be the same as the one in the ring standard sections but with a racetrack transverse shape
Both extremities of each ceramic chamber include a smooth shape transition and a short bellow that
lsquo
63
allows for RF finger contacts These bellows can accommodate thermal dilatations especially during
bake-out The ceramic chambers will be coated with a metallic deposit that will carry the bunch mirror
current and thus avoid an increase of the chamber impedance This coating needs to be very thin It will
consists of a titanium layer with a thickness le0050 microm The thickness has been calculated so as to
avoid the deterioration of the fast pulse shape and to avoid the temperature rise due to the mirror current
[314]
Figure 339 Time profile of the kicker pulse versus the titanium layer thickness
The kicker dimensions follow from
- the aperture needed by the stored beam H x V = 40 x 28 mm2
- the required deflection kick nominal 11 mrad maximum 15 mrad
- the beam energy nominal 50MeV maximum 70MeV
- the short revolution period of the stored e- bunch Trev = 56 ns
- the very short length of the injected bunch from 4 to 25 ps (rms)
With a ceramic thickness of 6 mm the effective magnet aperture must be H x V = 55 x 55 mm2 These
parameters determine the current and voltage pulse characteristics
The kicker magnets will be powered by a circuit delivering half-sine pulses [315] In fact because of
the e- bunch short revolution period (56 ns) the field pulse duration has to be less than twice the ring
revolution period (less than 112 ns) and thus must a have very fast rise time and decay time (both less
than a ring revolution period) On the other hand with such a very short bunch length (4 or 25 ps (rms)
whether one deals with the incoming bunch or the extracted bunch one may assume that the pulses do
not have to present a marked flat top
These considerations lead to the following kicker parameters
00000E
-2
17000E
-10
34000E
-10
51000E
-10
68000E
-10
85000E
-10
10200E
-9
11900E
-9
0000E+00
2000E-01
4000E-01
6000E-01
8000E-01
1000E+00
1200E+00
Thom-X injection kicker pulse field attenuation relating to Titanium thickness
Bext(t)
Bint(t) 002microm
Bint(t) 004microm
Bint(t) 005microm
Bint(t) 006microm
Bint(t) 008microm
Bint(t) 01microm
Bint(t) 02microm
Time
Fie
ld a
mp
litu
de
lsquo
64
Kicker beam energy deflection field integral
Operation at Enominal 50 MeV 15 mrad 25 mTm
Operation at Emax 70 MeV 15 mrad 35 mTm
field peak current estimated voltage
92 mT 406A 10 kV
175 mT 771 A 18 kV
A specific RampD program is necessary to guarantee that these constraints will be fulfilled by the power
supply and by the magnet Preliminary studies were based on the standard circuit used for such a pulsed
power supply (see Fig 340) involving a high voltage (HV) capacitor to store the energy in series with
a HV switch and a load whose inductance is close to the one of the kicker magnet In this circuit the
HV switch is a very fast MOSFET switch followed by a HV fast recovery diode module
Figure 340 Power supply circuit for the ThomX kickers
These first studies lead to unsatisfactory preliminary results
- The BEHLKE Ultra-Fast 17 kV 480 A MOSFET module can switch ON in a very short time less
than 8 ns Then the pulse shape is normally determined by the passive component parameters in the
circuit provided that the negative current is quickly interrupted either by HV diodes in series or by
a turn Off control of the HV switch
- But the different types of HV diodes used are not able to switch OFF the pulsed current quickly
enough Even the lsquofast recovery diodesrsquo have a reverse recovery time too much long to stop cleanly
the pulse at its zero crossing Thus it was necessary to work on other topologies and to find better
suited components
Further simulations were performed to see how the circuit could be improved It turned out that
controlling the switch both at the closing and at the opening can improves the shape of the current
pulse A prototype using the same Behlke MOSFET modules and including a coaxial transmission line
was built and tested A plot of the current pulse in the kicker coil that was obtained is shown below
lsquo
65
This plot was obtained with the following prototype circuit
As one can see more work is still needed to improve the shape of the current in the coil (namely one
should reduce the level of reflections improve the current pulse shape and work on the peak current to
meet the 70 MeV requirements) We are also looking for an other supplier who could provide controlled
HV switches ensuringr both ON and OFF fast operation
Mechanical and magnetic studies of the kicker magnets are ongoing
36 Storage ring RF system
The choice of 500 MHz for the RF frequency is a compromise between several constraints the cavity
fundamental and high order mode (HOM) impedances space requirements the availability of RF power
sources and other components The required accelerating voltage of 500 kV can be provided by only
one 500 MHz single-cell cavity with about 40 kW of RF power As shown in the following the choice
of the cavity design is mainly dictated by the need for a strong damping of the HOM
lsquo
66
361 HOM impedance thresholds as regard to beam instabilities
When a cavity mode is excited in resonance it generates beam oscillations with a rise time
approximately given by one of the two following expressions
(1τ)l = Ib α fm Rl (2 Qs Ee) in the longitudinal case and
(1τ)t = Ib βt fo Rt (2 Ee) in the transverse case
where fm is the HOM resonant frequency Rl [Ω] and Rt [Ωm] are respectively the longitudinal and the
transverse HOM impedances Qs = fs fo is the synchrotron tune (fs and fo are the synchrotron and the
revolution frequencies) βt is the beta function amplitude at the cavity location α is the momentum
compaction factor Ib is the average beam current and Ee is the beam energy in eV
Assuming typical values for the cavity HOM impedances (Rl fm = 01 to 1 MΩ GHz Rt = 1 to
10 MΩm) and the machine parameters listed in Table 315 the instability growth time is around 10 micros
in both the longitudinal and the transverse cases
Orbit length C 168 m
Electron energy E 50 MeV
RF frequency fRF 500 MHz
Revolution frequency fo 1786 MHz
Harmonic number h 28
Momentum compaction factor α 0015
β-function amplitude at the cavity location βxz 3 m 4 m
Relative energy spread σE E 40 10-3
Peak acceleration voltage VRF 300 (500) kV
RMS bunch length σs 27 (21) ps
Synchrotron frequency fs 358 (462) kHz
Bunch charge Qb 1 nC
Number of stored bunches Nb 1
Maximum current Ib 1786 mA
Table 315 Ring parameters used to estimate the rise time of the HOM induced instabilities
In a low energy ring like ThomX radiation damping is so weak (the damping time is ~ 1 s) that a
stationary state is not reached during the beam storage time (20 ms) On the other hand to ensure a
good ring operation it is sufficient to make all instability rise times much longer than this storage
period This can be achieved by attenuating the HOM impedances very strongly typically by a factor
104
362 Cures to the HOM induced instabilities
There are essentially two ways of coping with such HOM induced instabilities either by performing a
strong de-Qing of the HOM resonances or by tuning their frequencies away from the beam spectral
lines in order to prevent resonant excitation
In practice HOM de-Qing is achieved by extracting the cavity HOM power and transferring it towards
external dissipative loads But in this approach it is difficult to reach attenuation factors higher than
102 - 10
3 over a large frequency range and this technique requires heavy and cumbersome equipment
close to the cavity (coaxial lines with coupling antennas or ferrite loaded waveguides) The DAMPY
lsquo
67
cavity designed at BESSY [316] and which is used in ALBA [317] and the PEP II cavity developed
at LBNL [318] are two variants of this technique
The other method which consists in controlling the HOM frequencies is better suited to a small
circumference machine like ThomX where the beam spectral linesrsquo spacing (~ 18 MHz) is very large
compared to the HOM bandwidth (this bandwidth is of a few tens of kHz for Qo equal to a few 104) As
long as the HOM density is not too high and the modes can easily be tuned far enough from the beam
spectral lines (δf gtgt fm Qo) it should be possible to reduce their effective impedances (the ones lsquoseenrsquo
by the beam) down to tolerable levels ie Reff asymp Rs (2 Qo δf fm) 2 ltlt Rs
We are investigating the potential performance of this technique by applying it to an ELETTRA-type
cavity (see Fig 344) [319] which combines three frequency tuning means The HOM frequencies can
be precisely controlled by an appropriate choice of the temperature of the water cooling the cavity while
the fundamental frequency is maintained constant by a mechanism which deforms the cavity
longitudinally In addition a movable plunger tuner also called HOM frequency shifter (HFS) located
in the cavity equatorial plane provides an additional degree of freedom [320]
Tables 316 a and b list the characteristics of such a cavity fundamental and HOM resonances They
have been calculated with the 3D GdfidL code [321] One finds 18 significant HOM (9 monopoles and
9 dipoles) with resonant frequencies lower than the cavity cut-off frequency Figures 345 a and b
represent the corresponding HOM spectra fm Rl(f) and Rt(f) folded over a single base band of
18 MHz The horizontal dashed line in the graph represents the impedance threshold corresponding to
an instability rise time of 20 ms the beam storage time Although they remain somewhat critical these
results tend to confirm that it should be possible to find acceptable operating conditions as long as the
HOM frequencies can be shifted by a few MHz by combining temperature and HFS tuning
lsquo
68
Figure 344 Drawing and photo of the ELETTRA cavity
lsquo
69
Figure 345 (a) Longitudinal and (b) transverse HOM spectra folded over a single base band of
18 MHz for the ELETTRA-type cavity
τL = 20 ms
lb
os
I
eEQ
τα
2
ttob
o
fI
eE
τβ
2
τt = 20 ms
lsquo
70
a) Longitudinal modes fm [MHz] Q0 Rs Q0
[Ω]
Δfm (T)
[MHz]
Δfm (HFS)
[MHz]
L0 (fundamental) 499132 44164 765 000 000
L1 958618 48158 336 037 -100
L2 102781 57861 036 080 369
L3 14087 52562 220 174 021
L4 151528 69088 783 112 086
L5 155129 66375 850 240 721
L6 186912 55785 097 142 -092
L7 19495 81752 008 183 143
L8 203985 62729 086 400
L9 206896 87300 530 445 667
Table 316 Main characteristics of the monopole (a) and dipole (b) modes computed with GdfidL for
an ELETTRA-type cavity Δfm (T) and Δfm (HFS) are the effective temperature and HFS tuning ranges
assuming a constant fundamental frequency
It is worthwhile mentioning that in any event fast longitudinal and transverse feedback systems are
required to damp the beam oscillations resulting from injection transients (see section 333) They will
contribute to the HOM damping as well
363 RF power source
An RF power of about 40 kW is required to generate the desired voltage of 500 kV in a single 500 MHz
ELETTRA-type cavity For this frequency there exists a wide choice of power sources based either on
vacuum tubes (klystron IOT diacrode) or on solid-state technology The low operating voltage and
high modularity of the solid-state version offer significant advantages compared to vacuum tubes
Experience acquired at SOLEIL after few years of operation has definitely demonstrated all the benefits
of this technology [322] Figure 346 shows the 35 kW - 352 MHz solid-state amplifier used for the
SOLEIL booster synchrotron It consists of a combination of 147 modules Each one has a power of 330
W a gain of 11 dB and an efficiency of 62 This amplifier was commissioned about 7 years ago
During a running period of more than 35 000 hours it tripped only once and this trip was caused by a
human mistake
b) Transverse modes fm
MHz]
Qo Rsperp Qo
[Ω]
Rsperp[MΩm] Δfm(T)[MHz] Δfm(HFS)
[MHz]
T1 73701 50267 335 2599 04 075
T2 74606 52566 92 7557 155 285
T3 115202 47196 264 3006 049 -312
T4 125565 55386 51 7428 068 725
T5 130249 71674 36 7039 097
T6 169981 69325 103 2542 188
T7 173650 91039 165 5463 gt 2
T8 177337 59409 32 7061 gt 2
T9 183617 77697 55 1643 gt 2
T10 206142 88484 59 2254 gt 2
T11 222475 99092 11 5079 gt 2
T12 225933 56641 13 3484 gt 2
lsquo
71
This technology has already been extended to 476 MHz in the framework of a SOLEIL-LNLS
collaboration aimed at realizing two 50 kW amplifiers for the LNLS storage ring [323] More recently
the SOLEIL team has developed modules which can deliver up to 650 W at 500 MHz with a gain of
17 dB and an efficiency of 62 while still reducing the thermal stress [322] About a hundred of such
modules will be combined to obtain the 50 kW required by ThomX
Figure 346 35 kW - 352 MHz solid-state amplifier of the SOLEIL Booster (147 amplifier modules
and DCDC converters bolded on 8 water-cooled dissipaters)
364 Low level RF system and longitudinal feedback
The proposed low level RF (LLRF) system is conventional As shown by the synoptic diagram of Fig
347 it consists of three relatively slow loops (frequency amplitude and phase) aimed at controlling the
cavity voltage with an accuracy of about plusmn 02 in amplitude plusmn 02deg in phase and a response time
around 1 ms This scheme is suitable for compensating the pressure and temperature drifts as well as
the power supply ripples
Such a system is not fast enough to deal with the beam phase oscillations at the synchrotron frequency
fs on the order of 400 kHz that originate in injection transients These oscillations can produce an
undue emittance blow up and a phase jitter at the interaction point Beam dynamics simulations show
that preserving the beam quality requires a damping of these oscillations within a few tens of a micros (see
section 333) As the radiation damping is negligible during the storage time a longitudinal feedback
600 W 280 Vdc 28 Vdc converter
Power combination
330 W amplifier module
lsquo
72
(LFB) must be implemented It can be performed by a dedicated broad-band longitudinal kicker that
acts on the beam Since ThomX is operating with a single bunch we propose to use the main cavity as a
longitudinal kicker despite its narrow bandwidth of 25 kHz ltlt fs This difficulty can be overcome by
making the cavity effective bandwidth much larger than this value in fact larger than fs We will use a
high gain direct RF feedback (RFFB) that allows the operation of a fast phase feedback (ϕFB)
Assuming a cavity-to-amplifier distance of about 10 m the loop delay will be less than 150 ns Then
one expects an RFFB gain Go larger than 50 which leads to an effective bandwidth larger than 1 MHz
Simulations show that a LFB that combines such a RFFB with a ϕFB (see Fig 348) can damp injection
phase errors of up to 10 degrees within a few micros ie faster than the above specification
Figure 347 Basic LLRF system scheme with lsquoslowrsquo frequency amplitude and phase loops
lsquo
73
Figure 348 Schematics of the longitudinal feedback system combining a phase feedback with a direct
RF feedback
365 RF equipment status
For the ThomX commissioning we will use an ELETTRA cavity The design of all the components of
the 50 kW solid-state amplifier (600 W modules combiners power supplieshellip) was validated in 2011
The next two milestones are first the validation of a 9 kW unit (combining 16 modules) and secondly
the commissioning of the complete amplifier on a dummy load
Several versions of the LLRF system including LFB are under investigation either fully analogue (see
Fig 349) or mixed analogue for the fast feedback loops and digital for the slower ones (see Fig 350)
or even fully digital based on the new generation of fast ADC DAC and FPGA
366 RF system conclusion
For the RF system of the ThomX storage ring we plan to use a 500 MHz single-cell cavity of the
ELETTRA-type Maintaining at a tolerable level the impact of the RF cavity HOMs on the beam is a
challenge However this goal should be achievable with a proper control of the HOM frequencies It
can be done by combining three different means of tuning the cavity temperature control longitudinal
elastic deformation of the cavity and motion of a plunger The fast longitudinal and transverse feedback
systems - which in any event are required to damp the beam oscillations resulting from injection
transients - will contribute to the damping of the HOM excitations as well
In order to provide the required accelerating voltage of 500 kV the cavity will be powered by a solid-
state amplifier of the lsquoSOLEILrsquo type able to deliver a power of up to 50 kW The cavity voltage will be
controlled with an accuracy of plusmn 02 and plusmn 02 degree by means of conventional amplitude and phase
loops (either analogue or digital) with a few kHz bandwidths In addition a fast phase feedback system
combined with a high gain direct RF feedback will provide the longitudinal damping which is required
to preserve the beam quality
lsquo
74
Figure 349 Complete LLRF system longitudinal feedback included (analogue version with phase and
amplitude modulation ndash the option with IQ modulation is also considered)
Figure 350 Complete LLRF system longitudinal feedback included (mixed analogdigital version)
37 Transverse feedback
371 Specifications
Table 317 summarizes the instability growth time constants and the corresponding kicker strength
requirements for the main types of instability It shows that for Thom-X the betatron oscillations due to
the injection jitter in position and angle are the most critical ones They require a feedback damping
time constant of 5 micros or less (cf chapter 3)
lsquo
75
Source type growth time constant kicker strength
Beam pipe
geometry
TMCI
Head-Tail
-
160 micros
gt10 nrad
Resistive Wall 600 micros gt2 nrad
Ions lt 100 micros gt20 nrad
Injection Jitter 5 micros 2 microrad
Table 317 Overview of expected instabilities (1 nC 25 ps rms at 50 MeV)
372 Electronics
To cope with these instabilities we will use a SPring-8 based high performance digital transverse
feedback system which consists of a wideband detector (a BPM) an RF front-end an FPGA based
processor a power amplifier and a stripline kicker (see Fig 351) This system is capable of detecting a
coherent transverse motion and applying a counter kick to damp it bunch-by-bunch and turn-by-turn
with one or two bunches stored in the ring
Figure 351 Spring-8 based digital feedback system
Two BPMs (one for each plane) will provide the beam position information The RF front-end will
extract the betatron oscillation information which will be sampled by ADCs From the sampled data the
FPGA will compute a suitable kick to damp the oscillation The signal processor output will be
amplified by a 10 W wideband amplifiers before being fed to the kicker
In the RF front-end (see Fig 352) a baseband down-converter will be implemented in order to amplify
the beam signal and to minimize the jitter effect for the same reason the digital kick will also be
extended over four RF periods
lsquo
76
Figure 352 Front-end of the Thom-X RF electronics
For the commissioning phase a digital system (rather than an analog one) will be used to benefit from
the tuning flexibility provided by the digital parameters of the FIR filter Once the machine tune will be
fixed a more suitable low cost analog system will be installed
373 Kicker
The kicker is under development It consists of a stripline composed of four electrodes connected to
vacuum feedthroughs on each side (see Fig 353) The amplifiers will be connected to the downstream
ports whereas upstream ports will be used for signal detection and they will be equipped with 50 Ω
loads The same kicker will be used to act on the beam in both planes
Electrode line impedances must be adapted (50 Ω) Electromagnetic 3-D simulations are being
performed to optimize the bandwidth and shunt impedance (ie to maximize the stripline efficiency)
and also to minimize the kicker impedance to wakefields
Figure 353 Schematics of the Thom-X stripline kicker
lsquo
77
38 Vacuum
381 Transfer line vacuum
Concerning the design of the transfer line vacuum system three main constraints have been taken into
account
- Pressure of 10-7
Pa at the acceleration section output
- Ion pump operating at 10-6
Pa
- Ring pressure of 10-8
Pa
Simulations have been performed assuming that two 03 m3 s
-1 triode pumps will be installed one at the end of
the acceleration section and the other at the septum The vacuum chamber is a DN40 The septum output
conductance is 018 m3 s
-1 Without baking the outgassing rate would be σ τ = 2x10
-8 Pa m s
-1 and the
pressure at the ring entrance would be about 10-7
Pa Thus the septum and its environment must be baked to
bring τ down to 5x10-10
Pa m s-1
and obtain a pressure of 10-8
Pa at the ring entrance
Pressure in the line of transfer
The vacuum equipment envisaged for the transfer line is the following one
‐ 3 ion pumps 150 l s-1
with their baking systems and power supplies
‐ 1 ion pump 500 l s-1
with their baking system and power supply
‐ 4 Pirani gauges
‐ 4 cold cathode gauges
‐ 2 power supplies gauges
‐ 2 gate valves electro pneumatically actuated DN 40 (acceleration section output ring entrance beam
dump)
‐ 1 turbo pump (DN 63 ~ 80 l s-1
)
‐ 1 rotary vane pump (~ 12 m3 h
-1)
‐ 4 angle valves manually actuated for pre-pumping DN 63
lsquo
78
382 Ring vacuum
Concerning the ultra-vacuum to be maintained in the ThomX ring one must take into account first the
pipe relatively low specific conductance c = 452x10-3
m3 m s
-1 and secondly the pipe specific
outgassing rate θ τ = 108x10-10
Pa m2 s
-1 Also the pumpsrsquo effective pumping speed is reduced by the
T port with the grid Seff = 19x10-2
m3 s
-1 In a first stage the ionic pumps chosen for the simulations
are 0040 m3 s
-1 diode pumps This corresponds to S = 0030 m
3 s
-1 at 10
-8 Pa and with a grid pumping
port whose conductance is 00362 m3 s
-1 to an effective pumping speed Seff = 0019 m
3s
-1 The pressure
has been estimated by modeling a quarter of the ring including half of the non-baked optical cavity ie
in the most unfavorable case
Figure 354 Pressure distribution in the vacuum chamber
The calculated average pressure is 306x10-8
Pa It can be improved by increasing the grid pumping port
conductance by replacing the DN35 with a DN63 Monte-Carlo simulations show an improvement of ~
16 in efficiency
The entire ring must be baked Geometrical and temperature constraints (respectively a distance of only
07 mm between the magnet coils and the vacuum chamber a baking temperature of 150 degC for the
chamber while keeping the coilsrsquo temperature below 80deg C) are strict An ultra-thin baking system has
been constructed tested and validated
Figure 355 The vacuum chamber ultrathin baking set-up
lsquo
79
The following is the vacuum equipment needed for one quarter of the ring
- 3 ion pumps (40 l s-1
) with their baking equipment and their power supplies
- 1 residual gas analyzer (RGA)
- 2 turbo-pumps (DN 63 ~ 80 l s-1
)
- 2 rotary vane pumps (~ 12 m3 h
-1)
- 2 Pirani gauges
- 2 cold cathode gauges
- 1 gauge power supply
- 1 DN 63 gate valve electro-pneumatically actuated with an HF feed-through (one on each side of
the RF cavity and similarly for the septum) and the baking equipment
- the vacuum chamber baking equipment
- 3 angle valves manually actuated for pre-pumping DN63
- 3 T with the grid
lsquo
80
Chapter 4 INJECTOR TRANSFER LINE AND RING DIAGNOSTICS
Several diagnostic devices will be implemented in the ring to measure the electron beam parameters
listed in Table 41
Parameter unit value
Energy MeV 50
Average current mA 1784
Bunch charge nC 1
Energy dispersion 03 at injection to 06 at storage end
Emittance x mrad 5 e-8 at injection to 30 e-8 at storage end
Normalized emittance x mrad 5 e-6 at injection to 30 e-6 at storage end
Emittance y mrad 5 e-8
Normalized emittance y mrad 5 e-6
Bunch length ps-rms revolutions 1 ~ 4
revolutions 100-200 ~ 25
revolutions 200-300000 ~ 50
RF frequency MHz 500
Revolution frequency MHz 1784
Period ns 56
Injection repetition rate Hz 50
Ring circumference m 1680
(a) Ring parameters
Magnetic field T 05
Radius of curvature m 0352
R51 m 0 to -025
R52 m 0 to -045
R56 m 07 to -07
Beta x m 06
Beta y m 6
Eta x m 02 - 03
Transverse dimension x mm (rms) 06
Transverse divergence x mrad 2
Transverse dimension y mm (rms) 054
Transverse divergence y mrad 2
(b) Parameter values at the location of the bending magnets
Table 41 ThomX beam parameters
41 Charge measurement
411 Current transformers
The beam charge will be measured non-destructively with commercial in-flange high resolution
current transformers (ICT) Three devices will be installed
- one in the linac (LIDGICT010) located before the accelerating sections
- one at the transfer line entrance (TLDGICT010)
lsquo
81
- one at the transfer line exit (TLDGICT020)
In-flange ICTs are directly mounted in the beam line They are UHV compatible can be baked up to
150deg C (185deg C on request) and they are available for many pipe diameters (also with an elliptical
aperture or any other aperture shape) We will use the CF6rdquo model its internal diameter is 604 mm and
it is 60 mm long This device includes a ceramic gap a shield and a wall current bypass Bellows are
not required
The signal from each integrating current transformer will be processed by a BCM (beam charge
monitor) A built-in calibration generator provides an on-line check of the measurement reliability An
Integrate-Hold-Reset (BCM-IHR) device is suited for pulse repetition rates ranging from 10 kHz down
to single pulses and for bunch charges up to 2 nC The acquisition will be performed by using a 100
kHz 12 bit ADC
412 Faraday cups
Interceptive absolute beam charge measurements will be performed by Faraday cups There will be two
of them placed inside the two beam dumps (see Fig 2 of the Introduction) They should provide a
better resolution for small electric charges
42 Transverse profile measurements
421 Diagnostics stations
Five diagnostics stations will be installed to measure the transverse beam profile as shown in Fig 41
Diagnostics station 1 (DS1 ndash LIDGSST010) will be located after the RF gun at the beginning of the
linac Diagnostics station 2 (DS2 ndash TLDGSST010) will be located between the end of the linac and
the linac beam dump Diagnostics station 3 (DS3 ndash TLDGSST020) will be placed in the middle of the
transfer line Diagnostics station 4 (DS4 ndash TLDGSST030) will be located at the end of the transfer
line and Diagnostics station 5 (DS5 ndash ELDGSST010) will be located before the ring dump
lsquo
82
Figure 41 Location of the diagnostics stations (see text for the nomenclature)
The ThomX station design draws from the station built by LAL for PITZ (itself drawn from a SOLEIL
design) It is illustrated in Fig 42
Figure 42 3D view of a ThomX diagnostic station
Each station will have a motorized arm on which several screens will be mounted A stepper motor will
move this arm and insert the screens laterally The following screens will be available
- 1 YAG(Ce) screen in all stations
- 1 optical transition radiation (OTR) screen in all stations
lsquo
83
- 1 target to allow an online calibration of the camera in all stations
- one Cerenkov screen for bunch length measurements in station DS2 (described in more detail
below)
The YAG(Ce) and OTR screens and the calibration target will be tilted at 45deg with respect to the beam
axis so that the emitted or reflected light will travel downward and exit the vacuum through a DN60 CF
fused silica window A CCD camera will be located below the station to image the screens The
Cerenkov screen will be titled so that its light exits at the top of the diagnostic station Shutters will be
used to close either the Cerenkov window or the YAG(Ce)OTR window when not in use to prevent
measurement noise due to background photons Several LEDs located just after the bottom window will
be used to illuminate the inside of the chamber when needed
At the beginning of the Linac at station DS1 the beam size will be about 05-07mm (rms) in both
planes at the end of the Linac at DS2 the beam size will be 15 mm (rms) in both planes In the middle
of the transfer line at DS3 the beam size is expected to reach 25 mm (rms) in x and slightly less than
1mm in y At the end of the transfer line at DS4 it will be about 1 mm in both planes After the
extraction at DS5 the beam size will be about 05-07mm (rms) in both planes Given these beam sizes
we chose a screen diameter of 25 mm for all stations
Screen photon yield
The YAG(Ce) screen will be 100 microm thick Such screens have a light yield of 8 photons per deposited
keV [41] The YAG radiation length is 35 mm [42] assuming the usual model for energy deposition in
matter [43] each electron will deposit about 143 keV in the screen this corresponds to the emission of
~1100 photons Thus the total number of photons produced by an e- bunch crossing the screen will be
71 1012
We recall that YAG(Ce) emission occurs in all directions
The second screen will be of the OTR type The screen support design allows either the use of a 100 microm
aluminized silicon wafer or of an ultra-thin aluminized mylar film The silicon wafer has the advantage
of giving a better image quality while the mylar film allows a beam injection through the screen without
significantly affecting its quality The OTR yield within a cone of half-aperture θ for photons whose
wavelengths range between λ1 and λ2 is given by [44]
Thus for a standard CCD camera whose sensitivity ranges between 400 nm and 800 nm and assuming
an acceptance cone of 003 radian this expression gives ~0002 photon per electron at 50 MeV (station
DS2-5) A similar yield is reached at 5 MeV with an acceptance of 03 radians (station DS1)
To image the screens a lens will be located 500 mm away from them A small telescope made of two
lenses will be used to focus the light on the CCD camera At least one of the telescope lenses will be
movable to provide the needed dynamic range A Gigabit Ethernet camera with at least 600 x 400 pixels
will record the images The camerarsquos square pixels are about 74 x 74 microm2
With such a configuration one expects that a 50 MeV bunch passing through the YAG(Ce) screen will
emit 71x1012
photons and assuming that the CCD chip has an average efficiency of 50 1010
counts
will be recorded by the camera taking into account the solid angle covered by the lens
lsquo
84
The same bunch passing through the OTR screen will emit 82 107 photons within the lens acceptance
Thus 3 106 counts will be recorded by the camera
For a 5 MeV bunch (station DS1) the YAG(Ce) screen will emit 2 1012
photons and 109 counts will be
recorded by the camera The OTR screen will emit 82 107 photons within the lens acceptance and 3 10
6
counts will be recorded by the camera
Assuming a transverse beam size of 15 mm one expects 8 106 counts per pixels with a YAG(Ce)
screen and 25 105 counts per pixel with an OTR
If the 50 MeV beam is defocused (for example during a quadrupole scan) to produce a 5 mm spot size
these numbers become 8 105 for the YAG(Ce) screen and 2 10
4 for the OTR
Number of OTR
photons emitted
Number of
OTR
photons
recorded by
the camera
Number of
YAG
photon
emitted
Number of
YAG
photons
recorded
DS1 82 107 3 10
6 2 10
12 10
9
DS2-5 82 107 3 10
6 71x10
12 10
10
Assuming a YAG Cerium dopant density of 018 we get a Cerium density of 25 1019
cm-3
[45] and
a 15 scintillation efficiency It follows that the scintillating site density is 37 1018
cm-3
Therefore the
100 microm YAG(Ce) screen will saturate when the photon density reaches 36 1014
mm-2
This last figure
translates into an electron density of 32 1011
mm-2
for an e- beam energy of 50 MeV In these
conditions the YAG(Ce) screen will saturate if the beam is focussed on a spot smaller than 20 microm This
is five times less than the smallest beam foreseen at ThomX
Additional targets will be used to calibrate the cameras when necessary They will be made of stainless
steel plates with etched grids Each square of the grid will be 2 by 2 mm2 In some of these squares
smaller lines of known thickness will be etched Images of these targets will allow one to check the
whole optics
To minimize a possible radiation deterioration of the cameras lead-glass windows will be inserted
between the vacuum windows and the focusing lenses
43 Emittance measurement
The beam emittance will be measured with the quadrupole scan technique In this technique a triplet
(set of three quadrupoles) is used to find the minimum transverse beam size in each plane A fit of the
beam size variation versus the quadrupole strength enables one to retrieve the projected transverse
emittance
The sets of triplets and the diagnostic stations implemented along the ring allow an emittance
measurement at three different locations at the exit of the linac with DS2 in the transfer line with DS4
and in the ring with DS5 by using the extraction kicker The beam emittance measurement with DS5
lsquo
85
will provide a check of the turn-by-turn emittance growth thus bringing precious information on the
transverse beam dynamics
Pepper-pot measurements are also foreseen The advantage of this technique with respect to a
quadrupole scan is that it consists in a single shot measurement independent of machine operation
fluctuations
A pepper-pot set-up comprises a plate with evenly distributed holes Such a plate is placed in the beam
path together with a scintillating screen located downstream and an imaging system A pepper-pot
mounted on a movable arm could be located at the end of the linac between the last dipole and the
diagnostic station DS2 and another one could be located just before station DS5
At station DS2 the beam size will be about 15 mm (rms) and the geometric emittance about 5 10-8
m
rad Therefore the maximum beam divergence will be about 50 microrad By adjusting the quadrupoles one
may focus the beam down to a 20 microm spot and increase its divergence up to 250 microrad To measure the
transverse emittance one would have to position this focal point two meters before DS2 (or DS5) and
to position a pepper-pot mask with 100 microm holes or slits one meter upstream of DS2 (or DS5)
44 Beam position measurement
The beam position will be measured by a set of BPMs placed all along the ring Their locations are
shown in Fig 43
lsquo
86
Figure 43 Location of the BPMs along the ring (note the BPM located at the IP has been suppressed
since this drawing was made)
441 Beam Position Monitors (BPM)
Two types of BPMs will be used short circuited striplines in the linac and the transfer line and buttons
in the storage ring
Compared to button BPMs striplines generate a higher signal but need more space and contribute more
to the wake-field generating impedance For these reasons striplines are preferred for the linac and the
transfer line where we need single pass measurements and where wakefield is not an issue In the
storage ring button BPMs will be chosen for their small size and small contribution to the ring
impedance The lower signal produced by the small electrodes will be compensated by the high
repetition rate of the stored beam (averaging is possible)
BPM electronics detection frequency on the storage ring is usually equal to the RF frequency as it is the
only common frequency component present in the BPM data whatever the filling pattern is Even
though only one bunch will be stored in ThomX the same approach is chosen Therefore the storage
ring BPM electronics detection frequency is 500 MHz
Since it is more convenient to adopt the same electronics for all the BPMs (linac transfer line and
storage ring) stripline BPMs on the Linac and transfer line are optimized to give a signal with the
highest possible component at 500 MHz Their electrode length is thus adapted to this frequency which
means that it has to be equal to λ4 (150 mm 500 MHz) where λ is the RF wavelength
4411 BPM for the linac
The linac BPM equipment will be as follows
- Number of BPM 1
- Type short circuited stripline
- Electrode length 100 mm
- Required resolution 100 microm for a single pass measurement and a 1 nC bunch charge
Figure 44 Linac BPM
lsquo
87
Due to lack of space in the linac section where a stripline must be installed the electrode length cannot
be 150 mm but must be reduced to 100 mm Transfer impedance calculations show that at 500 MHz
the signal amplitude will be reduced by 13 compared to the optimal length of 150 mm (see Fig 45)
This amplitude reduction is acceptable
Feedthroughs will be soldered on CF16 flanges
Figure 45 Stripline transfer impedance versus frequency for various electrode lengths With an
electrode length of 100 mm the signal is reduced by 13 compared to its maximum value for a
frequency of 500 MHz
4412 BPMs for the transfer line
The transfer line will be equipped as follows
- Number of BPM 4
- Type short circuited stripline
- Electrode length 150 mm
- Required resolution 100 microm for a single pass measurement and a 1 nC bunch charge
lsquo
88
Figure 46 Drawing of the BPMs that will equip the electron transfer line
Feedthroughs will be soldered to CF16 flanges
4413 BPMs for the storage ring
The specifications relevant to the ring BPMs are as follows
- Number of BPM 12
- Type button
- Button diameter 107mm
- Required resolution ~1 microm 50 Hz and for a bunch charge of 1 nC
- Absolute precision plusmn 250 microm
In order to provide additional BPM signals for other applications (see below) 4 BPM blocks will be
equipped with a double set of electrodes In that case only 4 electrodes (among 8) will be used for
position monitoring The button size and the beam pipe size remain unchanged
Figure 47 Simple and double BPM blocks for the ThomX storage ring
The button positions and dimensions have been determined by simulations with Poisson Superfish (an
electrostatic simulation software) An evaluation of the impedance due to the button discontinuities has
shown that their effect will be negligible
Feedthroughs will be soldered directly to the BPM block They are bakable Connectors will be SMA
males to prevent any deformation during baking
lsquo
89
Alignment
BPM blocks will have an external mechanical reference This reference will be used for BPM alignment
during the installation phase and to position precisely the block in a calibration bench
Calibration
Storage ring BPMs will be calibrated by using a dedicated bench An antenna inside the block will be
fed with a 500 MHz wave of constant amplitude Calibration is done by measuring the signal output by
the 4 buttons A precise position of the antenna is not needed as position errors can be eliminated by
repeating the measurement with the BPM block turned by 180deg and averaging the results
The second halves of the blocks equipped with two sets of buttons will be dedicated to
‐ longitudinal (beam phase) feedback (one set of BPMs)
‐ transverse feedback (one set of BPMs)
‐ ion clearing To suppress ions which may generate beam instabilities BPM buttons will be
polarized with a high voltage (max plusmn200V) This electric field will deflect the ion trajectories
toward the vacuum chamber
Electronics
Electronics will be the same for all BPMs (whether equipping the linac the transfer line or the storage
ring)
Data rate
Revolution-by-revolution beam position measurements would be best But because of the high
revolution frequency (178 MHz) it might be difficult to get electronic devices that handle the
corresponding data rate Then one may work with averages over two turns This data will be provided
on demand and the acquisition will be triggered The buffer size should accommodate data collected
during at least 20 ms to cover full storage cycles Revolution-by-revolution data will be used for
machine physics studies and tune measurement The position measurement resolution must be better
than 100 microm
Slow acquisition
Slow data acquisition will be used for slow orbit correction and slow orbit feedback It will be obtained
by averaging the fast data flow with a sampling rate of ~10 Hz A data rate of 50 Hz would provide just
one measurement per storage cycle The resolution aimed at for this measurement is one micron or
better
Acquisition must be synchronous with the machine revolution frequency Moreover modules must be
synchronized together
Connections and cables
The four cables that carry the signals of each BPM should be put together in a strand Their electrical
lengths will be matched so that phase differences are kept below 13deg 500 MHz The cable attenuation
should not differ much from one BPM to another If the cable lengths cannot be the same for all BPMs
cable sections will be chosen in order to ensure the same attenuation for all cables To minimize both
lsquo
90
attenuation and cost cable lengths below will be kept below 30 m The LMR240 cable is an example
of what could be used its attenuation is ~6 dBm 500 MHz over a distance of 33 m
As radiation close to the machine can damage the cable insulator short (~1 m) pigtails (that can be
easily replaced if needed) will be inserted between the BPMs and the long cables
442 Orbit feedback system
A feedback system will keep the beam orbit stable within plusmn1 microm It will use the position measurements
provided by the 12 ring BPMs The correction rate will be comprised between 01 Hz and 10 Hz
Correction signals will be applied to the 12 correctors located along the ring The orbit feedback system
will be a matlab based application
443 Tune measurement and feedback
The tune will be obtained from the revolution-by-revolution data provided by the BPMs There is no
need for any kicker to excite betatron oscillations as the beam transverse position will naturally oscillate
after each injection (ie every 20 ms) If necessary a tune feedback will be implemented to keep the
tunes constant
444 Energy measurement
In the middle of the transfer line ie downstream of the dipole located at the end of the linac the
dispersive function is non-zero Therefore the beam energy and its spread can be deduced from the
electron horizontal distribution shown by the YAG screen in DS3 The same measurement will be
performed with DS4 using the dispersion induced by the extraction magnet
45 Bunch length measurement
The bunch length will be measured at two different places at the end of the linac and in the ring
Depending on the observation point two different physical processes are used to produce visible light
pulses which are then transported by dedicated optical lines through the ThomX shielding wall to a
double-sweep streak camera located in the experimental hall
451 Cerenkov radiation
Cerenkov radiation (CR) produced in a target placed at the end of the linac will be used to measure the
e- bunch length Cerenkov radiation is emitted when a charged particle passes through a medium at
velocity greater than the speed of light in that medium It is emitted at an angle θcr ndash with respect to the
particle momentum ndash determined by n(λ) the medium index of refraction and β the particle velocity as
follows
Above 10 MeV the emission angle is almost independent of the particle velocity Ncr the number of
radiated photons whose wavelengths range between λ1 and λ2 summed over a path of length d along the
charged particle trajectory is
( )
minus
minus
λnβλλπαd=N
cr 22
21
11
112
( )
minus
minus
λnβλλπαd=N
cr 22
21
11
112
( )( )λβn
=θcr
1cos
lsquo
91
where α=73 10-3
is the fine structure constant
Two types of radiator are usually used to produce Cerenkov radiation an aerogel or a solid target
Aerogels dramatically spoil the vacuum and therefore have to be mounted in a separate vacuum vessel
to maintain ultra-high vacuum conditions Solid targets are UHV compatible This is why they are
preferred
At the end of the linac when the bending magnet is off the beam goes straight into a beam dump
Before the dump several types of targets can be inserted in the beam trajectory for diagnostic purposes
For the production of Cerenkov radiation a sapphire target is adequate This materialrsquos index of
refraction is 177 With a 50 MeV beam the Cerenkov angle is θcr=5563deg Assuming that the window
thickness is 025 mm 16 photonse- are created with a wavelength ranging between λ1 = 300 nm and λ2
= 800 nm Their spectrum is given in Fig 48 For a 1 nC electron bunch 1011
photons are created The
energy deposited in such a target is negligible
Figure 48 Spectrum of the Cerenkov radiation emitted by a 50 MeV e- beam passing through of
sapphire window 025 mm thick
For simplicity the sapphire target is oriented so that the Cerenkov radiation exits in the vertical
direction For a Cerenkov angle θcr = 5563deg the window angle with respect to the beam axis must be θP
= 11475deg (see Fig 49)
8x10-8
6
4
2
0
In
ten
sity
[au
]
800700600500400300200
Wavelength [nm]
lsquo
92
Figure 49 Orientation of the sapphire target used to observe the Cerenkov light emitted by the e- beam
at the linac end
At the target location the beam dimensions will be σxy= 14 mm (rms) It will produce a light spot
whose dimensions will be σx = 14 mm (rms) and σy = 154 mm (rms) in the sapphire To intercept the
whole light beam and to leave some margin for the alignment we will use a target whose diameter will
be equal to 2 x 6 x σy ~ 20 mm
The Cerenkov radiation will exit the vacuum chamber via a UHV viewport made of fused silica (FuSi)
with a 90 transmission over the 300-800 nm wavelength range The viewport is mounted on a CF63
flange that provides a 68 mm clear aperture At the exit of the viewport a FuSi lens is used to focus the
light This collimated beam is then transported via a set of planar mirrors across the shielded area and
then through the ThomX shield to a dedicated hutch in the experimental hall (see sect 453)
452 Synchrotron radiation
Most of the synchrotron radiation (SR) emitted when the electrons circulate in a bending magnet (BM)
will be in the far infrared but visible light will also be present The cut-off wavelength of this radiation
corresponds to ~077 eV Thus the X-ray content is negligible This last feature makes the extraction of
the visible part of the spectrum easy The spectral flux per unit bandwidth per unit surface ΦBM
[Photons01bwmmsup2] is shown in Figure 410
Figure 410 On-axis spectral flux 1 m downstream of a bending magnet Photon energy in eV The
visible spectral range corresponds to [400 nm - 800 nm] or [155 eV - 31 eV]
The radiation is extracted at the exit of the BM ndeg5 through a porthole which provides a 63 mrad x 32
mrad aperture at 15deg with respect to the 0deg incident angle in the dipole The maximum power density is
lsquo
93
4 microWmmsup2 in the horizontal plane of this extraction porthole thus no dedicated absorber is required
(But for the porthole region the radiated power will be dissipated in the vacuum chamber wall) The
radiation will exit the vacuum chamber through a UHV viewport made of MgF2 (this material offers
the highest transmission coefficient over the 200-800 nm range) The synchrotron radiation distribution
at 5 eV one meter downstream of the bending magnet is around 6 mm (FWHM) So the spectral flux
integrated over a collecting area of 6 mm x 63 mm is about (around 5 eV)
Φe [Phs] = ΦBM [Phs01bwmmsup2] x 6 mm x 63 mm x 1000
Since photons of lower energy thus with a higher divergence will also contribute to the total spectral
flux this value gives a low limit estimate of the real total spectral flux Figure 411 shows the Φe
spectral flux
As was the case for the Cerenkov radiation a FuSi lens will be used at the exit of the viewport to focus
the beam The collimated radiation will then be transported via a set of planar mirrors across the
shielded area and then through the ThomX shield to a dedicated hutch in the experimental hall (see next
section)
Figure 411 Spectral flux Φe in a 63 x 32 mrad2 aperture 1 m downstream of a bending magnet
453 Transport of the Cerenkov and synchrotron radiations to the diagnostics hutch
Both the Cerenkov light and the synchrotron radiation have to be transported to the diagnostics hutch
located in the experimental hall Starting from different locations both radiations will be transported via
a set of planar mirrors to a lsquomeeting pointrsquo inside the shielded area A flip mirror (ie a double-
orientation mirror) will enable the selection of one of the radiations The one selected will be further
transported again with a set of planar mirrors through the shielding wall to the diagnostics hutch
Inside the diagnostics hutch the last lens will focus the radiation on the streak camera entrance The
lens aperture and its focal length will be adapted to the selected incoming radiation All the line
components and their parameters are chosen to minimize spatial aberrations and to maximize the
number of photons entering the streak camera
454 Streak camera for the bunch length measurement
We plan to use a double-sweep streak camera to measure the bunch length and to visualize its evolution
over short time scales Since the bunch length may vary from 4 to 20 ps (rms) a 2 ps (FWHM)
resolution streak camera is required
lsquo
94
The time resolution of the bunch length measurement depends on the radiation type (CR or SR) which
is used and involves several contributions the electron transverse beam size at the light emission point
chromatic effects etc All these contributions will be minimized as much as possible to maintain the
effective resolution as close as possible to the one of the streak camera
46 Bunch imaging using synchrotron radiation
The synchrotron radiation transported to perform the bunch length measurement (see section 45) can
also be used to image the bunch while it circulates For this purpose a beam splitter could be installed
at the radiation exit port the main part of the radiation would be sent to the streak camera while a small
fraction would be directed to a CCD camera mounted nearby A focusing lens placed between the
viewport and the CCD would enable the imaging of the stored beam
In the orbit section where the bunch emits the synchrotron radiation used for the above measurements
the e- beam transverse size is about 05 mm (rms) Since the size of a CCD chip is typically 5 mm the
lens gain should be about 10 Such a set-up would enable one to follow the evolution of the beam size
in real time (we will use a CCD whose acquisition rate is about 10 Hz) It would be a non-destructive
and cheap diagnostic Similar setups are currently used on storage rings to measure the electron beam
emittance assuming that the beta function is known
47 Beam loss measurement
Particle losses will be measured by using optical fibers [46] When a charged particle crosses a glass
fiber Cerenkov radiation is emitted within the fiber and propagates along the fiber It can be detected
by a photomultiplier that collects the light at the fiber end The measurement eg with an oscilloscope
of the pulse production (in the fiber) time enables one to recover the particle loss location
Two fibers will be installed One will be used to monitor e- losses along the linac and the transfer line
The second fiber will be used to monitor particle losses along the ring Each one of these fibers will be
connected to a fast photomultiplier (PMT) The PMT time resolution will determine the accuracy with
which the losses can be localized
A fused silica fiber has a yield of 227 photons per cm of a relativistic electron track [47 48] Therefore
a 5 mm thick fiber will have a yield of ~100 photons per relativistic electron which crosses it Such a
signal should be easily detected by a PMT
As it may be difficult to place the fiber close to the beam pipe in some parts of the ring the fiber may
have to be located at a distance d away from the beam axis Then the number of lost particles which will
cross the fiber will be reduced by a factor dsup2
48 Beam dumps
The ThomX set-up will include two particle dumps to stop the e- beam in such conditions that the
radioactivity then generated is minimal The first dump will be located at the linac exit The second
dump will be located downstream of the injectionextraction section The latter dump can be used to
stop the beam either at the exit of the transfer line or after any number of bunch revolutions in the ring
Both dumps are based on the SOLEIL design and can handle the 35 W power deposition (1 nC 70
MeV 50 Hz repetition rate)
lsquo
95
Chapter 5 SYNCHRONIZATION
51 Overall scheme
An ultra-low noise 500 MHz synthesizer will be used as the main clock for ThomX The 10 MHz
output of this synthesizer will be used to drive a second synthesizer (the so-called lsquolinac synthesizerrsquo)
operating at 3 GHz The RF output of the latter synthesizer will be used for the linac operation It will
be split in two lines The first one will be used to synchronize the gun laser oscillator (after appropriate
down conversion of the frequency) The second line will be used to drive the gun and the linac RF
The RF output of the main synthesizer (500 MHz) will be used for the ring RF it will be split in three
lines The first line will drive the ring RF (after appropriate phase shifting) The frequency of the second
line will be divided by 14 this line will synchronize the Compton cavity As in the MightyLaser
experiment [51] the laser used to inject power in the Compton cavity will be synchronized with that
cavity by using the PDH method The frequency of the third line will be divided by 28 and will
therefore be synchronous with the electron revolution frequency This line will be used to synchronize
programmable delays delivering turn-by-turn triggers and injectionextraction triggers To ensure that
injection is always taking place at a zero crossing of the mains the programmable delays will also be
synchronized with the mains A counter will reduce the mains frequency to the operations frequency
(when lower than 50Hz) The gun trigger will be generated when the mains will be at a zero crossing
and the ring RF at a preset phase that takes into account the time it will take for the electrons to travel
from the linac gun to the ring accelerating cavity
To ensure that the e- bunches will always be injected at a triple zero (50 Hz 500 MHz and 3 GHz) we
need to specify the tolerance on the 50 Hz zero crossing We may take 1 (ie 02 ms) for this
tolerance this means that the coincidence between the ring RF (~500 MHz) and the linac RF (~3 GHz)
must occur at a frequency greater than 5 kHz Therefore by choosing a heterodyne frequency (ie the
frequency difference between the linac RF divided by 6 and the ring RF) greater than 5 kHz we ensure
that there will always be a common zero crossing between the ring RF and the linac RF within 1 of
the mains zero crossing This sets a constraint on the ring radius of 10-5
ie 168 micrometres on a
168 m long orbit (see Table 2)
The RF bucket must be positioned with an accuracy of 3 ps in the ring RF Thus the phase noise
between the linac RF and the ring RF must be lower than 3 ps when integrated between 50 Hz and
500 MHz Most good-quality synthesizers achieve such a performance
The information collected by the ring diagnostics will be used to adjust the exact value of the frequency
(around 500 MHz) generated by the main synthesizer so that it matches the exact ring length This
information will also be used to adjust the phase between the injection trigger and the ring
To reach the correct signal level each RF line may have to be amplified An isolator will be added to
avoid reflections that may degrade the overall performance of the system
To ensure long-term low frequency synchronization a Network Time Protocol (NTP) server will be
available to allow all computers working at ThomX to synchronize their internal clock
lsquo
96
52 Specifications
Synthesizer
The main synthesizer will be used to generate a 500 MHz adjustable signal Its phase noise jitter
integrated between 50 Hz and 20 MHz must be less than a few picoseconds It must support a frequency
modulation of at least 100 kHz Typical mid-range synthesizers meet such specifications
Measurements done at LAL in collaboration with SOLEIL have shown that the addition of an external
frequency reference does not necessarily improve the phase noise of these synthesizers in the frequency
range of interest
The linac synthesizer specifications are the same ones except that it must be able to generate a
frequency of 3 GHz
Programmable delay
Two low jitters units with 8 channels each will be used
Phase shifter
The phase shifters must be voltage controlled and cover 360deg (two 180deg units)
Figure 51 Synchronization layout
lsquo
97
Chapter 6 OPTICAL SYSTEM DESIGN
Since the writing of the ThomX CDR three directions have been pursued improvement of the feedback
system improvement of the laser amplification and mechanical design of the optical resonator The
results are described in the following three sections
In the first section the ThomX feedback system is described During the past year a strong RampD
program has been developed on the locking of a TiSapph picosecond laser beam on a two-mirror high
finesse cavity at OrsayLAL Very good results in operating for the first time a 30 000 finesse cavity
in the picosecond regime were obtained The feedback techniques used for this task are directly usable
for ThomX and are reported in this section A 3000 finesse four-mirror cavity at KEK was also installed
and operated and we succeeded in producing γ rays off the ATF electron bunches (Mightylaser project
[61]) Hardware damages were however caused by the Japanese 2011 earthquake At the time of
writing this document we are recovering all our equipment to reach our final goal locking a 30000
finesse four-mirror cavity to produce a high γ ray flux at ATF Thanks to a contact with the SYRTE
laboratory we have acquired a new laser oscillator with a 1030 nm wavelength that we are testing and
that we plan to use for ThomX With this oscillator by embedding an intra-cavity EOM we are able to
increase our feedback bandwidth by more than one order of magnitude
The second section illustrates the laser amplification developments done at CELIA First a non-all-
connected fiber amplifier for the Mightylaser project was set up that allows us to extract an average
power of 50 W This amplifier was used to produce γ rays at ATF A new setup has been designed and
built to reach both the reliability and the power level required by ThomX An important activity is
devoted to the construction of a fully all-connected amplification chain As mentioned above the new
laser oscillator and the new laser amplification chain are being integrated in order to fill a 30000 finesse
high finesse cavity to conclude the ANR Mightylaser project
In the third section the details of the implementation of the cavity in the electron ring and the cavity
vacuum system are described with the opto-mechanical system that will be used for a precise alignment
of the cavity mirrors
61 Feedback system and results on a Fabry-Perot cavity locking
To produce a high X-ray flux the ThomX facility produces high intensity laser pulses and collides them
with electrons stored in a ring Hence the locking system (see figure below) has a double goal
- Stabilize the synchronization of the Fabry-Perot cavity (FPC) photon pulse with the electron bunch
- Stabilize the synchronization of the laser pulses with the FPC to obtain amplification
For this last point as will be seen in section 62 firstly the laser beam will be amplified with a CPA
technique using optically pumped fibers and then it will be amplified again by accumulation during
many round trips into a high finesse FPC (the target is a gain of 10000 which leads to a finesse of
30000)
This section describes the following topics related to the stabilization of the laser pulses relative to the
FPC
- Parameters to be stabilized repetition rate and carrier-envelop phase (CEP)
- Error signals the Pound-Drever-Hall (PDH) measurement technique
lsquo
98
- Actuators dynamic range and bandwidth
- Behavior of the digital feedback system (DFS)
General scheme for the locking system Detection and actuation elements are shown in red
The results on the locking of a TiSapph laser oscillator on a 30000 finesse cavity will also be
illustrated
Figure 61 Locking scheme for the synchronization of the electron bunch and the FPC pulse for the
ATF experiment at KEK
611 Electron and FPC pulse synchronization
This kind of stabilization has already been achieved for the MightyLaser experiment at KEK (ATF) a
1785MHz repetition rate A digital feedback system was used (i) to acquire two clocks one from the
laser and one from the master oscillator of the electron ring (ii) to perform a digital phase detection
(iii) to build a digital integrator filter to remove phase drifts and (iv) to react on a PZT inside the FPC
to change its length according to the measured phase drift (see Fig 61)
lsquo
99
612 Laser parameters to be stabilized
We will choose a laser with a repetition rate corresponding to twice the e- bunch revolution frequency
in the ring (two laser pulses per electron bunch revolution) around 500MHz14 = 357 MHz A phase
noise measurement on a similar product operating 1785 MHz was already carried out to characterize
the repetition rate phase noise and the drift (see Fig 62 and Fig 63)
Figure 62 Repetition rate phase noise spectrum Figure 63 Repetition rate drift measured
X-axis offset frequency from the carrier in Hz during about one hour X-axis repetition
Y-axis single side band phase noise power rate drift in Hz Y-axis time duration in
density in dBcHz seconds
The global shape of the phase noise in 1f will permit to use three feedback actuarurs a high bandwidth
low dynamic one (EOM for repetition rate and AOM or pump current for CEP) a middle
bandwidthmiddle dynamic one (PZT for repetition rate) and a high dynamiclow bandwidth one
(mirror motor and isochronic wedge motor)
For this kind of laser the harmonic content (index m) is well known and can be represented either in the
time domain or in the spectral domain (see Fig 63 and Fig 64)
( )2
CEL REP CE REPf m m f f m f
φ
π
Δ = + = +
lsquo
100
Figure 64 Time domain and spectral representation of the laser pulses
Since one is dealing with two parameters (the repetition rate and the CEP) one needs two different
sources of information to measure them The standard method to measure these two parameters consists
in using the Pound-Drever-Hall (PDH) technique [62] which gives the position of the laser harmonics
relative to the FPC resonances
( )FPC CAVf m m f=
One of the parameters is deduced from the whole beam signal using a central photodiode the second
one is derived from a part of the spectrum selected by a grating and a slit and measured by the grating
photodiode
If the repetition rate and the FPC round trip frequency are close enough the error signal coming from
the central photodiode (index m0) is proportional to the difference between the laser harmonics and the
corresponding resonances of the FPC
1 0 0
2
CEREP CAVm f m f
φε
π
Δ prop + minus
Frequency
domain
lsquo
101
As the locking system cancels the error the central laser harmonic m0 is perfectly aligned on the FPC
resonance but all other laser harmonics (m=m0+Δm) are slightly detuned from the corresponding FPC
resonances This introduces a correlation between the CEP and the repetition rate
( )( )
1 0 0
2 0
0 0
02
1 12
CEREP CAV
CEREP CAV REP CE
m f m f
m mm m f f f f
m m
φε
π
φε
π
Δ = rArr + =
Δprop minus minus = minus = minus
As a result of these equations the signal received by the PDH system located after the grating (ε2) is
directly proportional to the offset frequency (fCE) and then to the CEP Thus if we change the CEP of
the laser we should clearly see on the grating photodiode PDH signal ndash which measures the laser-FPC
detuning on the border of the spectrum (for an m index far from m0) ndash a linear relation between the error
signal and the CEP
This method well known for femtosecond lasers has been tested in our laboratory in the picosecond
regime Figure 65 shows our results (submitted to Applied Optics) on the locking of a 76MHz
TiSapph picosecond laser oscillator on a 30000 finesse cavity
Figure 65 Top experimental measurement of the CEP value of a TiSapph laser oscillator Bottom
measurement of the corresponding grating photodiode signal
In Fig 65 one sees that when the CEP is measured close enough to zero the grating photodiode
exhibits a linear relation with the CEP otherwise the photodiode is saturated (due to the PDH signal
saturation) In our experiment the ratio between the cavity reflected power and the incident beam
power was ~20 which leads to a value of 80 for the coupling of the incident beam power to the
cavity
These results demonstrate for the first time that a picosecond laser oscillator can be locked to a 30000
finesse cavity (implying a power enhancement factor of 10000) with a power coupling of 80
613 PDH Measurement technique
Having shown that we are able to measure independently both parameters of interest (the repetition rate
and the CEP) we will now describe the apparatus to transform these measurements into error signals by
lsquo
102
the PDH technique (see Fig 66) Side-bands to the laser optical harmonics are produced by an electro-
optic modulator (EOM) modulated by a generator coming from the digital system Side-bands and laser
harmonics are reflected by the FPC the first ones without acquiring any phase the second ones shifted
in relation with the discrepancy between the laser and the FPC frequencies All the harmonics and the
side-bands of the reflected laser are combined into the photodiodes and the front-end electronics giving
an error signal at the modulation frequency
Figure 66 Pound-Drever-Hall scheme with digital phase detection
The photodiode front-end (see Fig 67) amplifies these signals at a level compatible with the digital
feedback system (DFS)
Figure 67 Electronic scheme and picture of the PDH front-end
Inside the DFS the PDH signals are filtered by a band-pass filter to remove unwanted frequencies and
multiplied by the same modulating signal with a phase shift to allow for the phase detection A low-pass
filter at the end of the chain removes the image signal at twice the modulation frequency Figure 68
shows an example of the input and the output signals
lsquo
103
Figure 68 Modulated input (blue) error signal (yellow) FPC transmitted signal (pink)
614 Actuators
In the previous sections we described how the two parameters (the repetition rate and the CEP) of a
pulsed laser have to be controlled in order that all the laser harmonics match the FPC resonances and
we also detailed how to measure the variations of these two parameters We will now describe the
devices inside or around the laser on which we need to react to correct the variations of the laser
harmonics relative to the FPC resonant frequencies
Figure 69 Typical scheme of a laser with all its actuators
The actuators will produce an action on the two parameters to be controlled with different dynamic
ranges and different bandwidths (see chart in Fig 610)
Figure 610 Dynamic ranges and bandwidths of the laser actuators
lsquo
104
All of the actuators are located inside the laser except the AOM (see Fig 610) They are the key items
to achieve a good stabilization
All the actuators measurements have been made on the 1785MHz laser version used at KEK
- Mirror motor dynamic range precision and bandwidth
The motor is a step motor with a precision of 140 nm on the round trip variation (equivalent to 15 Hz on
Frep) Figure 611 shows the linearity and the maximum dynamic range available with the motor stage
while Fig 612 shows a step response which indicates the maximum bandwidth we can obtain
Figure 611 Mirror motor range Figure 612 Time response of Frep for
1k motor steps X-axis repetition rate
offset (Hz) Y-axis repetition rate
The repetition rate range is 178033 - 179244 MHz (gt 1MHz)
- Piezoelectric sensitivity dynamic range and bandwidth
The PZT sensitivity is approximately 3Hz V 28 nmV (length variation to be compared to the
length of a round trip of the FPC pulse)
The PZT can be operated up to 150V providing a total dynamic range of 42 microm (round trip length
change) Its first peak resonance has been measured and found above 40 kHz
lsquo
105
Figure 613 Repetition rate change produced by a Figure 614 PZT frequency response X-
PZT voltage variation from 0 to 28 V frequency range 01-5 kHz Y-axis
Attenuation (dB) ndash arbitrary offset
Figures 613 and 614 show the PZT sensitivity and its transfer function
- EOM sensitivity measurement
Measuring the sensitivity of the EOM is not direct thus we chose to apply a 10 V amplitude ndash 10 Hz
frequency modulation ndash to the EOM and to measure the power of the harmonics (see Fig 616)
Assuming a pure frequency modulation the power of the harmonics fis a Bessel series that depends on
the sensitivity Thus taking the ratio of two consecutive harmonics gives directly the sensitivity
Figure 615 EOM modulation spectrum used for sensitivity measurement X-axis frequency centered
on the repetition rate Y-axis relative optical power around the repetition rate due to the modulation
The table on the left gives the frequency and the power of each harmonic
0 5 10 15 20 25 300
10
20
30
40
50
60
70
80
90
100
PZT voltage (V)
Fre
p v
ariation (
Hz)
Frep variation vs PZT voltage
AJ0(z)
AJ1(z)
AJ2(z)
lsquo
106
We now check that the sensitivity calculated from the ratio of the first harmonic to the fundamental and
the one obtained from the ratio of the second harmonic to the fundamental are the same
∆
∆
~1
2
∆
10~ minus 2909 rarr ∆~07
∆
∆
~1
8
∆
10~ minus 6398 rarr ∆~07
Since the ratio is 07 Hz for an applied sinusoidal signal of 10 V the sensitivity is around 007 HzV
- Isochronic wedge
It is made of two glass prisms with different refraction indexes chosen to maintain a constant group
delay in order to avoid repetition rate variation
We measured that the maximum variation of the repetition rate for a full CEP range (several times 2π)
is about 300 Hz for a repetition rate of 1785 MHz (see Fig 616 which shows the measured repetition
rate shift plotted along the X-axis versus the date of the measurement plotted along the Y-axis) Thus
the influence of this actuator on the repetition rate is about 210-6
Figure 616 Measurement of the influence of wedge displacement on Frep
- Acousto-Optic Modulator (AOM)
Such an actuator has already been used with a laser which is not the same as the one used for the other
actuators namely a 76 MHz laser locked on a two-mirror FPC
In Fig 617 we present the closed loop measurement The lowest frequency part (lt10 kHz) is
attenuated by the loop gain If the loop were open this part of the transfer function would be quite flat
To estimate the AOM bandwidth limits we can use the frequencies where the gain starts to drop
significantly this bandwidth is about 300 kHz
300 Hz
lsquo
107
Figure 67 Frequency response of the AOM in closed loop
615 The digital feedback system
The feedback system will deal with several coupled inputs and outputs To ensure flexibility we have
chosen a digital system Since we also want to work with a large bandwidth in closed loop (gt 1MHz) a
low latency system (lt 100 ns) is needed A digital system featuring such performance does not exist on
the shelf Thus we decided to design and build such a system at LAL (see Fig 618)
Figure 618 Scheme of the DFS and picture of the DAC daughterboard
This system will perform several functions (see Fig 619)
103
104
105
106
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
Frequency (Hz)
Magnitude (
dB
)
Bode plot
lsquo
108
Figure 619 Organization of the digital feedback functions
- The first important function is to produce the error signal from the modulated signal coming from the
photodiodes It is a phase detection block (Digital PDH DPDH) the main problem being to reduce the
latency as much as possible This will be achieved by optimizing the low-pass filter and the modulation
frequency (see Fig 620 for a digital implementation)
Figure 620 Digital phase detection (DPDH) to produce the PDH error signals
- The second important function is to provide Infinite Impulse Response (IIR) filters to build
Proportional-Integrator (PI) filters in order to cancel the system drifts
lsquo
109
Figure 621 Digital IIR filter implementation with adders and multipliers
These filters can be programmed by the user to change the filter coefficients and it can be cascaded to
realize complex filters They have been cut in Second Order Sections (SOS) to minimize the
propagation of fixed precision errors due to quantized data and coefficients (see Fig 621)
- The last important function integrated in the digital system is performed by an identification block
which works like a network analyzer by (i) sending a small harmonic signal (ii) looking at the system
response in closed loop at this frequency and (iii) sweeping this small harmonic signal frequency to
cover all the range to be measured (see Fog 622)
Figure 622 Identification system
lsquo
110
This kind of identification system can provide information on (i) the feedback system itself (ii) the
system to be stabilized or (iii) the open and closed loop transfer functions by producing Bode or
BlackNichols diagrams (see Fig 623) These informations are very useful for system analysis and for
tuning or designing the feedback filters
Figure 623 Typical Bode (top-left magnitude bottom-left phase) and Black-Nichols (right) diagrams
produced by the identification system
62 High average power laser system and its coupling to a high gain FP cavity
The all-fibered laser system will draw from the system developed in the MightyLaser project It will
consist in an oscillator associated with three amplifying stages involving single and double clad Yb-
doped fibers (see Fig 625) At present the amplifiers are based on commercial components to facilitate
a potential replacement of damaged parts The architecture is designed to provide the mandatory
diffraction-limited beam This relatively simple and compact architecture will implement the lsquoChirp
Pulse Amplificationrsquo (CPA) scheme In order to limit non-linearity effects in the fiber the signal is
initially stretched in time thus reducing its intensity in the waveguide then amplified in the fiber and
finally recompressed to its minimal duration The difference between the MightyLaser and the ThomX
repetition frequencies (respectively 1785 MHz and ~357 MHz imposed by the different repetition rates
of the two accelerators) requires an adjustment of the laser design
Figure 625 Sketch of the all-fiber laser system
lsquo
111
621 The MightyLaser oscillator
This oscillator is an all-normal dispersion mode-locked fiber laser based on the nonlinear polarization
evolution technique equipped with a fibered output It uses an ytterbium doped fiber which allows an
emission centered around 1030 nm with a full width at half maximum of 10 nm (see Fig 626) It
delivers short pulses at a repetition rate of 1785 MHz with a low average output power (~10 mW) in
order to minimize the noise level (see Fig 628) The main advantage of the all-normal dispersion
regime resides in the significant peak power reduction provided by the pulse stretching during the
propagation along the cavity Non-linear effects are thus reduced allowing a high-energy extraction
The output pulses are naturally chirped with an estimated duration of 2 ps
Figure 626 Emission spectrum of the oscillator
The oscillator is passively mode-locked by taking advantage of the non-linear evolution of the
polarization This technique is based on the optical Kerr effect The polarization of a pulse propagating
in a fiber evolves non-linearly as a function of intensity Thus by combining polarization controllers
and a polarizer one may generate losses that depend on the intensity (see Fig 627) Beyond the
polarizer all the pulse components are linearly polarized The wave plates transform the linear
polarization into an elliptic one The latter polarization state then evolves in a non-linear way while it
propagates along the fiber This evolution results from the combined effect of the self-phase and the
cross-phase modulations induced on the two polarization components The polarization rotation of the
more intense central part of the pulse is larger than the polarization rotation of the pulse wings Since
the pulse center crosses the analyzer with a reduced loss compared to the wingsrsquo loss the pulse duration
is shortened
Figure 627 Principle of mode locking based on the non-linear polarization evolution
lsquo
112
Figure 628 Spectrum of the oscillator phase noise noise level (dBcHz) versus offset frequency (Hz)
622 Amplification stages
The amplification chain is divided in three stages in order to distribute the gain and to saturate the
second and the third amplifying stages The first and the second stages are based on the same
architecture It is an all-fibered amplifier whose central component is a polarization maintaining single
mode using an Ytterbium doped fiber with a core diameter of 6 microm and a clad diameter of 125 microm (see
Fig 629) A wavelength division multiplexer (WDM) is spliced on the doped fiber to transfer the pump
power into the gain medium and to couple the signal of the picosecond oscillator to the amplifier The
first stage is pumped by a single mode diode centered at 976 nm and delivering 200 mW The second
one is pumped at a higher average power namely600 mW The two amplifying stages are separated by
an optical isolator to avoid any parasitic feedback from one amplifier to the other
Figure 629 Schematic of the first and the second amplification stages
The average power level at the input of the first amplifying stage is about 1 mW A limitation is
imposed on the output of this stage which should remain below the damage threshold of the isolator
fixed at 100 mW Consequently we adjust the pump level in order to obtain 80 mW at the output of the
ytterbium-doped fiber The 3 dB losses of the isolator allow one to inject 40 mW into the second
amplifying stage The evolution of the average output power versus the pump level is shown in Fig
630 For the maximum pump level of 600 mW the second amplifying stage delivers 330 mW
lsquo
113
Figure 630 Second stage output power plotted versus the pump power
Beyond the second isolator the third amplifying stage (see Fig 632) is built by using as its central
component a polarization maintaining double-clad Ytterbium-doped fiber with a core diameter of 25
microm and a clad diameter of 250 microm The amplifier is clad-pumped through a special combiner 6+11
(see Fig 631) In practice this component has an input signal fiber 6125 microm directly spliced to the
previous amplification stages an output signal fiber 25250 microm spliced to the double-clad Ytterbium
doped fiber and six multimode fibers of 105 microm to couple the pump diodes Here only three pump
ports are spliced to multimode diodes emitting up to 55 W at 976 nm corresponding to a pump power of
at most 165 W The development of a mode field adapter was necessary to splice the pump combiner to
the double-clad Ytterbium-doped fiber because their numerical apertures are different Losses entailed
by this component are compensated by the last amplification stage
Figure 631 Pump combiner Figure 632 Schematic of the third amplification stage
Figure 633 is a plot of the average output power versus the pump level For a pump power level of
76W the last amplification stage delivers 52W This was the first objective The fiber laser power
stability was measured during 1 12 hour at this power level (see Fig 634) The residual pump power
the pump and signal feedbacks into the pump diodes are also plotted in this figure
lsquo
114
Figure 633 Third stage output power Figure 634 Laser stability at 52 W
versus the pump power
New measurements were made after optimizing the central wavelength of the multi-mode pump diodes
The efficiency was increased to reach 787 for an output power of 24 W (see Fig 635) The fiber
laser power has been tested at this level during two hours (see Fig 636) The observed power variation
is less than 01
Figure 635 Output power versus pump power Figure 636 Laser stability at 24 W
after wavelength optimization
The all-fibered amplifier setup has been integrated in a 19rsquorsquo rack as shown in Fig 637 An FCPC
connector placed in the front panel allows the connection of the fibered oscillator to the master
amplifier module Several 955 optical fiber couplers have been inserted in our setup at strategic
locations to monitor the power level in real time by using photodiodes they may also signal potential
problems The output fiber comes out of the module through its front panel The back-side is dedicated
to all the electronic controls (diode pump photodiodes and modulators for noise management described
elsewhere)
lsquo
115
Figure 637 Pictures of the 52 W all-fibered setup integrated in a 19rsquorsquo rack
623 Stretcher and compressor
A chirp volume Bragg grating (CVBG) is used in our setup to stretch and compress the pulse with a
very good efficiency of 94 The CVBG transmission bandwidth is around 2 nm (see Fig 638) leading
to a pulse duration ranging from 200 to 250 ps depending on the grating dispersion It is possible to
recompress the pulse after amplification by using a second CVBG Then the pulse length ranges
between 20 and 87 ps depending on the second CVBG dispersion (see Fig 639)
Figure 638 Transmission bandwidth of the CVBG
1‐2 couple 3‐2 couple 3‐1 couple
Figure 639 Time profile of the recompressed pulses for different grating pairs
624 ThomX development
Some changes must be made to the all-fibered lsquoMightylaserrsquo design in order to use it in the ThomX
project The oscillator repetition rate needs to be scaled down to match the frequency imposed by the
accelerator As far as the laser design is concerned the stretchercompressor couple must be modified to
limit non-linear effects The increase of the output power from 50 to 150 W imposes an upgrade of the
second amplification stage and eventually of the third one
lsquo
116
6241 Oscillator
The oscillator for ThomX will be based on the same principle as the one used for Mightylaser The only
difference is the repetition frequency imposed by the accelerator (357 MHz for ThomX) The low
average output power around 5 mW permits to limit the phase noise A particular attention will be paid
to the noise level of such a long cavity as no commercial products have been tested at this repetition
rate
6242 Amplification stages
For the ThomX project the lower repetition rate and the higher average output power require a change
of the second amplification stage architecture in order to limit non-linear effects and to saturate the last
amplification stage Consequently the second pre-amplifier will be based on a polarization-maintaining
double-clad Ytterbium doped fiber with a core diameter of 15 microm and a clad diameter of 130 microm (see
Fig 640) The amplifier will be clad-pumped through a special combiner 2+11 with an input signal
fiber 6125 microm directly spliced to the previous amplification stages an output signal fiber of 15130
microm spliced to the double-clad Ytterbium doped fiber and two multimode fibers of 105 microm to couple
the 9 W pump diodes
Figure 640 Schematic of the second amplification stage of the ThomX laser
Concerning the final amplification stage various fibers are available (see below) Micro-structured
fibers can be engineered and tailored but suffer from the lack of splicing techniques Hence their use in
an all-fibered setup will require additional developments dedicated to splicing (see Fig 641) and
integration If splicing is not possible we still have the possibility of proceeding with air propagation
and injection However this approach requires precise mechanics control and advanced engineering
The lsquocoherent beam combining techniquersquo will also be considered at this point In such a configuration
amplification would be supported by two parallel amplification stages dividing by two the energy level
in the gain medium Yet a first version will be developed with commercially spliceable flexible fibers
and preliminary tests of cavity locking with high power from these fibers will be performed to validate
or invalidate operating with a high level of non-linear effects
lsquo
117
Fig641 A micro-structured fiber
6243 Stretcher and compressor
Because of the high average output power required by ThomX we must modify the stretcher size Two
techniques can be considered First we may continue to use a CVBG which can stretch the pulse up to
450 ps with a maximum size of 25 cm (see Fig 642) It would be possible to stretch the pulse up to
900 ps with an efficiency of 88 through a double pass in a CVBG or a single pass in cascaded CVBG
The second way involves a chirp fiber brag grating (CFBG) as depicted in Fig 643 When associated
with an optical circulator it allows a pulse stretching with up to 800 ps in an all-fibered setup The
efficiency is around 70 (92 for the CFBG and 75 for the circulator) in this configuration This
solution is less efficient than the CVBG but the all-fibered configuration ensures the set-up robustness
and stability and the losses can be easily recovered by the amplifier chain However dispersion issues
have been reported for these devices and a detailed investigation is needed prior to a definitive
integration
Fig 642 Examples of CVBG Fig643 Example of CFBG
The high output average power forces us to use a CVBG to compress the pulse one or two units
depending on the stretcher configuration The final pulse duration can be varied between 1 ps and 50 ps
thus leaving the possibility to optimize the X-ray production by adjusting the pulse duration
We also performed simulations to evaluate the influence of the couple stretchingfiber core diameter on
the non-linearity with 3 different fiber cores 25 microm 30 microm and 40 microm Our results are summarized in
Table 61
lsquo
118
Fiber
dimensions
mode field
diameter stretching B-Integral
25250 20 microm 250 ps 47 rad
25250 20 microm 500 ps 263 rad
25250 20 microm 1 ns 118 rad
30150 22 microm 1 ns 09 rad
40200 30 microm 1 ns 0525 rad
Table 61 Pulse stretching as a function of the fiber core diameter (simulations)
A first configuration will be developed with a stretching of 500 ps If the non-linearity effects remain
too high to allow for an efficient locking on the Fabry-Perot cavity we will modify the setup to induce
a larger stretching factor or insert a fiber with a larger core diameter in the last amplification stage or
both
63 Mechanical design
Collisions between the laser pulse and the stored electrons will take place between two dipoles (see Fig
643) The constraints on the optical cavity mechanical design are as follows
- High stability level (vibrations)
- Good accessibility to the cavity mirrors
- Weak impact on the electron beam dynamics
- Ultra high vacuum (UHV) system with baking capability
- Motorized granite table for micrometric scan oof the electron beam
lsquo
119
Figure 643 Implementation of the cavity in the electron ring
The cavity mirror system is mounted on a massive granite table which can be driven to perform a
micrometric electron beam scan (see Fig 643)
lsquo
120
Figure 644 Scheme of the Fabry-Perot cavity mounted on the optical table
The Fabry-Perot cavity is composed of two cylindrical UHV vessels linked by a dipole chamber
Bellows are placed between each element to absorb the dilatation due to the backing (see Fig 644)
e‐
e‐
X‐Ray
1rsquorsquoMirrors
IP
Bellows
1rsquorsquoMirrors
Vessel
Vessel
Dipole
chamber
lsquo
121
Figure 645 On the left detail of the standard mirror assembly and on the right detail of the piezo
electric mirror assembly
The Fabry-Perot cavity is composed of two spherical and one flat mirror mounted in a standard
assembly and one flat mirror mounted on a special assembly with a piezo electric actuator for very
small longitudinal displacements (see Fig 645)
Figure 646 Detail of a truck system
Mirrors
Piezo
electric
actuator
Mirror
Gimbal
Actuators
lsquo
122
Figure 647 Cut of an UHV actuator
All the mirrors are mounted on a truck system that porvides three degrees of freedom (two rotations and
one translation) driven by three micrometric actuators These actuators are specially designed to be
compatible with UHV and backing constraints (see Fig 646 and Fig 647)
Figure 648 Cut of a vacuum vessel
Trucks (two trucks per vacuum vessel) and plugs with special cables composed of ceramic connectors
oxygen- free copper cables and ceramic pearls and spacers for electrical isolation are placed in a UHV
vacuum vessel One of the vacuum vessels has a beryllium window to extract the X-ray beam (see Fig 648)
Micrometric Actuator
Capsule
Trucks
Beryllium
window
lsquo
123
631 Optical cavity vacuum
A simulation was carried out in the most unfavorable case when the whole optical vacuum chamber is not
baked The area of one half of the optical cavity is about 2 m2 the conductance of the rectangular chamber
connecting the two cavity halves is C =70810-1
m3s
-1 (value for one half of that rectangular chamber) The
conductances of the slits connecting the optical cavity to the ring are respectively C = 45210-2
m3s
-1 (9 x 48)
and C = 16010-2
m3s
-1 (6 x 30) The outgassing rate of one optical cavity half without any baking (σ = 210
-8
Pams-1
) is Q = 410-8
Pam3s
-1 The outgassing rate of one rectangular chamber half (σ=10
-9 Pams
-1) is Q =
7610-10
Pam3s
-1 The outgassing rates of the two slits (σ=10
-9 Pams
-1) are respectively Q = 22810
-13
Pam3s
-1 (9x48) and Q=14410
-13 Pam
3s
-1 (6x30) The diode ion pump used for the optical cavity has a
pumping speed of 05 m3s
-1 DN 150 S = 0320 m
3s
-1 at ~10
-10 mbar The conductance of the pumping port
being C = 136 m3s
-1 the effective pumping speed is Seff= 0278 m
3s
-1 The diode ion pump used for one half
of the rectangular chamber has a pumping speed of 015 m3s
-1 DN 100 S = 0090 m
3s
-1 at ~10
-10 mbar The
conductance of the pumping port being C = 0046 m3s
-1 the effective pumping speed is Seff= 0075 m
3s
-1
The outgassing rate of the optical cavity without baking is very high 2 x 410-8
Pam3s
-1 (which is
approximately 40 times the outgassing rate of the whole ring) With such a rate one would need a pumping
speed of 8 m3s
-1 to reach the required pressure level of 10
-8 Pa (the present pumping speed is 07 m
3s
-1) The
conductance of the rectangular chamber that connects the two halves of the optical cavity (710-1
m3s
-1) and
the conductance of the two slits between the optical cavity and the ring (610-2
m3s
-1) are high These figures
do not allow a differential pumping between the cavity and the ring Simulations of the ring vacuum level
show that it is mandatory to bake the whole optical cavity
Figure 649 Location of the cavity pumps
The vacuum equipment needed for the optical cavity is as follows
‐ 2 ion pumps 150 l s-1
with their baking equipment and their power supplies
‐ 2 ion pumps 500 l s-1
with their baking equipment and their power supplies
‐ 2 gate valves manually actuated (pipe diameter DN100) and their baking equipment
‐ 4 Pirani gauges
‐ 4 cold cathode gauges
‐ 2 power supplies for the gauges
‐ 4 residual gas analyzers (RGA)
‐ 1 baking equipment for the vacuum chamber
‐ 4 angle valves manually actuated for pre-pumping (pipe diameter DN63)
lsquo
124
Chapter 7 X-RAY BEAM LINE AND EXPERIMENTS
Introduction
The equipment of the X-ray beam line should firstly ensure a continuous detection and monitoring of
the beam (mainly in the first optical hutch) and secondly allow a complete characterization of the
beam (in the experimental hutch) This characterization consists in measuring the beam absolute
intensity its total energy range its spectral and angular distributions and its degree of transverse
coherence Time analyses of the bunched X-ray beam will also be performed This line also aims at
confirming the ThomX assets in such fields as cultural heritage biomedical and materials sciences by
performing demonstrative experiments
71 Available extracted X-ray beam
The X-ray beam is extracted through a 50 mm diameter beryllium window as schematized in Fig 71
Because of limitations due to the dimensions and the location of the Fabry-Peacuterot cavity mirrors the X-
ray beam available for characterization and experimentation is a cone with a half-aperture of 16 mrad
Figure 71 The X-ray beam extracted from ThomX through a beryllium window The beam fills a cone
whose total aperture is 2 x16 mrad
In the case of a lsquoperfectrsquo beam (ie without taking into account the electron beam emittance) the X-ray
spectral and energy distributions are shown in Fig 72 as a function of the scattering angle for two
electron beam energies 50 and 70 MeV and a laser wavelength of 106 microm In Fig 73 the energy
bandwidth and the flux per unit solid angle are shown for on-axis Compton photons produced by either
a 50 MeV or a 70 MeV electron beam
72 Global view of the X-ray lne
As can be seen in Fig 74 the positioning of the machine in the Igloo imposes the splitting of the X-ray
line into two sections a first optical table (3x2 m2) located just after the beryllium exit window and a
second table (4x2 m2) located outside the Igloo in hall D1 Along its path and until the optical table 2
the beam is transported in the primary vacuum inside stainless steel pipes with the exception of a short
upstream section of a few tens of cm between the beryllium window and the optical table this
insulating section will prevent the transmission of harmful vibrations to the Fabry-Peacuterot cavity This
primary zone is kept oxygen free by a nitrogen flow that prevents ozone production
lsquo
125
The beam diameter at the entry of the first optical table is 45 cm After this first optical table the beam
reaches the second table via a 13 cm diameter beam pipe inserted in a hole which was drilled long ago
in the Igloo wall About 8 mrad beam angular divergence are preserved at the entry of the second
optical table where the beam diameter is 13 cm
Figure 72 X-ray spectral distributions (left panel) and X-ray energy (right panel) versus the scattering
angle for an e- beam of 50 MeV (black) and 70 MeV (red) The laser wavelength is 106 microm Grey
areas correspond to emission angles larger than 16 mrad
Figure 73 X-ray flux (left panel) and X-ray energy bandwidth (right panel) of on-axis photons per unit
solid angle versus the emission angle for an e- beam of 50 MeV (black) and 70 MeV (red) The laser
wavelength is 106 microm
lsquo
126
Figure 74 Global view of the X-ray line in ThomX
73 Chraracterization of the X-ray beam
731 Optical table 1
A global view of the optical table 1 design is presented in Fig 75 Just upstream of this optical table a
60 mm diameter beam shutter activated when one needs to take background data will allow stopping the
beam A few millimeter thick tungsten is sufficient to stop the full beam (tungsten attenuation mass
factor microen ~ 4043 cm-1
for 100 KeV photons) This shutter does not have to be cooled since the ThomX
radiated power will not exceed 100 mW A drawing of the shutter mechanical design is shown in Figure
76 (a)
Figure 75 Global view of the optical table 1 showing the location of the main devices placed on it
lsquo
127
Beam monitoring
The optical table 1 equipment (see Fig 75) is mainly devoted to the detection and the continuous
monitoring of the X-ray beam The first device monitoring the beam presence is a fluorescent screen
mounted on a two-position motor (up or down) as shown in Fig 76 (b) It is inserted in the beam path
and viewed by a CCD camera Its screen absorbs almost the totality of the beam and thus has to be put
in the up position when not in use Downstream of the fluorescent screen a device composed of two Si
PIN diodes measures the beam scattered by a thin Kapton foil inserted in the beam path This setup
shown in Fig 76 (c) will monitor the beam flux variations and beam position displacements The
Kapton foil is fixed at an inclination angle of about 60o with respect to the beam path The sum and the
difference of the two diode currents will provide a relative measurement of the beam flux and of the
beam position respectively This set-up which is nearly transparent to 20-90 keV radiations is a non-
intrusive monitor and can be used continuously Then a high precision double slit [71] intercepts the
X-ray beam and allows one to let through only the desired part of the Compton cone (see Fig 76 (d))
The minimum dimensions of the slit aperture are 10 microm in the horizontal and the vertical directions
Beyond the double slit a wire monitor [72] shown in Fig 76 (e) is inserted in the beam This device
consists in two tungsten wires stretched horizontally and spaced by a few tens of a millimeter This
monitor will be driven vertically through the beam The difference of the two wire signals allows the
measurement of the absolute beam position
Figure 76 Drawings of the X-ray line devices located on the optical table 1 (a) Beam shutter
(b) Motorized two-position fluorescent screen (c) Kapton foil and Si diodes (d) Double slit (e) Wire
profiler
lsquo
128
Focusing device
Downstream of the beam monitoring instruments a focalizing device will be installed (see Fig 75) to
make the X-ray beam parallel andor to focus it on a sample It can be either a device based on
compound refractive lenses [73] (a so-called lsquotransfocatorrsquo) or two bendable mirrors in the Kirkpatrick-
Baez configuration (KB Optics) or a capillary optical device These three techniques will be
investigated to evaluate the advantages and disadvantages (aperture efficiency focusing strength etc)
of their eventual implementation in the ThomX facility Such a focusing apparatus has to be located as
close as possible to the X-ray source because of the rapid growth of the beam transverse dimension
Finally at the end of the optical table 1 some space is kept free for any specific study which might
concern optics detectors or samples
732 Beam transport
As shown schematically in Fig 77 beyond the optical table 1 the X-ray beam crosses the Igloo wall
inside a beam pipe that runs through a 15 cm diameter hole and reaches the D1 hall where the second
optical table is installed in the lsquoexperimental hutchrsquo At the entrance of the wall hole a second beam
shutter in lead around 15 cm in diameter and 25cm thickness is installed This shutter is connected to
an interlock system that ensures radioprotection It absorbs radiations coming from the electron
machine which propagate through the Igloo cylindrical hole and allows X-ray users to work in the
experimental hutch without stopping the electron machine
Figure 77 Schematic of the X-ray beam transport across the Igloo wall from the optical table 1 located
in the Igloo until the optical table 2 located in the D1 hall
733 Optical table 2
Several kinds of instruments will be placed on the optical table 2 The goal is to study in detail the X-
ray beam produced by ThomX firstly to determine how the beam characteristics depend upon the
machine operation parameters and secondly to prepare the demonstration experiments A global view
of the optical and detection instruments placed on that table is provided by Fig 78
lsquo
129
Figure 78 Global view of the optical table 2 indicating the main instruments that will be used to
characterize the beam
Absolute and spectral flux measurement
The measurement of the X-ray beam absolute flux and of its energy spectrum will be performed by
using two detectors a calibrated Si diode [74] and a cooled high-purity Ge solid-state detector (SSD)
[75] The Si diode will be bought already calibrated [76] A lsquogoodrsquo Ge SSD has an energy resolution
(ΔEE) around 1 at 20 keV 06 at 45 keV and 045 at 90 keV [77] The energy calibration of the
Ge SSD will be carried out in the X-ray beam line by using the Kα fluorescence lines of reference
materials (see Table 71)
Element Ge Mo Sn Cs Ce
EKα1 (keV) 9886 17479 25271 30973 34719
EKα2 (keV) 9855 17374 25044 30625 34279
Element Eu Er W Pt Pb
EKα1 (keV) 41542 49128 59318 66832 74969
EKα2 (keV) 40902 48221 75981 65112 72804
Table 71 Kα fluorescence lines of chemical elements used to calibrate the Ge SSD in the 10-90 keV
energy range
The two detectors will be mounted alternately on a (X Y) translation stand to measure a large number
of points of the emission spectrum
- the mean emission energy E (Ge SSD)
- the energy spread ΔE (Ge SSD)
- the absolute flux (photon numbers) (Si diode)
These beam characteristics will be measured as a function of the size of the selected X-ray sample the
selection being carried out with a double slit device (see Fig 76 (d)) Our goal is to check
lsquo
130
experimentally the theoretical angularspectral distributions shown in Fig 12 of the Introduction by
measuring the number of Compton photons per second per mrad2 and per 01 of energy bandwidth
Time analysis of the bunch structure
The X-ray source has a time structure which involves two main scales a 50 ns period which is the
interval between two successive collisions between the laser pulse and the electron bunch and the 20
ms e- storage cycle To perform a pulse by pulse analysis we will use a high resolution time detector
(~1-5 ns) with a high repetition frequency (~10-100 MHz) eg a micro-channel plate (MPC) detector
[78] or an avalanche photo-detector (APD) Such time measurements have already been performed at a
similar facility by Sakaue et al [79] MCP Hamamatsu devices can reach a 07 ns (FWHM) time
resolution which is good enough not only to resolve each one of the collisions between the laser pulse
and the electron bunch but also to measure their relative intensity accurately from the beginning of the
e- storage cycle to the end These measurements will be performed on the X-ray beam axis They will
have to be synchronized with the ThomX clock
Transverse coherence of the beam
Some simple diffraction and interference measurements will be performed in order to analyze the
source spatial coherence The diffraction of the X-ray beam by a slit of a known aperture or the
interference effect produced by the insertion of a phase-shifting object in the beam allows one to
quantify the transverse coherence of the beam [710] In these experiments the on-axis photon intensity
distribution will be recorded with a 2D high resolution detector (such as for instance an XPAD CdTe
camera with 130 microm pixels) This measurement will be repeated at several positions along the beam
The pattern analysis will inform us about the degree of coherence of the source A monochromator will
be inserted upstream of the diffractive slit or the phase-shifting object to select a well defined energy
Later on Young double-slit experiments could also be carried out [711] to measure the beam
coherence length beam
74 Demonstration experiments
In parallel with the complete characterization of the ThomX beam described in the previous
sections some simple experiments will be performed in order to demonstrate the potentialities
of the ThomX source In this phase we will focus on medical and materials science
applications An important aim of the ThomX project is to perform medical and materials
imaging exploiting the high and tunable energy of its beam A second aim of the ThomX
project is to show its capability in the structural investigation of materials using X-ray
diffractionscatteringspectroscopy techniques
741 Experiments using the full X-ry beam
Three experiments issued from the expertise of the ESRF-INSERM teams in medical applications of
synchrotron radiation have been chosen to be carried out as demonstrators on the ThomX prototype
The goal of setting up such experiments is to demonstrate (i) the suitability of ThomX of performing
both imaging and therapy experiments (ii) the tenability of the source adjustable at energies
corresponding to the K edge of high Z elements routinely used in medicine (Gd I) or of those of new
experimental radiotherapy adjuvants (Au Gd nanoparticles CisPt) (iii) suitability of the source for
phase contrast imaging (iv) exploitation of the advantage of using a 2D beam with respect to the typical
laminar synchrotron X-ray beam Some imaging and therapy experiments in the biomedical field
require a wide 2D beam Using the natural divergence of the ThomX beam a sample of a few
lsquo
131
centimeters in diameter located at 10-20 meters from the ThomX interaction region can be fully
illuminated by the a conic beam with no need of optical elements Calculations have been performed in
order to show the potential of using the ThomX source in comparison with a hospital X-ray generator
(fluoroscopy unit type) as well as in comparison with the beam from the biomedical line ID17 at ESRF
Results of these comparisons are given below for each medical experiment we intend to perform with
ThomX
An imaging experiment
For imaging experiments the energy range of the beam has to around 60-90 keV According to Fig 73
the corresponding half-aperture will be 5 mrad leading to a photon flux of 61012
phs When the beam
reaches the sample stand placed on Table 2 at 20 m from the ring interaction region it will have a
diameter of 20 cm This conical beam will then have a cross section of ~300 cm2 and permit
tomographic acquisitions of a full human head using a 2D detector This wide 2D beam is a major
advantage of the ThomX source for biomedical imaging with respect to the laminar synchrotron
radiation beam The dose rate (DR) in Gys can be expressed as
(71) DR = 16 10-13
Ф[phscm2] E[keV] (microenρ)[cm
2g]
where Ф is the photon flux per cm2 E is the mean energy of the beam and (microenρ) is the mass
attenuation coefficient For an X-ray beam of 75 keV (ie the average of our 60-90 keV energy range)
the mass attenuation coefficient microenρ for soft tissues is 0028 cm2g leading to a dose rate of 67 mGys
delivered to a 300 cm2 area The corresponding expected lsquodose area productrsquo (DAP) of the ThomX
source for such imaging experiments is DAP = 20 Gyscm2 For comparison at the ESRF ID17 line a
dose rate of 800 mGys is delivered to an area of 150x2 mm2 corresponding to a DAP of 24 Gyscm
2
As far as conventional sources are concerned typical laboratory devices used in 2D fluoroscopy deliver
a DAP of 06 Gyscm2 [712]
A feasibility experiment will be performed by using a standard human head phantom (see Fig 79)
which will be mounted on a rotating stand that allows a 360deg rotation It will be possible to translate this
stand in three directions (along the X-ray beam axis and in the two other perpendicular directions) to
optimize the sample position in the beam Acquisition will be performed using the 60-90 keV pink
beam before and after the insertion of a high-Z element solution (typically iodine or gadolinium) in a
slot available in the phantom head Several images will be acquired using different concentrations of
high-Z element chosen in the range of concentrations that can be typically reached into a tumor (in
humans the high Z element is injected intravenously) The study will also aim at finding for each high-
Z element the lowest concentration that can be measured with the ThomX imaging system
Figure 79 Typical phantom featuring a human skull from X-ray absorption point of view
lsquo
132
The detector will be placed downstream of the sample A typical detector that may be used is a large
field of view 2D detector equipped with Si or CdTe sensors with ~100-200 microm pixels [713] Finally a
standard computed tomography (CT) algorithm will be used to reconstruct the tomography images and
to calculate the concentrations (in mgml) of the given high Z element in the skull The results will be
compared to previous work done at the ESRF [714]
A radiotherapy experiment
For this experiment the targets will be rats implanted in their right brain hemisphere with rodent brain
tumors (F98 9L) at an appropriate tumor stage (typically at D10 or D14 after implantation) For the
irradiation we need a pink beam with a 10 keV energy bandwidth and a median energy above the K-
edge of the high-Z element to be injected as a radiosensitizer (ie in the 45-80 keV range for both
iodine and gadolinium) The beam size will be a few centimeters in diameter Figure 710 shows the X-
ray beam energy and beam flux generated by a 69 MeV electron beam versus the photon back-
scattering angle X-rays of energy 80 keV within a 10 keV bandwidth are contained in a cone of half-
aperture 27 mrad and their flux is ~31012
phs At the sample location on optical Table 2 at 9 m from
the ring interaction region the beam has a diameter of 5 cm ie a cross section of ~20 cm2
The center of the ratrsquos right hemisphere will be placed on the rotation axis of the irradiation system and
slits will limit the beam transverse extension in order to define a precise field of irradiation Assuming
an X-ray flux (Ф) of 31012
20 phscm2 with an average energy (E) of 80 keV a mass attenuation
coefficient of 0025 cm2g (soft tissue 80 keV X-rays) Eq (71) leads to a dose rate DRe of 48 mGys
close to the tissue region where the X-ray beam penetrates Before reaching the tumor the radiation is
attenuated by the bone (~ 02 cm thickness (microρ)att = 022 cm2g ρ=192 gcm
3) and by the skull healthy
tissues (~ 15 cm thickness (microρ)att = 018 cm2g ρ = 10 gcm
3) Thus the dose rate DRt delivered at
the tumor is DRt = TDRe (where T = exp (-02219202) exp (-0181015) = 070) and is
expected to be 337 mGys The dose will be delivered while the rat undergoes a 360deg rotation The total
dose to be delivered to a tumor ranges between 10 to 20 Gy Therefore according to the expected rate
of 337 mGys the irradiation will take typically from 5 to 10 minutes with the big advantage of not
having to move the rat A dosimetry protocol (such as the one used at ID17 [715]) will be set to check
the homogeneity of the irradiation and to measure the absolute dose with a 2 to 5 uncertainty
Figure 710 X-ray energy (left panel) and X-ray flux (right panel) versus the photon back-scattering
angle for stored electrons of 69 MeV The laser wavelength is 106 microm
For humans and a tumor placed at 8 cm into the skull the transmission coefficient becomes T= 019
Therefore the expected dose rate DRt delivered by the ThomX source at the tumor is 91 mGys One
may compare with the ESRF ID17 beam-line where 800 mGys can be delivered for irradiation on a
lsquo
133
surface of 2x50 mm2 ie with T=019 a dose rate DRt of 015 Gys on 2 mm in height at the tumor
which is equivalent to a rate of 6 mGys to cover 5 cm of height with a scanning rate of 50 mms
ThomX could therefore deliver doses for human skull irradiation larger than those available at
synchrotron radiation facilities and also perform the irradiation without having to move the irradiated
sample Concerning the conventional hospital devices they deliver low doses deposited by X-rays
whose energy spectrum is broad and moreover they are not made to operate continuously for such long
time irradiation
Also the tunability of the ThomX beam will allow testing several high Z drugs containing iodine
gadolinium platinum or gold proven to be promising radiosensitizers [716]
Conformal radiotherapy
A final step in this sequence is performing conformal radiotherapy experiments ie doing both imaging
and therapy at the same time In fact one major issue of radiotherapy is to spare as much as possible
healthy tissues located around the tumor and at its border In this regard it is a key issue to adapt the
field of irradiation to tumor geometry size and positioning by performing first a tomography
Phase contrast imaging
The absorption medical imaging technique is based on the fact that different parts of a sample absorb X-
rays differently But the fact that X-ray absorption by biological material is weak constitutes also the
limitation of this technique To enhance or replace absorption-based imaging refraction diffraction and
interference of X-rays can also be used A better visibility at the edge of different regions of a specimen
can therefore be obtained by using the information contained in the phase of an X-ray wave For this a
spatially coherent X-ray source is required and was available only since the development of modern
synchrotron sources Experimentally the simplest way to visualize phase-contrast imaging is provided
by the propagation-based imaging (PBI) method In this method a spatially quasi-coherent radiation
illuminates an object which gives rise to a spatially varying phase shift in areas with spatial variations
of the refractive index As the radiation propagates beyond the sample parts of the wavefront which
have experienced different deflections on both sides of an edge of the sample produce a serie of fringes
due to the classical Fresnel-edge diffraction phenomena [717]) This gives rise to a characteristic phase
contrast pattern which is then recorded by a 2D detector placed at a proper distance downstream of the
object (see Fig 711 left panel) The actual phase-contrast signal is convolved with the detector point
spread function which has to be sufficiently narrow to avoid the smearing of the interference pattern
Additionally the phase contrast signal can be visualized if and only if the transverse coherence is
sufficient ie if Lcoh = λRs is larger than the details to be imaged (here R is the source-sample distance
and s is the source size) In contrast the requirements in terms of longitudinal coherence
(monochromaticity) are rather weak even a beam with a bandwidth of several keV does not lead to a
significant smearing of the phase contrast signal [717]
Assuming s = 70 microm R = 15 m a sample-detector distance of 5 m and a detector point spread function
of 50 microm the phase contrast signal expected when the ThomX beam illuminates a nylon wire of 300
microm diameter ranges from 10 to 3 when the X-ray energy increases from 20 to 80 keV (see Fig
711 right panel)
lsquo
134
Figure 711 Left panel The set-up for propagation-based phase contrast imaging Right panel The
simulated signal produced by a nylon wire of 300 microm diameter illuminated by the ThomX source
(simulation)
The experiment with ThomX will comprise two parts a planar radiography and a computed
tomography For the radiography three kinds of samples will be used (i) a custom-made phantom
composed of nylon wires of different diameters (ii) the ACR (American College of Radiology)
accredited phantom used in mammography quality (iii) a 4 cm thick formalin-fixed human breast tissue
obtained from a hospital bank The X-ray energy will be chosen in the 20-80 keV range For each
sample and for several energy values images will be acquired at two different sample-detector
distances in the absorption regime for comparison purposes (the detector will be in contact with the
sample) and at 5 m A silicium detector with 50 microm pixels will be used Images will be acquired for
different doses (measured with a standard PTW dosimeter) in order to obtain the signal-to-noise ratio as
a function of the dose These measurements will allow one to qualify the phase contrast imaging set-up
at ThomX
Computed tomography will be applied to (i) a meacutethyl polymeacutethacrylate (PMMA) custom-made
cylindrical phantom containing insets of different compositions and densities (water ethanol paraffin
etc) (ii) a formalin-fixed human breast tissue (5 cm in diameter) Images will be acquired using the
same parameter values as the ones chosen for the radiography Samples will be installed on a rotating
stand positioned on a double cradle that will permit an alignment with the beam and with the detector
Images will be taken during the sample rotation over 360 degrees
The final step will consist in applying standard algorithms to the images in order to calculate the phase
and to extract quantitative information about the samples This data will be compared with equivalent
images that will be acquired at the ESRF
742 Experiments using the central part of the beam
Most specimens from cultural heritage and functional materials are heterogeneous these lsquorealrsquo
materials often exhibit complex hierarchical structures involving various levels of order and
heterogeneity To understand the properties of such materials better one needs at least a description of
their structure at the atomic scale and at the macroscopicmesoscopic scale which takes into account
weak structural signals (relating to disorder minor constituents microstructure hierarchical orderhellip)
lsquo
135
This requires much probing as already demonstrated with synchrotron radiation sources A major goal
of materials science is to link global properties to local atomic features (orientation strain structural
distortion grain size) of some components in relation with neighboring domains and to deduce from
that the macroscopic properties As the functionalities of these materials often stem from the association
of different phases or constituents or of specific inter-relation between them (microstructure) highly
selective structural probes need to be used to characterize them Also in the particular domain of
cultural heritage many specimens require non-destructive methods and probes In order to understand
how they were made and to preserve them from degradation over time their physical and chemical
properties must be determined There is an increasing demand for material probing with complementary
techniques Fluorescence emission techniques excel in revealing the elemental composition of the top-
most layers of samples Diffraction is suited for studying quantitatively the phase composition the
structural details of the constituent phases and the characteristic distribution of sizes and orientations of
crystallites Tomography techniques excel in revealing the 3D mapping of the chemical andor the
density heterogeneities within a sample This is why the cultural heritage community is today strongly
involved in the development of new instruments that may characterize patrimonial artifacts by mapping
not only the chemical composition of a variety of samples but also their physical phases
Experiments described in section 741 require a large 2D beam and exploit the natural divergence of
the ThomX beam In this section diffraction scattering and spectroscopy experiments require a
focalized or a parallel beam and it should also be monochromatic or lsquopinkrsquo The optics will therefore
select the central part of the beam ie its higher energy part Let us give an order of magnitude
assuming a perfect 50 MeV electron beam a collimator of 2 mm diameter located at 2 meter from the
IP will cut the X-ray conic beam at 05 mrad leading to a flux of ~8 1010
phs with an energy of 45 keV
and an energy bandwidth ΔEE on the order of 10-3
(see Fig 73) In practice this bandwidth will be
broadened and will reach a few percent because of the actual e- beam divergence and energy spread For
experiments requiring a narrower energy bandwidth an additional selection will be performed with a
monochromator For instance selecting an energy bandwidth of 10-4
would lead to a flux on the order
of 109 phs
Since the goal of the first series of demonstration experiments is to qualify ThomX as an X-ray source
these experiments will be performed with samples previously characterized with other sources such as
synchrotron radiation sources
Diffraction and scattering
Diffraction and scattering experiments are major ways of performing materials structural and micro-
structural analyses This concerns highly ordered samples as well as powders or ill-ordered samples
Complete scattering analyses are based not only on Bragg diffraction but also on diffuse scattering in
order to look beyond the average structure and to investigate the local structure or short-range order
The crystalline quality of powders depends on the preparation methods Grain sizes vary from about 10
nm to several hundreds of microm In the case of nano-materials Bragg peaks are strongly broadened and
X-ray diffraction performed with standard laboratory sources does not provide suitable data for
structural investigations As illustrated by Fig 712 laboratory X-ray sources do not allow the
identification of minor phases in a poorly crystallized powder Figure 713 shows an example of a
diffraction pattern obtained with an excellent resolution The sample used for this analysis is an
Egyptian cosmetic powder dated to 2000-1200 BC The patterns were obtained with the ESRF
synchrotron source (bottom panel) and with a classical laboratory source (top panel)
lsquo
136
Figure 712 Powder X-ray diffraction patterns obtained by using a laboratory X-ray source Top panel
Poorly crystallized Prussian blue sample Bottom panel Prussian blue crystals From [718]
Figure 713 Powder X-ray diffraction patterns obtained by using a laboratory X-ray source (top panel)
and the ESRF synchrotron source (bottom panel) Arrays indicate minor phases From [719]
We intend to use the ThomX source to record diffraction patterns and scattering plots in order to
characterize short-range ordering and identify minor phases in such heterogeneous samples
For the first demonstration diffractionscattering experiments we will study ill-ordered Prussian blue
and white lead samples Pigments used for paintings or frescos have various compositions Since their
usage followed a definite chronology such pigments can sign an era Pigment characterization can also
help the undertaking of a restoration while a thorough knowledge of pigment synthesis through
centuries can help to apprehend the degradation mechanisms better
lsquo
137
For ordered samples diffraction will be performed with a single crystal by using a monochromatic
beam but also by using a lsquopink beamrsquo as done in the Laue method For ill-ordered samples the whole
scattering pattern will be recorded and analyzed with the Pair Distribution Function (PDF) method This
analysis technique (often called the ldquototal scattering methodrdquo) can be used alone or associated with
diffraction For experiments using the lsquopink beamrsquo Laue method the beam will be focused directly on
the sample (by using convergent refractive lenses (CRLs) or a capillary optic system as mentioned in
Section 731) To determine powder crystalline parameters diffraction analyses require the
monochromatization of the beam in order to reach ΔEE ~ 10-4
In the optic hutch (optical Table 1) the
diverging beam will first be made parallel by the CRLs or the capillary optic method and then
monochromatized in the experimental hutch (optical Table 2) by a monochromator The set-up for these
measurements needs a goniometer to align the sample and a low noise 2D-detector sensitive to high
energy photons
Fluorescent spectroscopy
X-ray fluorescence spectroscopy experiments allow one to determine the chemical composition of
materials and the mass concentrations of elements Indeed depending on the atomic number of a given
chemical element energies of the fluorescence lines emitted after an X-ray excitation are a signature of
the element An illustration of this approach is presented in Figure 714 where the fluorescence
tomographic images of a Globorotalia inflata specimen are shown and compared with a classical
radiography image
Figure 714 Fluorescence tomographic imaging (a) Set-up (b) Radiography (c) Fluorescence
spectrum measured at a given angleposition of the sample (d) 3D tomography reconstruction From
[720]
We plan to carry out fluorescence experiments since the energy of the ThomX beam can reach the
excitation energy of most chemical elements usually involved in material science for instance the
critical excitation energy of lead regarded as a heavy element is 88 keV) For this experiment the
excellent energy resolution of cooled Ge detectors will be an asset to detect the fluorescence light (the
detectors will be oriented at 90o with respect to the incident beam)
lsquo
138
K-edge and XANES spectroscopy
In K-edge experiments two images of a given element present in some sample are performed one at an X-ray
energy just below the K edge of the chosen element and the other at an energy just above Then they are
subtracted (see Fig 715 left panel) The K-edge imaging technique is used in particular in the medical and
the cultural heritage domains In XANES (X-ray Absorption Near Edge Structure) the absorption by a
material is measured over an energy range of about 100 eV on both sides of the absorption edge This
technique provides information about the local structure prevailing close to a particular chemical element (see
Fig 715 right panel)
Figure 715 Left panel K-edge subtraction imaging Right panel Principle of the XANES method
The first XANES spectroscopy experiments with ThomX could exploit the K-edge of heavy elements
such as W Pt or Pb By first selecting the central part of the ThomX beam with an energy spread of a
few 10-2
a monochromator could then provide a beam with an energy spread of 10-4
Samples will be
aligned by using a goniometer and the transmitted intensity will be collected by a low noise 2D-
detector
Combined diffractionabsorptionfluorescence tomography
A major limitation of diffraction methods comes from the fact that only average properties can be
obtained while most lsquorealrsquo materials are heterogeneous and complex Thus structural studies of such
materials are difficult to carry out A new tool for structure mapping of materials by means of X-ray
diffractionabsorptionfluorescence combined with tomography allows one to perform non-destructive
3D structure selective mappings of heterogeneous samples This way one may identify minor phases
and determine crystallite orientation and grain size Furthermore the selectivity of the method allows
one to separate and locate the origin of the diffuse or diffracted patterns to identify amorphous phases
In archaeology such complementary micro-structural analyses could shed light on the mechanical
history of artifacts
A recent experiment carried out with a micrometer beam at synchrotron radiation sources have
established the validity of such a local probe by analyzing a mixture of chalcedony and iron pigments
containing haematite [721] This experiment illustrated in Fig 716 consists in (i) performing a
structure tomography mapping directly from the diffraction data (top panel) (ii) obtaining selective
structure patterns of unknown minor phases from these maps (bottom panel) This study was realized by
the Neacuteel institute and the C2RMF and was one of the first diffraction tomography experiments
performed with a micro-beam
With the ThomX source we will use poorly crystallized samples and combine in the same experiment
and simultaneously tomography diffraction with fluorescence and absorption measurements Diffraction
lsquo
139
tomography is beam-time consuming it will be performed to demonstrate its feasibility at ThomX The
experimental set-up will be similar to the one used for multimodal analyses (cf the top panel of Fig
716) The sample will rotate and this rotation will be combined with a translation to provide a 2Θ scan
of 2D diffraction patterns obtained with a 2D detector sensitive to high energy photons The diffracted
2D fluorescence patterns will be recorded with our Ge detector while the sample absorption will be
recorded with our calibrated Si diode
Figure 716 Top panel Experimental set-up for multimodal spectroscopydiffractiontomography
analysis Insets Images of the chalcedonyndashFe mixed-pigment sample are reconstructed by combining
the various signals X-ray absorption (bottom) Fe X-ray fluorescence (top left) and chalcedony
diffraction (top right) Bottom left panel Reconstructed diffraction image based on the whole
diffraction diagram (black) compared with the calculated pattern for pure chalcedony (red) Bottom
right panel Reconstructed image that only uses the iron pigment scattering contribution (black)
compared with the calculated pattern for hematite and siderite grains (red) From [721]
75 Summary
As shown in this chapter two types of beams will be available at the ThomX compact X-ray source
(i) wide beams well adapted for imaging (conventional or phase contrast imaging) and therapy in the
biomedical field and (ii) almost monochromatic beams that can be either focused or made parallel for
diffraction and spectroscopy experiments in the field of materials science
The X-ray line is designed
- to ensure a continuous monitoring of the X-ray beam
- to measure the beam brightness and its transverse coherence
- to demonstrate the potential of this new type of compact source and identify the benefits it can
bring to the currently available X-ray sources
lsquo
140
Chapter 8 CONTROL SYSTEM
Introduction
The ThomX control system combines those hardware and software elements involved in the
management of all the facility components But more than a system dedicated to carrying out operating
processes (processes launched from the X-ray beam control room in order to undertake some specific
task such as a performing an imaging experiment or a material chemical analysis) it must also be an
advanced tool for various studies (data processing new equipment integration operating mode
evolution) and scientific experiments which will be performed with the X-ray beam line Hence the
control system has to be a versatile tool easily scalable by various users (scientists operators IT
technicians)
In the following a lsquoSub-systemrsquo is a physically connected set of hardware items sharing a global logical
function The ThomX sub-systems are as follows the linac (LINAC) the transfer line (TL) the ring
cell 1 (RI-C1) the ring celle 2 (RI-C2) the optical cavity (OC) the extraction line (EL) the section
of the X-ray line located in the Igloo (XLI) and the section of the X-ray line located in the X-ray hall
(XLH) A lsquoDomainrsquo is a set of hardware items related by a common purpose The ThomX domains are
as follows alignmentmetrology (AM) control-command (CC) diagnostics (DG) infrastruture (IF)
magnetic elements (ME)n optics (OP) pulsed elements (PE) power supplies (PS) radio-frequency
(RF) safety (SA) synchronization (SY) and vacuum (VA)
81 General principles
Some general principles drive the design of such a control system In broad terms the following criteria
will serve as guidelines
- Since monitoring is a major role of a control system comprehensive monitoring interfaces must be
provided to all users Moreover the monitoring system must take into account sub-systems specificities
dedicated GUIs must be designed for every need for instance in synoptic form to give a global system
status overview or as charts to show the time evolution of some specific parameters of a particular
device (eg pressure readings on TPG300)
- Operating processes management a control system also aims at the prioritization and sequencing
of processes It has to provide high-level functions in order to remotely launch complex operations (eg
machine startstop interlockshellip) Computing and data analysis tools which could launch such particular
procedures will be integrated or developed to maximize the automation of the machine
- Default management the facility control system must be able to manage operation defaults It will
be necessary to design graphical tools to quickly identify defaults when they occur Color display of
warnings and errors according to a criticality-based convention will be implemented in addition to the
recording of running process logs
- Safety for obvious reasons the ThomX radioprotection and safety system must standalone It should
not use any data generated by the control system Nevertheless each end-user must be able to make
sure that the ThomX facility does not present any danger whenever the accelerator is running This
information must be available in every control room through a continuous display of all the safety
equipment status (radioprotection sensors door openingshellip) A full historical record of safety-related
lsquo
141
events will be made available for analysis and a procedure of detailed acknowledgement (operatorrsquos
name action to close an incident) will be defined in order to guarantee safe operations
- Ease of use The ThomX control system is intended for various usages therefore it will concern
various users including operators who are not familiar with software development Hence it is crucial
to make it modular through relevant to sub-systems GUIs with various visualization interfaces control
buttons graphs sub-system overviews in synopsis or detailed display of subset status The definition of
new interfaces will be possible through visual development frameworks to fit new needs
82 Technical considerations
In order to guarantee a proper accelerator operation to make its use as easy as possible and to limit
costs some technical points have to be taken into account
- Distributed architecture an accelerator is a complex structure which results from various designs in
different fields The hardware part of the control system relies on dedicated IT devices (industrial PCs
programmable logic controllers) whose role is to drive sub-systems Each of these nodes will be fully
and remotely controlled by the system software from the control rooms (or from any authorized user
terminal in the ThomX network) independently of possible other issues All of these components must
communicate with each other when required by the operation needs Moreover the supervision part of
the control system must be available in different places (main control room X-ray line control room
perhaps others) The chosen system must meet these requirements and must run on several terminal
types
- Standardization of the hardware items and of the software codes is necessary in order to simplify
their integration in the control system (eg same brandmodel for all the stepper motors) Then it will be
possible to mutualize high-level controlling tools whether pre-installed or homemade Additional
benefits of this approach concern the overall maintenance the deployment of new releases and the
replacement of hardware parts
- Reliability The service continuity of the control system must be guaranteed A spare strategy must be
set up to make sure that the replacement of defective parts will be done on short notice In particular
storage memory redundancy will guarantee that none of the machine parameters or of the collected data
will be lost Periodic backup of main software systems (device databaseshellip) will be performed right
from the start Uninterruptible power supplies are foreseen to protect major elements of the control
system from electrical failures
- Upgradability Special attention will be paid to communication features when choosing hardware
devices Ethernet is now one of the major protocols and must be preferred when possible For the PLC
driven devices the fieldbus Profibus [81] is recommended since one single master interface can
manage more than 100 slave devices without data losses This product is also valuable as far as cabling
cost is concerned
- Safety Since several users may wish to perform various machine-related tasks (integration
maintenance measurementshellip) a policy must be established to control the access to the ThomX
network and its control system The management of these access permissions will be provided It will
take into account the role and the field of expertise of each project member
lsquo
142
83 The TANGO control system software
Tango [82] is the software chosen to be the backbone of the ThomX control system It is an open-
source software initially developed at the ESRF It is now used in several Institutes to control huge
accelerator facilities like the SOLEIL synchrotron In fact the SOLEIL staff is playing a major role in
maintaining and developing new Tango features
831 Software bus
An accelerator equipment is usually highly heterogeneous Thus its control system must enable various
devices to communicate with each other even if they use different protocols or communication
standards like Ethernet RS232 USB PCI DAQ Cardshellip
The first asset of Tango is the communication standardization that it provides between any type of
devices Tango implements a CORBA-based communication protocol so that a common network is
virtually created for the whole hardware If needed the parameters of a given device are thus available
to every other element of the control system This is achieved by adding an extra Tango-compliant
software layer above the hardware low-level libraries This layer is specific to a device brand and model
and is a simple executable file which can be instantiated as many times as required by the number of
devices This key element of the Tango system is called a lsquoDevice serverrsquo
832 A control system for accelerators
For basic reasons the Tango core and many of the tools developed around this software bus are open
source tools It is therefore an accessible system fully customizable for specific ThomX needs and a
system which can be maintained Since Tango is Linux and Windows compliant issues originating in
driver availability for the Linux OS can be avoided while Tango enables the use of scientific
applications which are often developed for Linux
Furthermore Tango is developed and maintained by a community which involves several accelerator
Institutes Even if Tango is not an accelerator-dedicated tool a number of developments have been
carried out or are being carried out that deal with devices currently used at such facilities Thus a large
Tango library of device servers is presently available to any new user
Finally more than a simple tool which provides a network access to a set of devices Tango is also a
framework that provides high level operational services such as configuration tool logging archiving
services and alarms permission management For developers specific tools are provided for the
generation of codes that fit in the common structure required by all device servers Other tools allow
easy GUI development As a result the integration of new devices or the upgrade of control and
supervision windows do not need in-depth coding As far as the compatibility with commercial software
is concerned dedicated bindings can be freely used eg applications developped for Matlab or
Labview can be used to perform a variety of tasks involving the ThomX devices
lsquo
143
84 Control system architecture
841 Equipment to control
On a complex machine like ThomX a wide range of devices and control signals must be taken into
account motors cameras scopes diodes or valves will deal with analog and digital inputsoutputs
complex signals transmitted through GPIB USB Ethernet or profibus interfaces
The inventory of devices to be controlled is presented in the following table Safety equipment is not
mentioned as it is a fully standalone sub-system
LINAC TL Ring OC XL
Power Supplies
Coils 2
Steerers 2 4 24 2
Dipoles 5 4 8 3
Dipole chicane 1
Quadrupoles 7 24 3
Sextupoles 12
Pulsed elements
Septum 1
Kickers 2
Synchronization
3GHz synthesizer 1
500 MHz synthesizer 1
Phase shifter 5
Programmable delays 2
Counter 1
Vacuum
Ion pump 8 4 17 4
Gauge 8 8 22 8 1
Valve 4 4 4 2
Primary pump 1
Diagnostics
BPM 1 4 12
Low frequency Signal
generator 2
ICT 1 2
Stepper motors 1 3
Camera CCD 2 3 1
RF
Klystron coil 1
Modulator 1
Amplifier 1
Canon 1
Feedback 2
Source laser
Photodiode 1
Camera CCD 1
lsquo
144
Motor 1
Mirror 2
Laser ndash FP cavity
Camera 4
CCTV Camera 10
Motor 16
Thermal control unit 2
Laser equipment 3
Scopes 3
FPGA 1
X‐ray Line characterisation
Xpad camera 1
Si PIN diode 3
Ge diode 1
Motor 13
Stepper motor 25
Others
Translation table 1 1
Slot 1
Table 81 Devices to control (TL=Transfer line OC = Optical cavity and XL = X-ray line)
842 Inputsoutputs
All devices listed above communicate with each other by digital inputsoutputs (DI DO) ie one bit
signals analog inputsoutputs (AI AO) or more complex signals as n-bit digital signals (n = 8 12 or
16) Several communication protocols mentioned in sect832 above will be used
They are detailed in Table 82
PLC InputsOutputs Bus Ethernet
connections
DAQ (ADCampDAC)
Vacuum 26 DI 13 DO 26 profibus
Diagnostics 2 gpib 30 3 x ADC 12 bits 100 kHz
Linac 2 DI 2 DO 5 AI 3 ADC 8 bits 1 GHz
Magnets 35 profibus
Feedbacks RF (x2) 10 DI 10 DO 10 AI
10 AO
USB
Ampli RF 64 DI 8 DO 32 AI 16
AO
RS232 2 4 channels ADC 12 bits
1MHz
Modulator RF 23AI 4 AO 69 DI 5 DI 5 DO
8 channels ADC 8 bits
Pulsed Elements (x3) 12 DI 3DO profibus ADC 12 bits
Laser ndash FP cavity RS232 22
Timing 2 gpib 14 15 channels DAC 16 bits
X Line 26 DI 26 DO 7
Table 82 Equipment IO to control (not complete at the time of writing)
lsquo
145
843 Front-end machine architecture PLCs and industrial PCs
The front-end architecture will comprise equipments individually dedicated to sub-systems These
machines are controlling nodes which will be located in sub-system server racks as close as possible to
the equipment to be controlled (when required)
Two kinds of front-end machines will be used
- PLCs programmable logic controllers are automation OS-based machines able to manage digital and
analog IO and to control processes carried out by devices connected to a field bus (eg Profibus) or
specialized devices They are reliable devices designed to operate in an industrial environment A
proprietary operating system runs on these machines which are powerful tools (eg automats) to
implement logic appropriate to experimentation Interconnection with the other Tango network
equipment is made through an Ethernet adapter interface
- Compact PCIs (industrial PCs) these crates can embed complex operating systems like a Linux
distribution or some Microsoft Windows Operating Systems hard drives Ethernet connections and
USB adapters Such crates are based on a PCI bus backplane to interconnect specific cards (DAC
ADC counters optoelectronics convertershellip) cPCI is a standard widely used in an industrial
environment it is compatible with numerous card suppliers These front-end machines will also allow
the running of Tango processes or specific computing tasks locally installed in a sub-system
The ThomX distributed control system will be build on these controlling nodes This structure will
allow the bandwidth optimization of the network main links and will provide cabling simplifications
The control equipment may be placed in the accelerator hall as well as in the technical rooms where
most of the electronics that must be controlled will be clustered in 19rsquorsquo standard server racks
‐ Some devices need not be controlled through these front-end machines They will be directly
connected to the control network (eg ethernet cameras or electronics for motor control) Three
ways to interconnect such devices are shown in Fig 81
‐
Figure 81 Control architecture for one sub-system
lsquo
146
These acquisition and controlling units can be Profibus PLC or a cPCI crate Each driven element will
be controlled through one of these acquisition and controlling unit
The overall control system will be composed of more than 25 front-end machines as described in Table
83
PLC cPCI Crate
Linac (wout vacuum diags modulator
magnets) 2 1
Vacuum 2
Diagnostics 2
Feedbacks 2 2
RF amplifiercavity 1 1
RF modulator 1 1
Pulsed elements 3 3
Fabry-Peacuterot cavity 1 1
X-ray line 1
Synchronization 1
Magnets 2
Total 13 14
Table 83 List of front-end machines (not complete at the time of writing)
85 IT infrastructure
- Network The ThomX local network will be organized around an IP backbone It will be based on the
Ethernet TCPIP standard in order to link user terminals front-end machines specialized servers etc
Because of the large number of devices and the requirements bearing on the bandwidth (eg for image
storage) 1 Gbitss compliant equipment will be chosen (LAN adapters switcheshellip) Depending on the
actual number of cameras an image-dedicated network could be added On the cabling side shielded
copper cables will be used as interconnection wires to limit electromagnetic interferences as much as
possible
- Data storage During the ThomX commissioning or for the operation monitoring or for the X-ray
experiments it will be crucial to store a large amount of data It is foreseen to implement the three
storage processes provided by Tango historical (long term storage at a slow rate) temporary (short
term storage at a fast rate) and snapshot (storage of parameters on operator demand or event triggered)
It will also be possible to meet the X-ray line needs for the storage of large size images during short
periods (at most for several days) This specific storage will not be necessarily on local servers it could
be convenient to move these data and store them on servers close to the final users possibly outside the
ThomX network
Two storage-dedicated servers will be installed for analyses replay of accelerator operation scenarios
and for some specific needs of the X-ray line These storage units will be specific machines which
prevent data outages or losses by redundant power supplies stored data redundancy (RAID) hard disk
hot swap etc The first of these two storage units will be a 2 To server equipped with a disk array
dedicated to the accelerator data management The second one will have a 10 To capacity it will be
available for the large size images of the X-ray line
lsquo
147
- Equipment of the control rooms Users will work in control rooms on system administration
accelerator control development and integration (device servers GUIs high level computing software
safety analysis tasks) Administration and end-user terminals should be provided in sufficient number
(see below)
The main control room will be equipped with
- 5 PCs 1 for the Tango core system 2 for operations 1 for development 1 for safety (dedicated to
visualization storage and analysis of safety related events)
- 1 server to run high-level applications and device servers
- 2 screens one displaying the ThomX status with synopses and the other one for the safety
supervision
- camera monitors for some direct controls
- 1 UPS (uninterruptible power supply)
The X-ray line control room equipment will consist of
- 2 PCs for the operation equipment and for macro development
- 1 server to run high-level applications and device servers
- 2 screens dedicated to the monitoring of the machine status to safety synopses and to the display of
safety related events
- Software needs
- Both the Linux and Windows OS must be installed in the control room PCs in order to run Tango and
many other operating processes (see the Introduction above) for development purposes and to use
configuration software Together with the Tango core and specific tools high-level software will be
installed to meet usersrsquo needs Labview and Matlab for science applications SPEC to control the
motors and for the analysis of the X-ray line data
- Data bases will run on the MySQL open-source database managing system
- An open-source http server Apache will allow the implementation of WEB interfaces for the control
system authorized users this way these users will be able to remotely visualize experiment data or
equipment status
- Finally all end-user stations will provide standard office tools
The following Figure gives an overview of the control room equipment
lsquo
148
Figure 82 Schematics of the equipment foreseen for the main control room and the X-ray line control
room
Overall the ThomX control system ndash designed to ensure harmonization and standardization and based
on open source tools ndash should be cost effective The investment cost is estimated around 200 keuro
including IT small equipment not described in this document cables a gateway to provide internet
access extra screens to display the ThomX status in the laboratory etc
lsquo
149
Chapter 9 SAFETY AND RADIOPROTECTION
91 Introduction
The ThomX facility will be sited in an existing building called lsquoIgloorsquo where a source of hard X-rays was
operated until 2003 by the Laboratoire pour lrsquoUtilisation du Rayonnement Synchrotron The ThomX linear
accelerator and storage ring will be built inside this building while the facility control room and the one of the
X-ray line will be located respectively in the D1 and D2 experimental halls The D4 hall will be devoted to
experimental tests
The ThomX operation will produce high-energy electrons and other particles such as X-rays and neutrons
which generate ionizing radiations through their interactions with matter An adequate protection against these
radiations must be ensured for people working at the facility The safety logic adopted is the so-called
lsquopositive safetyrsquo which means that any system dysfunction will immediately cause a complete stop of the
accelerator Furthermore redundant safety devices will be implemented allowing various actions that will
guarantee the staff safety such as
- Shining the beam into an area shall be submitted to prior people evacuation and to the interlocking of this
arearsquos doorways
- Access to a sensitive area shall be submitted to the prior stop of the upstream beam by using at least two
different means independent of each other
Complementary to these two procedures safety measures will be applied involving hard-wired logic safety
PLCs shielding beam dumps shutters personnel exclusion interlocks radiation monitoring of the
experimentation areas staff dosimeters and radiation hazard training of the personnel In case of dysfunction
radiological monitoring hard-wired logic or safety PLCs will act through interlocks that will break
connections to the mains voltage (Pre-Amplifier modulator or electron gun depending on the situation) in
order to stop the accelerator
92 PLC safety
The safety hard-wired logic works in association with an automaton PLC system via two completely separate
circuits A Safety Programmable Logic Controller (PLC) is a special purpose machine which is used to
provide a critical control and safety applications to automaton users This controller is an integral part of
lsquosafety instrumented systemsrsquo (SIS) which are used to detect situations involving potentially dangerous
processes When such a situation occurs the SIS is programmed to bring the process to a safe state
automatically
A safety PLC is specifically designed to accomplish two important tasks (ensured by a diversitary redundant
architrecture)
- lsquodo not fail but if this cannot be avoidedrsquo
- lsquofail only in a predictable safe wayrsquo
Hard-wired logic is a safety circuit fully independent of automatons It takes in charge in particular the
access door interlocking feedback contacts which provide a large number of checks bearing on the magnetic
field the high voltage the pre-amplifier status etc and the running control panel
The PLC safety level must be as high as the one of a positive safety hard-wired logic It manages access doors
emergency stop buttons safety rounds and reports issue authorizations to operate from a command-control
system
lsquo
150
93 Radiation monitoring
Concerning the radiation protection the ThomX facility design satisfies the following principles
- No one will receive a dose of radiation in excess of the regulatory dose limits as a result of normal
operation
- Professionnal occupation exposures will be kept As Low As Reasonably Achievable (ALARA)
- All practical steps will be taken to prevent accidents with radiological consequences and to minimize the
radiological consequences of any accident
Arrangements will be made for the monitoring of working areas such as the Igloo the bunker and the Optical
hutch For this purpose several monitoring modes will be available such as thermo-luminescent dosimeters
measurements made by the qualified person in charge of radiation protection (Competent Radiological
Protection Person) and a modular system for continuous monitoring A continuous monitoring of gamma rays
and neutrons will provide radiation dose rates and will manage alarms its measured values will be transmitted
to a central data acquisition unit Placed in the Compton room a special treatment unit will also be
implemented that will provide a local display of the dose rate signage and will manage alarms
94 Radiation protection zoning
The D1 D2 and D4 halls are not classified lsquosupervised arearsquo therefore their access is free to people who
satisfy access requirements In contrast the Igloo hall is classified lsquosupervised arearsquo
The D1 hall contains the X-ray line and technical equipment
- The X-ray line is not a classified area with the exception of the Optical hutch
- The Optical hutch is alternately classified lsquoprohibited arearsquo and lsquonot classifiedrsquo area depending on the
permission conditions issued by functional elements important to safety (shutter position permanent
radiological level)
The D2 hall contains the ThomX control room and a room which contains the laser equipment
The D4 hall is devoted to tests and mechanical assembly
The Igloo hall contains the electron accelerator located inside a bunker The Igloo is classified lsquosupervised
arearsquo therefore only workers subject to individual dosimetry are admitted However in exceptional
circumstances public access might be authorized as long as regulations and procedures are respected
The Igloo bunker on the one hand and on the other hand the basement area devoted to the facility are
alternately lsquoprohibited arearsquo during the accelerator operation and respectively lsquosupervised arearsquo and lsquonot
classified arearsquo when the facility operation has stopped
The accelerator energy is high enough to expect nuclear reactions of photons and neutrons with matter
leading to the activation of structural materials This is why it is particularly important to use appropriate low
activation structural materials or structural materials that are activated for short periods of time only
Furthermore one must ensure a nuclear waste management which will require a specific room to store and
monitor activated materials This room should be a lsquosupervised arearsquo located in the Igloo
95 Access control
951 General statement
It is a matter of absolute necessity to have appropriate procedures and devices in place to identify and screen
all persons entering the ThomX facility Consequently the LAL access control should be extended to the D1
D2 and D4 halls and to the Igloo
lsquo
151
952 Particular requirements concerning the lsquorestricted areasrsquo
The validation of door contacts as well as the validation of the position of protective shields and real-time
radiation measurement are prerequisites to access the restricted areas (ie Optical hutch and the accelerator
area) As it applies to all functional elements important to safety signage and door statements (openedclosed
lockedunlocked) from classified areas must be transmitted to the control room Local indications are
displayed alternately with the status messages The measured values of the dose rate and alarm symbols are
shown permanently
953 Access to the accelerator area and to the Igloo basement
The accelerator and the storage ring will be located in a concrete enclosure that will provide an efficient
shielding to protect people against ionizing radiations inside the igloo in line with regulations Prior to the
validation the floor load-bearing capacity must be checked by experts
The concrete containment structure will have a removable roof made of staggered concrete slabs Civil
engineering will be needed for the anchorage and supports to be used to adapt the bunker concrete blockds to
the igloo circular wall which will be part of the enclosure and to make sure that the roof of the whole
lsquoprohibited arearsquo provides a complete shielding Since doors will be made of standard steel the bunker access
will be indirect ie through (short) mazes to ensure protection against ionizing radiations
Access to the bunker and to the basement area which is part of the ThomX facility will be determined by the
equipment that guarantees peoplersquos safety in accordance with the facility operation As mentioned above
these devices ensure redundancy by using wired logic safety PLCs and also special safety door locks which
are simple but effective devices A mechanical system called lsquotrapped key lockrsquo will be implemented the key
is trapped in the lock when the door is open and it can be recovered only once the door is properly locked
Turning on the beam in lsquoprohibited areasrsquo is allowed only if the safety systems have been unlocked which
implies that the collection of access keys is complete as will be checked by the control board To meet the
safety requirements new access doors must be installed and some existing doors must be modified
954 Access to the X-ray line and to the Compton room
The D1 hall will not be a lsquoclassified arearsquo and will be open to authorized people The X-ray line sections will
be in open access during normal operation with the exception of the Optical hutch To meet the radiation
protection requirements the Optical hutch partition and its roof will be made of thick metal sheets A thick
lead shield will be installed at the end of the X-ray beam axis
The Optical hutch will be alternately classified lsquoprohibited arearsquo and lsquono classifiedrsquo area (under the worst-case
conditions it might become a lsquoregulated arearsquo) depending on permission conditions issued by safety functional
items External and internal light-signaling warning will indicate the zoning status Thus access to the Optical
hutch is firstly subordinated to the removable shutter position and secondly to the local permanent
radiological measurement values The shutter is dimensioned to protect people from high radiation rates
effectively A fail-safe closer device triggers the shutter closing automatically in case of electrical power
failure too (compressed air power supply) When a door is open the shutter command is prohibited
A local radiation detector located in the Compton room will check he radiation exposure and the dose levels
down to as low as reasonably achievable values Measured high dose rates will automatically shut down the
accelerator and activate the evacuation alarm The validation of these safety devices is mandatory to grant
access permission
lsquo
152
96 Beam dumps
The ThomX facility will comprise tow beam dumps made of concrete that will contain high energy particles
They are very important to ensure an adequate protection of people from the ionizing radiations that would be
produced if they were not used One will be aligned with the linac axis and the other one will equip the
extraction area These shields will be installed as close as possible to the radiation source to absorb ndash or at
least to moderate ndash as much as possible of the produced radiations Their size and thickness will be
sufficiently large to reduce efficiently all secondary radiation (X-rays and neutrons) produced by the stopping
of the e- beam They will be made of a lined lead shield designed to moderate back-scattered radiation placed
in a cavity made of concrete blocks or high-density polyethylene Ne peut-on pas ecirctre plus preacutecis donner des
nombres sur les dimensions et preacuteciser les mateacuteriaux utiliseacutes avec une figure
97 Interlocks
Depending on the dysfunction origin the safety system will operate and break command circuits such as the
pre-amplifier trigger the modulator high voltage the electron gun trigger or its high voltage Feedback
information from the control systems may be gradually requested according to the anomaly
98 Accelerator emergency shutdown
Accelerator Emergency Shutdown (AES) are signal and alarm devices located in specific areas A sufficient
number of AES should be installed in prohibited zones namely the accelerator area the bunker basement and
the Optical hutch For the users to spot them easily AES are devices equipped with flashing lights which are
turned on whenever the local zone is closed
If people are still present in a dangerous zone when a machine run is about to start (ie when the access is
going to be forbidden after a short while) they should signal their presence by pushing an AES This action
prevents the accelerator operation The system reactivation must follow a specific procedure
99 Control console
Placed in the control room the control console collects all access keys The accelerator may be turned on if
and only if all keys are inserted in that console
The safety circuit validation (PLC safety hard wired logic) allows one to launch the accelerator running
procedure and triggers audible warning devices for 30 seconds before operation becomes possible
910 Evacuation of dangerous areas
A specific procedure must be followed for the evacuation of the bunker and its basement Operators should be
trained in methodical evacuations with the validation of all checkpoints (roundsman function) Non
compliance with the procedure will lead to the non validation of the facility operation
A sufficient number of roundsman functions should be located in the accelerator area Also an exit path
should be marked leading to those areas that provide a possibility to regress
911 Audio system and horns
Loud-speakers and horns will be installed in the Igloo basement (in the so-called lsquorotundarsquo) and pits and
inside the bunker The sound system must be of good quality speeches and broadcast messages must be
intelligible Horns must be loud enough 80 dBA at least
lsquo
153
912 Safety notices
Local notices Posters should be put up in lsquosupervisedrsquo and lsquocontrolled areasrsquo to remind people of the
regulation requirements Such posts maintain the workerrsquos awareness whenever they are in a radiological
environment Access doors to zones with multiple classifications should be equipped with highly visible
luminous warning panels Also measured radiation levels and messages could be flashed regularly Regarding
the radiation measured level in the Optical hutch an alarm symbol must be permanently displayed that
informs of this level
Signal boards at zone entrances and within the zones will specify the real-time safety conditions display
warnings (such as presence of live voltage ongoing operation mode etc) and the access permissions (such as
open access prohibited zone etc)
913 Radiological monitoring
The control of the radiological environment will comply with the ALARA principle It will be performed by
- using bimonthly exposure dosimeters based on radio-photo-luminescence (RPL) and neutron sensitive
dosimeters
- PCR controls with live measurements
- a centralized modular system which will continuously monitor exposures feeding a data acquisition system
that can handle up to 10 sensors
This centralized system will perform dose rate and integrated dose measurements manage alarms test
measurement channels check settings and calibrations It will allow a special device placed in the Optical
hutch to display in real time local safety information
The ALARA principle is a commitment to keep radiation exposures and doses at a level as low as reasonably
achievable (social and economic factors being taken into account)
lsquo
154
Chapter 10 MANAGEMENT OF THE THOMX PROJECT
Introduction
The management of the ThomX project will have a simple structure that takes into account the three
project phases 1) The construction 2) The exploitation 3) The technology transfer Three boards will
govern the ThomX activity covering all project aspects the machine design the purchasing and the
assembly the commissioning the usersrsquo access the communication and the relationships with the
industries This management board will comprise
1) A steering committee that will supervise the project activity and define the collaboration scientific
policy
2) A scientific committee composed of the scientific coordinators of the collaboration partners It will
assess the usersrsquo projects and prioritize the beam-time allocation This committee must also define the
scientific policy of the machine exploitation and RampD of the ThomX project
3) A partnership and valorisation office This body will evaluate the cultural and the economic
potential of the ThomX innovations It will stimulate transfer initiatives and check their outcome It will
work in close contact with the steering and the scientific committees
The composition of these three bodies is given in Fig 101
Figure 101 The ThomX project committees
In parallel the communication group (see Table 101) will be in charge of all activity concerning
communication and reach-out in the ThomX project It will organise events realise the publications
relating to the project status and progressrsquo and coordinate the official announcements of the
collaboration
LAL Orsay
Achille Stocchi
CELIA Bordeaux
Philippe Balcou
UMPC
Philippe Walter
Institut Neel
Alain Schuhl
INSERM Grenoble
Franccedilois Esteve
Synchrotron SOLEIL
Amor Nadji
ESRF
Jean Susini
THALES
Yves Trinquet
Alessandro Variola
LAL Orsay
Alberto Bravin
ESRF
Geraldine Le Duc
ESRF
Jean-Louis Hazemann
Institut Neel
Philippe Walter
UMPC
Alexandre Loulergue
Synchrotron SOLEIL
Francois Esteve
INSERM Grenoble
Eric Cormier
CELIA Bordeaux
Alessandro Variola
LAL Orsay
Pierre Bordet
Institut Neel
Philippe Deblay
Synchrotron SOLEIL
Yves Trinquet
THALES
Annabelle Alves
CNRS DR4
Edward Mitchell
ESRF
Fabian Zomer
LAL Orsay
lsquo
155
The project management is organized according to a matrix structure The ThomX facility has been
divided in five sub-systems Linac Ring and transfer lines X-ray line Laser and Fabry-Peacuterot cavity
There is a coordinator responsible for the design installation and operation of each sub-system All
these sub-systems involve various technologies which have been grouped in expertise domains
Mechanics and integration RF systems Magnets Diagnostics Power supplies and Pulsed elements
Vacuum Synchronisation Controls Beam Physics Alignment Cabling Nomenclature and
Infrastructures There is a coordinator for each one of these domains These coordinators will be in
charge of the activity harmonization within each technical field and of the maintenance during the
facility exploitation (or ldquoscientific phaserdquo)
The project leader his deputy and a project engineer are responsible for the project overall
coordination Fig 102 describes the project structure
Figure 102 The project structure
Project and Quality Engineer Technical
coordinator
AVermegraves Francois Wicek
LAL Orsay
Deputy Project leader
A Loulergue
Synchrotron SOLEIL
Project Leader
A Variola
LAL Orsay
Systems
LINAC RRouxLAL Orsay
Ring and Transfer Lines A Loulergue Synchrotron SOLEIL
X line M Jacquet LAL Orsay
Laser E Cormier CELIA Bordeaux
Fabry-Perot Cavity F Zomer LAL Orsay
Groups
Mechanics and Integration MLacroix LAL Orsay
Beam Physics ALoulergue Synchrotron SOLEIL
Alignment ALestrade Synchrotron SOLEIL
RF systems P Marchand Synchrotron SOLEIL
Magnets ME Couprie Synchrotron SOLEIL
Diagnostics L Cassinari Synchrotron SOLEIL
Infrastructures BPieyre MLanglet LAL Orsay
Control PGauron LAL Orsay
Vacuum C Preacutevost LAL Orsay
Power Supplies - Pulsed Magnets P Lebasque Synchrotron SOLEIL
Synchronisation N Delerue LAL Orsay
Cabling JN Cayla LAL Orsay
Nomenclature PCornebise LAL Orsay
lsquo
156
In the following Table the support structures are presented
Support Structures
Administration Communication Quality
Administration Brigitte Renard LAL
Orsay
Procurements Anngaeumllle Mardam
Beck LAL Orsay
EDMS Support Agnes Vermes LAL
Orsay
Secretary Chloe SevestreLAL Orsay
DR04 Support Edouard Dorion
Dominique Bony LAL Orsay
Marie-Pauline Gacoin
Synchrotron SOLEIL
Claus Habfast ESRF
Sandrine Agnes Vermes
LAL Orsay
Table 101
The ThomX collaboration needs an important support from all administrative services For this an
administrative cell has been set up which is headed by the person responsible for the administration the
purchasing office the project database management the secretariat and the contact with the regional
CNRS delegation support Last but not least a quality engineer plays a major role in the project
101 Partnersrsquo complementarity
The ThomX project requires a multidisciplinary environment given the complexity of the facility and of
the set-up operation Each partner contributes its competences to reach the final goal The ThomX
collaboration was organized so as to meet the requirements of a multicultural scientific environment
where different communities are called upon to share their know-how and their knowledge to develop a
common scientific program This was already the case ndash and still is the case ndash for the MightyLaser
project In this project LAL and CELIA teams have developed a fully integrated set-up composed of a
fibre laser and a high gain optical resonator which is presently installed at ATF (KEK Japan) There the
complementarity between the two teams was crucial LAL has a solid experience in developing optical
resonators in an accelerator environment while CELIA is among the leading laboratories in fibre
amplification technology This program aims at a progressive increase of the light beam power stored in
an optical cavity in order to maximize the Compton back-scattering rate The final goal of this RampD is
to demonstrate the feasibility of a γ factory based on Compton backscattering It started as a
demonstrator for Compton-based polarized positron sources needed for the next generation linear
colliders The interest shown by other scientific communities in this technology made it clear that a low
energy Compton source would be very useful in various research fields This conclusion led to the
launch of the ThomX collaboration
The ThomX CDR has already been published and the present TDR is about to be widely circulated For
the TDR writing it was mandatory to develop close connections between the various communities
lsquo
157
participating in the ThomX project in order to merge technological developments and usersrsquo
requirements in a single document All partners have contributed to the scientific case elaboration the
study of the facility feasibility and the definition of its main components
- LAL and SOLEIL contributed the description of the Compton backscattering characteristics and
carried out the study of the accelerators and the Fabry-Peacuterot resonator
- CELIA Bordeaux Lyon provided their expertise for the laser dimensioning
- THALES provided its experience in industrial components and sub-systems
- UMPC and ESRF enriched the scientific case
This task sharing results from the multidisciplinary approach to the ThomX project where half a dozen
institutes play an important role
The writing of this TDR provided a unique opportunity of collaborating for laboratories that belong to
different fields LAL specialized in fundamental high energy physics SOLEIL and CELIA in advanced
technology in laser and accelerators LC2RMF in social sciences and compact detectors ESRF in
medical sciences In the future the diversity of this set of teams will still increase with the participation
of the Neel Institute and the GIN-INSERM in Grenoble
After a prototyping phase ThomX will operate as a mini-synchrotron whose beam line can be accessed
not only by the facility partners but also by other communities Moreover the aim of the
industrialization phase is to provide the high technology scientific and industrial with compact and
reliable sources that can be operated with little or no accelerator and laser expertise This should open
the way to a large community of CBS source users that would benefit from the complementarity
between techniques developed at synchrotron sources and Compton back-scattering facilities Imaging
techniques biological tests on cells and animals metallurgy fast chemistry lithography are examples
of future possible applications of these unique instruments
102 ThomX larger framework
ThomX is one of the EQUIPEX projects submitted for approval to the French government by the P2IO
Consortium (Physique des deux Infinis et des Origines) of the Paris-Saclay Campus This Consortium is
composed of eight laboratories or institutes (CSNSM IAS IMNC IPNO IRFU LAL LLR LPT)
some of which are among the largest French laboratories All together their staffs comprise 2000
scientists engineers and technicians working in particle physics nuclear physics accelerator science
astroparticle physics astrophysics and their interfaces P2IO represents between 20 and 90 of the
national potential in the various sub-domains Although obviously not all of the eight laboratories
participate in each EQUIPEX project each project ndash including ThomX ndash is fully supported by P2IO and
will contribute to the very ambitious common technological platform that P2IO wants to create in the
coming years in the framework of the Paris-Saclay Campus initiative Once equipped with these
EQUIPEX facilities this campus will represent a unique opportunity for French researchers and
students
lsquo
158
References
11 Geoffrey A Krafft Gerd Priebe Compton Sources of Electromagnetic RadiationlowastReviews of Accelerator
Science and Technology Vol 3 (2010) 147ndash163
12 VBesson et al NIMA 608 (2009) S75-S77
13 T Akagi et al 2012 JINST 7 P01021
14 CDR ThomX LAL RT 0928 SOLEILSOU-RA-2678 editors AVariola FZomer ALoulergue
15 PRussbueldt et al Optics Express Vol 17 Issue 15 pp12230-12245 (2009)
16 IPupeza et al OPTICS LETTERS Vol 35 No 12 June 15 2010
17 M Y Shverdin et al 2009 OSAIPNRANLOSL 2009
18 CVaccarezza et al IPAC Conference proceedings New Orleans USA2012 TUOBB01
19 AVariola IPAC Conference proceedings San Sebastian Spain 2011 WEOAA01 1903-1905
21 S Doumlbert ldquoIntegration of the PHIN RF Gun into the CLIC Test Facilityrdquo Proceedings of EPAC 2006
Edinburgh Scotland 2006 MOPLS129
22 R Roux S Cavalier M Bernard G Bienvenu MJoreacuteBLeblondBMercier BMoutonCPrevost A
Variola ldquoPHIL a Test Beamline at LALrdquo Proceedings of EPAC08 Genoa Italy 2008 WEPP078
31 J Payet Beta Code CEA SACLAY
32 JB Murphy et al Longitudinal Wakefield for an Electron Moving on a Circular Orbit Vol 57
Part Acc 1997
33 K Bane M Sand The Short-Range Resistive Wall Wakefields SLAC-PUB 95-7074
34 E Forest Phys Tech Part Photon Beam
35 J Zang et al lsquoDipole fringe field effects in the Thom-X ringrsquo Proc IPAC13
36 R Nagaoka et al Recent Studies of Geometric impedance At SOLEIL PAC2006
37 Z Huang RD Ruth Phys Rev Lett 80976 (1998)
38 S K Mtingwa African Physical Review 2 (2008) 001
39 P Chen G Horton-Smith T Ohgaki AW Weidemann K Yokoya
NuclInstrumMethA355107-1101995
310 K Ohmi Phys Rev E 55 7550 (1997)
311 S Haifets D Teytelman Phys Rev Spec Top Acc and beams 8 064402 (2005)
312 E Weihreter Compact synchrotron light sources Series On Synchrotron Radiation Techniques
And Applications - Vol 3 World Scientific
313 P Lebasque et al Synchrotron SOLEIL lsquoEddy current septum magnets for booster injection and
extraction and storage ring injection at Synchrotron SOLEILrsquo EPAC 2006 Edinburgh Scotland
314 P Lebasque et al Synchrotron SOLEIL lsquoOptimisation of the coating thickness on the ceramic vacuum
chambers of SOLEIL storage ringrsquo EPAC 2006 Edinburgh Scotland
315 P Lebasque et al Synchrotron SOLEIL lsquoFour matched kicker systems for the SOLEIL storage ring
injection a full solid-state solution of pulsed power supplies working at high currentrsquo EPAC 2006
Edinburgh Scotland
316 F Marhauser E Weihreter (BESSY) ldquoImpedance Measurements of the HOM-Damped 500 MHz
Metrology Light Source Cavityrdquo BESSY Gmbh Germany 08 January 2007
317 M Langlois P Sanchez M Cornelis F Perez (ALBA) ldquoMeasurements on the RF cavity for the ALBA
storage ringrdquo Proceedings of EPAC08 Genoa Italy
318 R A Rimmer (Lawrence Bekeley Lab) et al ldquoDevelopment of a high-power RF cavity for the PEP-II B
factoryrdquo DE-AC03-76SF00098 (LBL)
lsquo
159
319 P Fernandes R Massarino R Parodi ATarditi (INFN Italy) A Massarotti (Sincrotrone Trieste Italy)
rdquoThe Design of the RF Cavities for ELETTRArdquo IEEE 1989
320 M Svandrlik A Fabris C Pasotti (Sincrotrone Trieste) rdquoImprovements in curing coupled bunch
instabilities at ELETTRA by mode shifting after the installation of the adjustable higher order mode frequency
shifter (HOMFS)rdquo IEEE 1998
321 W Bruns web site httpwwwgdfidlde
322 P Marchand et al Development of high RF power solid state amplifiers at SOLEIL Proceedings of the
IPAC2011 San Sebastian Spain p 376
323 C Pardine et al Status of the 476 MHz 50 kW solid state amplifier Proceedings of the IPAC2010
Kyoto Japan p 3972
41 YAG(CE) data sheet Saint Gobain Crystals httpwwwdetectorssaint-gobaincomYAG28Ce29aspx
42 SPI Suppliesreg YAGYAP Comparison Table
httpwww2spicomcatalogscintillsintillator_properties_comparison_tablehtml
43 K Nakamura et al (Particle Data Group) J Phys G 37 075021 (2010)
44 Wartski et al Detection of optical transition radiation and its application to beam diagnostics IEEE
TransNuclSci20544-5481973
45]Saturation of YAG Beam Profile Monitors Measured at ATF A Murokh J Rosenzweig V Yakimenko X
J Wang
46 Radiation Detection by Cerenkov Emission in Optical Fibers at TTF E Janata M Koumlrfer Tesla-Report
2000-27
47 CERN-2009-005 p273
48 CERN-2009-005 p272
51 J Bonis et al 2012 JINST 7 P01017 doi1010881748-0221701P01017
61 J Bonis et al Non-planar four-mirror optical cavity for high intensity g ray flux production by pulsed laser
beam Compton scattering off GeV-electrons JINST SISSA Vol 7 January 2012
62 R V Pound lsquolsquoElectronic Frequency Stabilization of Microwave Oscillatorsrsquorsquo Rev Sci Instrum 17 490ndash
505 (1946)
71 httpwwwjjxraydkhome
72 RW Alkire et al Nucl Inst and Meth A 350 (1994) 13-16
73 B Lengeler et al J Synchrotron Rad 6 (1999) 1153-1167
74 RL Owen et al J Synchrotron Rad 16 (2009) 143-151
75 MS Nogueira et al Radiation Protection Dosimetry 111 (2004) 105-110
76 M Krumrey et al AIP Conf Proc 705 (2004) 861-865
M Gerlach et al Metrologia 45 (2008) 577-585
77 httpwwwcanberracompdfProductsUltra-LEGe-SS-M3843pdf
78 K Sakaue et al Rev Sci Instrum 80 (2009) 123304
79 httpsaleshamamatsucomassetspdfparts_FF9892-31_-32_TMCP1039E03pdf
710 O Chubar et al Proc DIPAC (2001) CT09
711 D Paterson et al Opt Com 195 (2001) 79-84
712 M Mahesh RadioGraphics 21 (2001) 1033-1045
713 K Medjoubi et al JINST 6 C01080 (2011)
httpwwwvariancomusxrayproductsdigital_radiographypaxscan_productshtml
714 H Elleaume et al Physics in Medicine and Biology 47 (2002) 3369-3385
lsquo
160
715 Y Prezado et al Medical Physics 38 (2011) 1709
716 I Martiacutenez-Rovira and Y Prezadoa Medical Physics 38 (2011) 4430
717 L Samain Thesis Liege university (ULG) 2012
718 P Martinetto Thesis Joseph Fourier university (UJF) 2000
719 P Bleuet et al Analytical Chemistry 29 (2010) 518-527
720 P Bleuet et al Nature Materials 7 (2008) 468-472
81 httpwwwprofibuscomncdownloadspecifications-standardsdownloadsprofibus-standard-dp-
specificationdisplay
82 httpwwwtango-controlsorgDocumentstango-kernel
lsquo
161
Table of content
INTRODUCTION 2
1 The ThomX project 2
2 Scheme and options 3
3 General layout of the machine 4
4 Machine parameters 6
5 Summary of the TDR main technical points 8
Chapter 1 THE THOMX INTERACTION REGION 9
11 Electron beam and laser pulse characteristics in the interaction region 9
12 Expected flux and its characteristics 10
13 Position stability 11
14 Timing stability 12
15 Energy stability 13
Chapter 2 INJECTOR 14
21 Injector specifications 14
22 Technical description of the linac components 16
221 Photo‐injector 16
222 Solenoids 17
223 Laser 18
23 Acceleration section 20
231 The RF source 21
232 RF network 22
24 Beam dynamics 23
241 Nominal operation 23
242 Alternative operation modes 24
Chapter 3 TRANSFER LINE and RING 28
31 Transfer line 28
32 Extraction line 31
33 Storage ring 31
331 Optics and lattice 31
332 Non‐linear dynamics 35
333 Collective effects feedbacks 37
334 CBS and IBS effects 42
335 Scattering on the residual gas 46
336 Ion effects 47
337 Electron beam injection and extraction 48
338 Orbit corrections 50
341 Dipole magnetic design 53
342 Quadrupole magnetic design 54
343 Sextupole magnetic design 56
344 Corrector magnetic design 58
345 Injection dipole 59
lsquo
162
35 Power supplies 60
351 Pulsed magnets 60
36 Storage ring RF system 65
361 HOM impedance thresholds as regard to beam instabilities 66
362 Cures to the HOM induced instabilities 66
363 RF power source 70
364 Low level RF system and longitudinal feedback 71
365 RF equipment status 73
366 RF system conclusion 73
37 Transverse feedback 74
371 Specifications 74
372 Electronics 75
373 Kicker 76
38 Vacuum 77
381 Transfer line vacuum 77
382 Ring vacuum 78
Chapter 4 INJECTOR TRANSFER LINE AND RING DIAGNOSTICS 80
41 Charge measurement 80
411 Current transformers 80
412 Faraday cups 81
42 Transverse profile measurements 81
421 Diagnostics stations 81
43 Emittance measurement 84
44 Beam position measurement 85
441 Beam Position Monitors (BPM) 86
442 Orbit feedback system 90
443 Tune measurement and feedback 90
444 Energy measurement 90
45 Bunch length measurement 90
451 Cerenkov radiation 90
452 Synchrotron radiation 92
453 Transport of the Cerenkov and synchrotron radiations to the diagnostics hutch 93
454 Streak camera for the bunch length measurement 93
46 Bunch imaging using synchrotron radiation 94
47 Beam loss measurement 94
48 Beam dumps 94
Chapter 5 SYNCHRONIZATION 95
51 Overall scheme 95
52 Specifications 96
Chapter 6 OPTICAL SYSTEM DESIGN 97
61 Feedback system and results on a Fabry‐Perot cavity locking 97
611 Electron and FPC pulse synchronization 98
612 Laser parameters to be stabilized 99
lsquo
163
613 PDH Measurement technique 101
614 Actuators 103
615 The digital feedback system 107
62 High average power laser system and its coupling to a high gain FP cavity 110
621 The MightyLaser oscillator 111
623 Stretcher and compressor 115
624 ThomX development 115
63 Mechanical design 118
631 Optical cavity vacuum 123
Chapter 7 X‐RAY BEAM LINE AND EXPERIMENTS 124
Introduction 124
71 Available extracted X‐ray beam 124
72 Global view of the X‐ray lne 124
73 Chraracterization of the X‐ray beaugrave 126
731 Optical table 1 126
732 Beam transport 128
733 Optical table 2 128
74 Demonstration experiments 130
741 Experiments using the full X‐ry beam 130
742 Experiments using the central part of the beam 134
75 Summary 139
Chapter 8 CONTROL SYSTEM 140
Introduction 140
81 General principles 140
82 Technical considerations 141
83 The TANGO control system software 142
831 Software bus 142
832 A control system for accelerators 142
84 Control system architecture 143
841 Equipment to control 143
842 Inputsoutputs 144
843 Front‐end machine architecture PLCs and industrial PCs 145
85 IT infrastructure 146
Chapter 9 SAFETY AND RADIOPROTECTION 149
91 Introduction 149
92 PLC safety 149
93 Radiation monitoring 150
94 Radiation protection zoning 150
95 Access control 150
951 General statement 150
952 Particular requirements concerning the lsquorestricted areasrsquo 151
953 Access to the accelerator area and to the Igloo basement 151
954 Access to the X‐ray line and to the Compton room 151
lsquo
164
96 Beam dumps 152
97 Interlocks 152
98 Accelerator emergency shutdown 152
99 Control console 152
910 Evacuation of dangerous areas 152
911 Audio system and horns 152
912 Safety notices 153
913 Radiological monitoring 153
Chapter 10 MANAGEMENT OF THE THOMX PROJECT 154
Introduction 154
101 Partnersrsquo complementarity 156
102 ThomX larger framework 157
lsquo
1
ThomX
LAL ndashpub etc etc
EDITORS Alessandro Variola
Jacques Haissinski
Alexandre Loulergue
Fabian Zomer
[THOMX TECHNICAL DESIGN REPORT] [Tapez le reacutesumeacute du document ici Il srsquoagit geacuteneacuteralement drsquoune courte synthegravese du document Tapez le
reacutesumeacute du document ici Il srsquoagit geacuteneacuteralement drsquoune courte synthegravese du document]
lsquo
2
INTRODUCTION
1 The ThomX project
The fast performance evolution of laser and particle accelerator systems has opened the way to compact
radiation sources based on Compton backscattering (CBS) [11] While CBS-based sources do not
compete with the more classical synchrotron sources as far as the photon total flux and beam brightness
are concerned the CBS scheme has several attractive characteristics it provides the most efficient
photon energy boost so that hard X rays can be produced with a comparatively low energy electron
beam resulting in very compact and low cost devices This is extremely important in view of the
integration of such a radiation source in a lsquonon-laboratoryrsquo environment like a hospital or a museum
Another attractive feature of CBS sources resides in the energy versus emission angle correlation This
correlation makes it possible to use a simple setup with a diaphragm to obtain a quasi-monochromatic
beam with a bandwidth on the order of one percent Moreover the CBS scheme allows the tuning of the
photon energy range in several ways by varying the laser wavelength the beam energy or the collision
angle Since the laser polarization is conserved in the electron-photon collision a CBS-based machine
may provide polarized photons
The ThomX project is taking advantage of the preeminent French technology in accelerator and laser
fields The goal is to design and build a demonstrator with cutting edge performances compared to
similar projects either in operation or planned A flux between 1011
-1013
γs in the hard X-ray range is
expected and the photon energy tunability will provide a Compton edge that can be set between 50 and
90 keV
This project is a direct outcome of the sustained effort made by several laboratories to achieve a high
amplification of laser pulses by stacking them in a passive optical resonator These studies resulted in a
record amplification efficiency in the picosecond regime obtained in the PLIC experiment [12] They
are presently pursued within the MightyLaser program at the ATF accelerator facility at KEK (Tsukuba
Japan) where a vacuum-compatible four-mirror cavity is installed [13]
The ThomX collaboration gathers experts in accelerator physics coming from SOLEIL and LAL
experts in laser systems from CELIA (Bordeaux) in optical resonators from LAL and in X-ray line
instrumentation from the Neel Institute in Grenoble The ThomX community includes representatives
from the medicine field and from the cultural heritage field The ESRF and INSERM (Grenoble)
together with the CNRS C2RMF will design and perform a series of demonstration experiments to
check the assets of such a radiation source Furthermore the ThomX overall characteristics give to this
machine a strong industrial potential the latter will be investigated by an industrial partner THALES
Electron Devices which will play an active role in the ThomX collaboration
As a first step in this program a CDR has been written [14] It describes the basics of the Compton
backscattering effect and gives a first estimate of the ThomX performances based on simulations The
main technological solutions are also described in this document The machine lattice and the entire
machine layout are provided together with the design of the optical resonator and its integration in the
collisions region The main items which need RampD were identified A global integration plan and a
machine cost evaluation were also given
The present TDR provides a detailed description of all the technical sub-systems together with the
machine integration scheme It also provides the results of complete beam dynamics simulations which
take into account the ring characteristics and all the effects resulting from the collisions between the
lsquo
3
electrons stored in the ring and the photons stored in the optical cavity These simulations lead to
precise predictions concerning the X-ray yield The last part of the TDR deals with the project structure
and its management Finally the updated cost estimate is presented
2 Scheme and options
X-ray sources based on Compton backscattering can be designed with various configurations
depending on the main performance goals (average flux peak flux brilliance monochromaticityhellip)
Once this choice is made one may envisage either a high or a low electron-photon collision frequency
In the first case the electron beam is recirculated in a storage ring or in an ERL in CW mode and
continuously interacts with the laser Another option is to operate directly a high duty cycle or CW
super-conducting Linac Correlatively the laser system has to provide a high average power in the CW
locked mode [15] In this case to amplify the laser pulse energy one may envisage the use of a passive
optical resonator with a high gain [16] If a high peak instantaneous flux in single bunch or multi-
bunch pulsed mode is required the laser system will be a high power laser with a low repetition
frequency Recirculation of the laser beam is possible for a few passages by means of passive or
regenerative cavities [17] As far as the accelerator technology is concerned the warm Linac
technology can be made to work in burst mode [18] This allows the production of short low emittance
bunches to increase the source brilliance Because of the Compton energy-angle correlation this
scheme can also provide an excellent performance as far as monochromaticity is concerned by using a
diaphragm which selects a fraction of the X-ray beam
The ThomX source is designed to maximize the average X-ray flux Another goal of this project is to
provide a compact reliable and tunable source which can be operated in hospitals or in museums in a
user-friendly way
These constraints imposed the choice of a high collision rate scheme and of the warm RF technology
Thus the ThomX accelerator system is based on a 50 Hz warm S band (2998 MHz) linac whose energy
is tunable up to 70 MeV an injection line and a compact electron storage ring whose RF cavity will be
warm and will operate at 500 MHz The ring revolution frequency is 178 MHz The laser is a fully
integrated fibre laser which provides a high average energy Its power is amplified by stacking the
pulses in a high gain four-mirror Fabry-Peacuterot resonator with a 30000 finesse The laser and the optical
cavity repetition frequency is twice the ring revolution frequency ensuring an electron-photon collision
every e- bunch revolution
The basic operating scheme of ThomX is illustrated in Fig 1 A 1 nC electron bunch is produced by an
RF gun and then accelerated up to the ring injection energy by an S band section A transport line
ensures the bunch transfer from the linac exit to the injection section of the storage ring After injection
the electron bunch is recirculated for 20 ms in the ring The ring optics and the RF peak value are such
that the beam size is very small (in all directions) at the interaction point (IP)
In parallel a high power fibre laser produces light pulses of ~ 14divide28 microJ energy at ~ 12 eV These
pulses are stacked in a four mirror optical resonator at twice the ring revolution frequency The
expected energy gain per pulse is 103 divide10
4 corresponding to a beam power of ~ 70divide700 kW in the
optical cavity
The synchronization system allows a near head-on collision (the crossing angle is two degrees) between
the electrons and the laser pulse every revolution of the electron bunch These collisions produce hard
X rays which are backward emitted in a cone whose angular opening is ~1γ (as a result of a relativistic
effect)
lsquo
4
Fig1 Schematic diagram of the ThomX source
At the end of a storage period of 20 ms the electron bunch is extracted from the ring and dumped in a
dedicated beam dump to avoid the background noise that would be produced by uncontrolled particle
losses At the same time a new lsquofresh beamrsquo is injected in the machine
3 General layout of the machine
As previously mentioned the ThomX design takes into account a strong constraint coming from the
goal of a small size machine Nevertheless this design allows for the possible integration of two
collision regions [19] Figure 2 shows the general layout of the machine The linac includes an electron
RF photogun (1) followed by a section dedicated to diagnostics (2) the acceleration structure (3) and a
quadrupole triplet (4) to allow for a three-gradient measurement in the direct line dump (5) where a
diagnostic chamber will be located The injection line consists of four 45 degree dipoles to steer the
beam four quadrupoles and two chambers (6 7) for energy spread measurements and beam
characterization before injection The injection is performed by a septum (8) and a fast kicker (9)
Another kicker (10) is inserted in the same line to provide together with the septum a fast extraction
Once extracted the beam is transported towards a dedicated beam dump (11)
lsquo
5
Figure 2 ThomX layout
The ring design ensures a four-fold symmetry based on a double bend achromat (DBA) optics It
includes 8 dipoles 24 quadrupoles and 12 sextupoles for chromatic corrections The bunch storage
time is too short (20 ms) and the beam energy is too low for synchrotron damping to play a role in
regard to the Compton back-scattering recoil effect nor in regard to collective effects such as CSR IBS
and ion instabilities For this reason an lsquoELETTRArsquo type RF cavity (12) will be used not to restore
energy but to bunch the electrons and to perform a longitudinal feedback Transverse position
diagnostics and feedback will be provided by a strip-line and by BPMs A synchrotron radiation
chamber will be used for transverse and longitudinal optical diagnostics The required ultra-vacuum
will be maintained by ion pumps distributed along the ring and by two dedicated pumps in the Fabry-
Peacuterot resonator
In the ring lattice two of the achromatic lines have been shortened by removing all the quadrupoles (see
Fig 3) so that the Fabry-Peacuterot cavity (13) can be easily integrated between two dipoles By minimizing
the beta functions it was possible to locate the collision region (14) in such a short section
Furthermore this design provides the possibility of implementing a second interaction region in the
future
lsquo
6
Fig 3 Layout of the interaction region and of the optical cavity integration
4 Machine parameters
The ThomX machine is composed of four main systems the injector the storage ring the laser and the
Fabry-Peacuterot resonator As mentioned previously we foresee some flexibility in the machine operation
so that the X-ray energy can be varied For this all four systems are dimensioned to withstand an
operating energy up to 70 MeV Nevertheless to characterize the machine a baseline configuration has
been chosen at 50 MeV In Tables 1 2 3 and 4 the parameter values which are given below correspond
to this baseline configuration
Table 1
Injector
Charge 1 nC
Laser wavelength and pulse energy 266 nm 100 microJ
Gun Q and Rs 14400 49 MWm
Gun accelerating gradient 80 MVm 5 MW
Normalized rms emittance 44 π mm mrad
Energy spread 04
Bunch length 43
lsquo
7
Table 2
Table 3
Laser and FP cavity
Laser wavelength 1030 nm
Laser and FP cavity Frep 3568 MHz
Laser power 50 ndash 100 W
Laser pulse energy 14 ndash 28 microJ
Fabry-Perot pulse energy 28 mJ
Fabry-Perot pulse length (rms) 5 ps
FP cavity finesseGain 3000-30000 1000- 10000
FP waist 70 microm
Power circulating in the FP cavity ~ 007 ndash 07 MW
Ring
Energy 50 MeV (70 MeV possible)
Circumference 168 m
Crossing-Angle (full) 2 degrees
βxy IP 01 m
εxy just after injection 5 10-8
m
Bunch length just after injection (rms) 4 ps
Bunch length at the end of a 20 ms storage
cycle
50 ps (rms)
Beam current 1784 mA
RF frequency 500 MHz
Transverselongitudinal damping time 1 s 05 s
RF Voltage 300 kV
Revolution frequency 1784 MHz
σx IP (just after injection) 70 microm
Tune xy 317 174
Momentum compaction factor αc 00136
InitialFinal relative energy spread (with IBS
and Compton back-scattering)
0406
lsquo
8
Table 4
Source
Photon energy cut-off 46 keV (50 MeV) 90 keV ( 70 MeV)
Total Flux 1011
-1013
photons
Bandwidth (with diaphragm) 1 - 10
Divergence 10 mrad (1γ) without diaphragm 50 MeV
5 Summary of the TDR main technical points
After the publication of the CDR much work has been put in simulations design and technical choices
The goal of this TDR is to provide a reference document in view of the machine construction as well as
a more precise planning and a more precise costing
The first chapter deals with the machine performances and the tolerances to be met so that the collisions
between the electron bunch and the laser pulse do take place in the conditions specified in the CDR
Then the main systems are described and our technical choices are presented This last section is
subdivided as follows
1 Linac
2 Beam transfer and injection section ring
3 Laser and Fabry-Peacuterot cavity
4 X-ray user line
A special attention is given to the integration of each system and to the global integration of the ThomX
machine The lsquoOptical systemrsquo chapter includes a short report on recent results obtained in the laser and
Fabry-Peacuterot cavity RampD program both in the laboratory and on a beam line at KEK-ATF (Japan)
Special chapters are dedicated to diagnostics the control of the main systems and their synchronization
safety and radioprotection The last chapter presents the project management
lsquo
9
Chapter 1 THE THOMX INTERACTION REGION
11 Electron beam and laser pulse characteristics in the interaction region
The ThomX injection frequency has been fixed at 50 Hz (20 ms period) by taking into account the
Compton scattering recoil effect which continuously deteriorates the beam longitudinal emittance This
effect is all the more important as synchrotron damping is strongly suppressed since the beam energy is
low Because of this lack of damping the Compton scattering recoil effect and eventual beam
instabilities must be controlled by longitudinal and transverse feedback systems
We stress the fact that in ThomX the beam does not reach a permanent state during a storage cycle its
properties are continuously varying To describe the Compton interaction dynamics we define a set of
nominal beam characteristics which are those prevailing just after the injection of a fresh e- bunch
Simulations have been carried out to characterize the X-rays emitted during a single crossing of the
electron bunch and the photon pulse (stored in the optical cavity) as a function of the collision
geometric parameters
These nominal collision parameters are as follows
Parameter value unit
Crossing angle 2 degree
Electrons energy 50 MeV
Electrons energy spread 03
Number of electrons 625 109
Fabry Perot pulse energy 28 mJ
Repetition rate 1784 MHz
Laser pulse transverse dimension (rms) 35 microm rms
Electron bunch transverse dimension (rms) 70 microm rms
Laser pulse duration (rms) 5 ps rms
Electron bunch length at injection 4 ps rms
Table 11 Nominal collision parameters
To extend these results to other geometric configurations one has to take into account the parametric
dependence of the Compton collision rate The X-ray flux is the product of the Compton cross-section
and the luminosity It is proportional to the electron bunch intensity and to the laser pulse power The
geometry dependence is given by the luminosity formula which involves the geometric 6D size and the
crossing angle Assuming Gaussian bunches and neglecting the hourglass effect the ring luminosity is
given by the following expression
Eq 11
L = NeNγ f cosφ
2π
1
σ ye
2 +σ yγ2 σ xγ
2 +σ xe
2( ) cos2φ + σ ze
2 +σ zγ2( ) sin2φ
lsquo
10
where σeγxyz represent respectively the two transverse dimensions and the longitudinal one of the
electron bunch and of the laser pulse f is the collision frequency Ne and Nγ are the number of
electrons per bunch and photons per pulse Φ is half the crossing angle
The X-ray energy cut-off is higher for head-on collisions and for photons which are exactly back-
scattered In this case the photon energy gain is 4 γ2
This calculation provides a useful reference to assess the efficiency of other configurations but it
remains an approximation since exact results must take into account the beam evolution during a
storage cycle and should be obtained from a full revolution by revolution simulation
12 Expected flux and its characteristics
In nominal conditions the expected flux is 78 1012
X-rayss whether one uses Eq 11 or the monte-
carlo simulation code CAIN Figure 11a shows the energy spectrum of the scattered photons and its
cut-off at 45 keV As shown in Fig 12b most photons are emitted very close to the interaction region
axis within a small cone with a 2 mrad opening angle They may be selected by using a small
diaphragm The energy spectrum of the X-rays emitted in a cone of 225 mrad opening angle is shown
in Fig 12a with a FWHM energy spread of 25 keV
Figure 11 Results of simulations performed with CAIN using the parameters listed above a)
Energy spectrum b) Transverse size of the source c) Angular emission distribution d) Horizontal (x-
xrsquo) phase space of the scattered X-rays
lsquo
11
The spot size at the source is dominated by the electron bunch of 7 10-5
m as illustrated in Fig 11b
The dimansions are assumed to be the same in the horizontal and vertical plane The total divergence
of the source is around 15 mrad given by the electron beam energy (ie the 1γ parameter) with an rms
size of 4 mrad (Fig 11c) The emittance of the photon source shown in Fig 11d is 03 mm mrad The
energy-angle correlation and the collimation effect are illustrated in Fig 12 As expected the high-
energy X-ray flux is peaked with a 1γ opening angle The diaphragm which is assumed for this
simulation selects emission angles and thus photon energies resulting in a ~ 10 monochromatic
photon beam
Figure 12 Results of simulations performed with CAIN using the parameters listed above
Left panel Energy spectrum of photons emitted in a cone with an opening angle of 225 mrad
Right panel Number of photonsmradsup2s versus energy and angle (the color bar indicates the photon
number)
13 Position stability
To optimize the X-ray flux a precise transverse alignment must be acieved in the interaction region It
can be performed either by steering the electron beam or by adjusting the optical table position These
procedures will be monitored by means of a double BPM which is integrated in the Fabry-Peacuterot cavity
vacuum chamber exactly at the interaction point The resolution of this technique is a few microns A
simulation has been performed to calculate the flux reduction resulting from a possible mismatch
between the electron bunch orbit and the laser pulse trajectory Figure 13 shows the number of X-rays
produced per bunch crossing and per keV as a function of a transverse displacement of the optical
cavity axis with respect to the electron orbit from zero to 140 microns
lsquo
12
Fig13 Number of back-scattered photons and their energy as a function of a transverse alignment
error This figure clearly shows that the losses entailed by a displacement comparable to the BPM
sensitivity are negligible The color bar indicates the number of X-rays produced at each crossing
14 Timing stability
A precise synchronization between the electron bunch and the laser pulse at the interaction point is
another important requirement A timing mismatch introduces a longitudinal shift between the two
bunches at the IP This can be due to an error in the location of the Fabry-Perot cavity with respect to
the e- storage ring or to a jitter in the synchronization system Fig 14 shows the variation of the X-ray
flux as a function of a longitudinal displacement
The flux is not so much reduced as long as the jitter is less than 10 ps This number fixes the
synchronization tolerance and thus the performance of the longitudinal feedback
Fig 14 Flux and energy spectrum of back-scattered photons as a function of the relative
synchronization of the beams (longitudinal displacement)
lsquo
13
15 Energy stability
The nominal ThomX injection energy is 50 MeV Phase rotation in the RF source or phase mismatch
will generate a jitter in the injection energy Such a jitter entails synchrotron oscillations which will be
damped by the longitudinal feedback However before this feedback has stabilized the longitudinal
oscillations the collision conditions will not be nominal This situation has been simulated the results
are illustrated in Fig15 One can see that it is in the higher part of the X-ray energy spectrum that the
loss is more important
Fig 15 Flux and spectrum of the back-scattered photons as a function of the e- energy
lsquo
14
Chapter 2 INJECTOR
The most important components of the linac injector are the electron gun and the acceleration section
which brings the beam to the desired energy Other components are the laser used for the e- gun the
magnetic elements used to transport the beam the diagnostics the vacuum system the waveguides and
the radio-frequency (RF) source Each component is detailed in the following First of all the injector
specifications are given since they lead to the choice of some of the linac components
21 Injector specifications
The project goal namely to produce a high flux of X-rays of energy 50 keV leads to the following
specifications for the linac
Energy 50 MeV
Charge 1 nC
Number of bunches per RF pulse 1
Emittance (rms normalised) lt5 mm mrad
Energy spread rms lt1
Bunch length rms lt5 ps
Average current 50 nA
Repetition frequency 50 Hz
Table 21 Nominal linac parameters
First of all one notices that the low average current 50 nA does not justify the use of cold technology
(ie of a superconductor) for the acceleration structure But there is still a large choice of possibilities
for an RF structure operating at room temperature For economic reasons and because of the long
experience accumulated in the field we have chosen an RF structure operating at 3 GHz This
frequency is a European standard
Furthermore the above specifications led us to choose a photo-injector for the electron source since a
low emittance beam is required Again to minimize risks we chose to build the same gun as the one we
constructed for the CLIC lsquoTest Facility 3rsquo at CERN [21] This gun has been running successfully for 4
years To increase the current per bunch we will use a metallic magnesium photocathode which can
deliver more than 1 nC with a laser pulse energy of a few tens of a microJ at a wavelength of 260 nm The
gun is made of 2 12 copper cells magnetically coupled to the waveguide To get 1 nC with an
emittance lower than 5 mmmrad an accelerating field of 80 MVm is required which means an RF
power of 5 MW in a 3 micros pulse The electron beam energy output by the gun is around 5 MeV
We will use an industrial laser to extract the electrons from the cathode by photoelectric effect The
main specifications of this laser are the following ones Wavelength 260 nm energy per pulse 100 microJ
pulse duration around 5 ps It must be synchronized with the RF frequency and triggered at 50 Hz This
laser will be installed in an optical hutch located outside the concrete shielding which will enclose the
accelerator Therefore its beam must be transported with dielectric mirrors and lenses over a distance of
about 20 meters
Concerning the acceleration section SOLEIL a partner of the project will lend us a lsquoLILrsquo section This
section was made by LAL for the pre-injector of LEP at CERN It has been kept as a spare for the
SOLEIL linac The ThomX project will provide an other section in case SOLEIL needs one The
section length is about 46 m To reach a final energy of 50 MeV the energy gain in the section must be
lsquo
15
45 MeV The latter value implies an electrical field of 125 MVm and an RF power of about 10 MW
However the accelerator should reach higher energies in order to produce X-rays beyond 50 keV Since
the maximum targeted X-ray energy is 90 keV the linac design should allow a beam energy of 70
MeV corresponding to an RF power of 20 MW Thus taking into account the RF system efficiency the
specification for the total RF power is set at 35 MW
This specification lead us to choose a klystron producing 35 MW in a 45 micros pulse To feed the
klystron a modulator is needed that provides high voltage pulses of typically 240 kV through a high
voltage transformer soaking in an oil tank The two technologies which can be used for this component
are discussed in detail below
For the electron beam steering two coils and two magnetic correctors are required The coils are placed
around the gun Opposite sign currents circulate in these coils (-310 A 336 A) so that the magnetic field
vanishes at the cathode location Its amplitude is 023 T roughly 7 cm downstream from the cathode
Concerning the magnetic horizontalvertical correctors once a good mechanical alignment is achieved
experience has shown that beam trajectory errors are not larger than a few mm between the gun and the
acceleration section Hence the correctors must produce a beam deflection of up to 10 mrad For the
model operating at LAL [22] two bipolar power supplies of 10 A per corrector are adequate
For the diagnostics three different instruments will be installed between the RF gun and the
acceleration section An Integated Current Transformer (ICT) will measure the charge per bunch
Beyond this device a beam position monitor (BPM) will measure the transverse positions (x y) of the
electron beam We will use a BPM based on striplines which was designed for SOLEIL The third item
comprises a vacuum chamber equipped with an arm which can be translated by compressed air in order
to introduce a YAG screen to intercept the beam The YAG crystal fluoresces when ionized by the e-
beam The emitted light shows the beam transverse shape A short optical transport line and a CCD
camera allow the measurement of the transverse dimensions LAL designed and constructed the
diagnostics chamber It has already been successfully tested in the PHIL facility A drawing showing
the linac layout is shown in Fig 21
lsquo
16
Figure 21 Linac layout In the upper figure the different components of the LINAC are illustrated
starting form the gun and subsequently passing trough the stripline the ICT for current measurements
the laser introduction chamber the diagnostics station the steerers and the accelerating section
22 Technical description of the linac components
221 Photo-injector
It is a copy of a gun built by LAL for the CLIC Test Facility 3 of CERN the so-called lsquoprobe beam
gunrsquo[22] Simulations have shown that the performances of this gun fulfill the ThomX specifications
We are thus benefiting from all the technical drawings which already exist and all the fabrication
processes are mastered by LAL from the machining to the brazing This RF gun is comprised of 2 12
cells The output cell is magnetically coupled and the other end is short-circuited The main parameters
are given in Table 22
Frequency 299855 MHz
Rs 50 MΩm
Q 14500
β 1
E 80 MVm
PRF 5 MW
Table 22 RF characteristics of the photo-injector at 30degC in vacuum
The photo-cathode will be made of magnesium It has the big advantage of producing the specified
charge without requiring an in-situ photo-cathode preparation chamber which is a heavy and expensive
lsquo
17
installation Preliminary tests performed at LAL have demonstrated the possibility to obtain good
quantum efficiency with such cathodes
The main difficulty expected in operating this device is the laser cleaning of the photocathode surface
when polluted by an exposure to air To perform this cleaning a laser intensity of 160 microJmm2 is
required together with an RF electric field of at least 50 MVm The magnesium disk holder is not
designed yet it could be press fitted screwed or brazed
222 Solenoids
The gun is equipped with two solenoids one is the focusing coil while the other is the bucking coil
which cancels the magnetic field at the photo-cathode location They have been modeled by usning
POISSON A schematic is shown in Fig 22
Figure 22 Schematic of the gun with its coils
Dimensions
For both solenoids the inner diameter is 160 mm and the outer one is 500 mm The copper part is 30
mm long for the bucking coil and 56 mm for the focusing coil
The conductor used for these magnets is a standard one with a 6x6 mm2 square shape It is cooled by
water circulating in a 4 mm diameter channel One can easily calculate the current the resistance the
dissipated power and the temperature rise assuming that the cooling water is supplied at 20degC and 8
bars Values are provided for each lsquopancakersquo since the conductors are wound by columns of two The
bucking coil Bbc has two pancakes and the focusing coil Bfoc four
Bbc Bfoc units
Number of coils 84 168 MHz
Number of pancakes 2 4
Current -274 326 A
P 23 33 kW
R 306 31 mΩ
Flow 164 164 lmn
ΔT +20 +28 degC
Table 23 Solenoid electric and cooling parameters lsquoNumber of coilsrsquo refers to one pancake
Two power supplies are required one for Bbc (400A 30 V) and one for Bfoc (400A 60 V)
lsquo
18
223 Laser
The laser extracts a bunch of electrons from the photo-cathode surface by photo-electric effect Table
24 specifies its characteristics
Parameter value unit
l 260‐270 nm
Emax 100 plusmn 05 microJ
Beam transverse quality M2 M2 lt 11
Gaussian Pulse width 5 ps
Transverse profile square
Pointing stability 10 of beam diameter
Oscillator frequency 74963750 externally triggered with jitter lt 1 ps MHz
Amplifier repletion rate 50 externally triggered Hz
Table 24 Laser specifications for the RF gun photocathode
The laser architecture is shown in Figure 23
Figure 23 Architecture of the laser proposed by industry
An oscillator provides a signal at the specified frequency It can be either a fiber or a TiSaphire laser
Then the pulse is stretched amplified (with one or two amplifiers) recompressed and converted in the
UV range
The laser will be installed on an optical table in a hutch whose temperature and hygrometry will be
regulated
224 Laser injection chamber
The laser injection chamber is located at a distance of 8011 mm downstream of the photo-cathode It
contains a metallic aluminum mirror placed 30 mm away from the accelerator axis It provides an 80
reflectivity The angle of incidence of the laser beam on the photocathode is 214deg This mirror is fixed
To adjust the laser beam position on the photo-cathode and its incident angle we will use a motorized
mirror with a remote control placed on an optical table just beside the laser injection chamber
The size of this optical table will be 1x1 m2 On it there will be
Oscillator Stretcher Preamplifier Amplifier Compressor
nJ microJ
mJ
Frequency
shifter
Clock
Syncro
Line syncro
Shaper
lsquo
19
- 2 dielectric mirrors
- 2 beam splitters
- 1 UV photo-diode to monitor the laser intensity
- 1 CCD camera to observe the laser spot on a screen (in paper or lanex) facing the photo-cathode
- An optical beam expander to adjust the laser spot size on the photo-cathode
This is the minimum number of optical items that will be put on the table some changes may be made
depending on the very final geometry of the accelerator and laser transport lines The set-up design can
be finalized during the last stage of the accelerator construction since it involves standard components
only
225 Beam steering coils
The e- beam trajectory will be adjusted by sets of two steering coils which produce dipolar magnetic
fields over short distances along the X and the Y axes One such device will be located at the gun exit
and the other about 800 mm downstream Only very small position and angle errors must be corrected
Figure 24 shows the LAL design for such a coil Experience with the PHIL photo-injector at LAL [22]
showed that an integral of Bmiddotdl of 52 G m is sufficient The latter figure has been validated by
calculations made by a private company With a current of 5 A a Bmiddotdl of 58 G m was obtained that
results in a 100 mrad deviation of a 5 MeV beam Thus a 100 mm lateral displacement of the beam can
be produced at the entrance of the acceleration section
Figure 24 Schematic of the beam steering coils Dimensions are in mm
All together the steering coils need 4 bipolar DC power supplies Their maximum current is 10 A
lsquo
20
23 Acceleration section
The acceleration section is shown in Fig 25 Its characteristics are given in Table 25
Figure 25 Picture of the SOLEIL LIL acceleration section
Length 45 m
Number of cells mode 135 2π3
Frequency 299855 at 30degC MHz
Rs 63divide74 MΩ
Q 14800
Attenuation 74 dB
Energy gain 15 MW 60 MeV
τ 135 micros
Ez max 182 MVm
Table 25 Characteristics of the acceleration section
In a second stage of the ThomX project a call of tender will be issued for a new acceleration section
that should be shorter in order to make the machine more compact
lsquo
21
231 The RF source
Figure 26 shows the RF source schematic
Figure 26 Schematic of the RF source
The RF timing and its synchronization are described in chapter 5 A clock signal is sent to the linac
which uses it to trigger a 3 GHz synthesizer Its output is fed to a preamplifier which brings its power to
350 W The preamplifier is triggered at 50 Hz Then the 3 GHz signal is amplified by a klystron whose
gain can reach 53 dB to produce a 35 MW RF wave as required by ThomX This power is distributed
via a waveguide network which is described below
To drive the klystron one needs a modulator delivering a pulsed high voltage (HV) whose
specifications are given in Table 26 The klystron cathode must be heated by a power supply that
provides a current rising slowly from 0 to 25 A and a voltage rising from 0 to 30 A in 20 minutes A
precision of 1 on the current value is required The klystron is generally equipped with focusing coils
and their power supply whose standard parameters are 40 A 120 V
Repetition rate 50 Hz
Plateau length 99 gt 3 micros
Plateau ripples lt plusmn 05
Pulse duration at 50 6 micros
Rise time 10 divide90 lt 05 micros
Fall time 90 divide10 lt 1 micros
Pulse to pulse stability lt 05
Voltage range 0divide300 kV
Peak current 285 kV 300 Hz
Table 26 Modulator specifications
The classical and well-proven modulator technology relies on the high voltage (40 kV) loading of a
delay line made of inductances and capacitances The delay line is switched by a thyratron A new
technology is presently emerging based on solid-state amplifiers Each amplifier produces a 100 V
lsquo
22
pulse Thus to produce a 40 kV pulse roughly 400 amplifiers are put in a rack and their output pulses
are added The performances of both technologies are similar
232 RF network
The RF power is transported and distributed to the electron gun and the acceleration section by a
waveguide network shown in Fig 27
Figure 27 Schematic of the RF waveguide network
The WR 284 waveguide network is pressurized at two bars with SF6 between ceramic windows at the
klystron output and at the acceleration structure entrance A 5 dB bidirectional coupler splits the power
in two channels In the branch which runs toward the electron gun there is a 10 dB attenuator and a
circulator to protect the klystron from the RF power reflected by the electron gun In the branch running
toward the acceleration structure there is a 10 dB attenuator and a phase shifter to adjust the relative
phase between the gun and the acceleration section Various RF loads (five in total) ensure the system
safety by absorbing of RF power of the reflected waves
lsquo
23
The network also includes three couplers to measure the incident and the reflected RF power One is
placed at the klystron power exit one is placed at the gun entrance and the last one is placed at the
acceleration section entrance The RF gun is equipped with a loop that measures the stored RF energy
All of these loops used for measurements are weakly coupled at -70 dB The signals are measured with
crystals or C boxes
24 Beam dynamics
241 Nominal operation
The beam dynamics was simulated with the PAMELA tracking code for the ThomX nominal
parameters ie a bunch charge of 1 nC at 50 MeV The 50 MeV can be obtained with an electric field
of 80 MVm in the RF gun and 125 MVm in the acceleration section It is clear that a different set of
field values can lead to the total of 50 MeV In particular one may wish to operate the gun at a higher
field in order to improve the beam quality We will operate the linac with this possible improvement in
view
The beam will be accelerated on crest Thus the coil magnetic fields are the only parameters savailable
to minimize the transveres and the longitudinal emittances Simulations have shown that for precise
magnetic field values (see Table 27) the e- beam emittances are optimized as illustrated in Fig 28
20
15
10
05
00
σxy
(m
m)
6005004003002001000
z (cm)
20
15
10
5
0
em
it (
mm
mra
d)
6005004003002001000
z (cm)
20
15
10
05
00
ΔE
E (
)
6005004003002001000
z (cm)
46
44
42
40
38
36
34
32
30
σz (p
s)
6005004003002001000z (cm)
lsquo
24
Figure 28 Beam characteristics (rms values) as a function of the distance along the accelerator (z
coordinate) Upper left panel Transverse size upper right panel normalized emittance lower left
panel Energy spread lower right panel Bunch length
The beam parameter values at the linac end are given in Table 27
Beam parameters value unit
Energy 50 MeV
Beam size 17 mm
Normalised Emittance 44 π mm mrad
Energy spread 04
Bunch length 43 ps
Table 27 Beam parameter rms values at the linac end
The accelerator parameters which should ensure the above beam characteristics are given in Table 28
RF structure parameters value unit
Ez in the gun 80 MVm
PRF in the gun 5 MW
Ez in the section 142 MVm
PRF in the section 14 MW
Solenoid parameters value unit
Bz max 2500 G
Ibc -274 A
Ifoc 326 A
Laser parameters value unit
Energy impinging on a Mg photocathode 10 microJ
Square transverse profile diameter 2 mm
Variation of the intensity from axis to border 14
Table 28 Summary of the linac parameters
242 Alternative operation modes
Concerning the injector other operation modes can be envisaged One at least will occur namely an
operation at 70 MeV in order to extend the X-ray energy range up to 90 keV Such an option has
attractive applications In this mode the electron beam performances should remain exactly the same as
the ones prevailing at 50 MeV (cf Table 27) For the accelerator parameters the only change is the RF
power required for the acceleration section it must reach 195 MW instead of 14 MW at 50 MeV
The operator can also be tempted to change the RF phase The one used in the simulations gives the
minimum emittance A phase shift of -10deg of the RF gun with respect to the laser could lead to an
energy spread reduction by a factor 2 at the expense of an emittance degradation Whatever the goals
the linac operator must be aware of the high sensitivity of the beam parameters to the RF phase This is
illustrated in Fig 29 An error of 5deg in the acceleration section phase results in almost a doubling of the
energy spread
lsquo
25
Figure 29 Energy spread at the linac end as a function of the RF phase in the acceleration section all
other parameters keeping their nominal values
The RF electrical field in the gun also plays an important role in the beam performances The results of
simulations of the emittance variation as a function of the gun electric field are presented in Fig 210 If
this field reaches 120 MVm the emittance is reduced by 20 with respect to the 80 MVm case
However the possibility to operate the gun at such a high gradient can only be assessed experimentally
Figure 210 Injector beam emittance as a function of the electric field in the electron gun (Gaussian
laser beam 1 nC e- bunches)
The injector performances as a function of the bunch charge are summarized in Table 29 Three modes
will be available in order to reduce the effect of collective instabilities at injection
Parameters Mode 1 Mode 2 Mode 3 Unit
Q 1 05 01 nC
σxy 165 15 08 mm
εxy 47 33 11 π mmmrad
σz 44 4 33 ps
σEE 04 03 02
Table 29 Linac beam performances (rms values) as a function of the bunch electric charge
As one can see in the above Table reducing the bunch charge by a factor 10 leads to an emittance
reduction by a factor 4 Although the X-ray flux is reduced the source brightness ie the ratio of the
flux to the divergence is improved by a factor 16 As a consequence a higher number of X-rays can be
20
18
16
14
12
10
08
06
04
02
δΕ
Ε (
)
151050-5-10
φ (deg)
10
9
8
7
6
5
ε (πmmmrad)
1401201008060
E0 (MVm)
lsquo
26
transported to a sample (number per unit area) Whatever the eventual benefits to the X-ray production
the advantage for the accelerator to operate at a lower charge per bunch is clear the reliability of the
accelerator is much enhanced the laser energy requirement is reduced and it becomes also possible to
use a copper photo-cathode instead of a magnesium one easier to handle But the main advantage lies
in the ring operation Indeed a 1 nC bunch whose length is only 4 ps induce a strong wakefield
stemming from the interaction between the beam and the vacuum pipe Such a wakefield perturbs the
beam and may cause oscillations of the bunch centroid possibly instabilities and thus a reduction of the
X-ray production rate
25 Photo-injector and acceleration section vacuum
Simulations of the photo-injector vacuum have been performed assuming a pressure of 3 10-8
Pa at the
cathode location and requiring a vacuum time constant (VS) that ensures an easy gun conditioning This
constraint must be respected to maintain a 10-7
Pa pressure at the acceleration section entrance where the
system will be baked to obtain an outgassing rate τ = 5x10-10 Pa m s-1
(N2)
Pressure in the line of injection
The use of a semi-conductor cathode (Cs2Te) puts severe constraints on the vacuum and requires a preparation
and a transfer chamber To avoid these complications some tests have been made on a metallic cathode (Mg)
The first attempt ran into difficulties (it took 12 h to reach a gradient of ~ 60 MV m-1
) we observed pressure
rises and sparks In presence of only the RF power we observed hydrogen CO and CH4 outgassing (as the
main components) A slight reduction in the quantity of water and oxygen was noticed due to chemical re-
composition When the laser beam was introduced to extract electrons a pressure increase by a factor of ten
was observed The 24th
25th
and the 26th
peaks also significantly increased for the HxCy compounds but no
Mg was measured
lsquo
27
Gas analyzes during the conditioning of cathode Mg
On the basis of simulations and experimental results we identified the need for the following items
‐ 2 ion pumps 40 l s-1
(upstream of the cathode amp injector) with their baking systems and power
supplies
‐ 2 ion pumps 55 l s-1
(for the RF network) with their baking systems and power supplies
‐ 1 ion pump 150 l s-1
(for the laser introduction chamber) with its baking system and power supply
‐ 1 ion pump 300 l s-1
(for the acceleration section) with its baking systems and power supply
‐ 1 gate valve manually actuated DN 100 (injector- upstream) and a baking valve
‐ 1 gate valve electro-pneumatically actuated DN 63 (injector - downstream) and a baking valve
‐ 1 gate valve electro-pneumatically actuated DN 40 (injector- downstream) and a baking valve
‐ 3 Pirani gauges (before the injector injector laser introduction acceleration section)
‐ 3 cold cathode gauges (after the injector injector laser introduction acceleration section)
‐ 2 power supply gauges
‐ 1 residual gas analyzer (RGA)
‐ 1 turbo pump (DN 63 ~ 80 l s-1
)
‐ 1 rotary vane pump (~ 12 m3 h
-1)
‐ 4 angle valves manually actuated for pre-pumping DN 63
‐ 2 T with RF grids
lsquo
28
Chapter 3 TRANSFER LINE and RING
31 Transfer line
The transfer line (TL) has to transport the beam from the linac to the ring and to match the optics of
these two devices In addition a set of beam diagnostics needed for an efficient injection in the ring are
localized along the line as sketched in Fig 31 The transfer line main characteristics are summarized in
Table 31
Figure 31 Layout of the injection and extraction transfer lines
A particular attention is paid to ensure a versatile optics that allows the matching of different linac
beams to different ring optics Simulations have been carried out using the BETA code [31] The
transfer line optics is optimized in order to accommodate some emittance degradation which may arise
from collective effects
Screen station + dump
Linac emit + pulse length diags
Screen + Scraper
Linac energy diags
Screen station
TL emit diags
Extraction line dump
lsquo
29
Figure 32 Transfer line optics including the ring The vertical line indicates the ring entrance point
The nominal transfer line optics is shown in Fig 32 from the linac to the ring together with the ring The
nominal input bunch Twiss parameters are x = z = 34 x = z = 42 for Q = 1 nC bunch length = 4
ps (rms) and an optimized normalized emittance of about 5 πmmmrad The strong bend produced by the four
dipoles of the transfer line increases the risk of emittance degradation under collective effects but simulations
show that the emittance increase is only of about 20 including coherent synchrotron radiation (CSR [2]) and
3D space charge effects (see Fig 33)
Figure 33 Emittance increase due to chromatic (cont lines) CSR (dashed) and space charge (dot dashed)
effects for 1 nC and 4 ps (rms) bunches at 50 MeV along the TL and the first revolution in the ring
The magnetic elements are identical to those of the ring
lsquo
30
Parameter value unit
Length 14 m
Nominal energy 50 MeV
Beta max x z 70 40 m
Maximum dispersion 1 m
Dipole numberfamily 4 1
Quadrupole number 7
Biplane corrector number 5
Table 31 Transfer line parameters and components
Besides ensuring the matching between the linac and the ring the transfer line must provide the measurement
of the beam emittances the beam energy and the energy spread Care has been taken to locate the diagnostic
stations in a way that ensures accurate measurements of these important parameters
The emittance measurements will be done by means of the lsquothree-gradientrsquo scheme (The pepper-pots
alternative is also foreseen) For instance with an adequate refocusing that produces a double waist (vertical
and horizontal) at the screen it is possible to scan both planes at the same time by varying only one
quadrupole gradient (see Fig 34) Other scan variations are also possible The following figures show the
optics retuning as well as the beam spot size variation at the screens of both stations the one at the linac end
and the one downstream of the U-turn
Figure 34 Optics retuning and beam spot size scan as observed on the screen (simulation)
The electron energy and its spread are measured in between the dipoles by a dedicated optics Note that it is
also possible to retrieve the bunch length by measuring the energy spread as a function of the linac phase
(three-phase scheme)
lsquo
31
32 Extraction line
To dump the residual beam at the end of the storage time (typically from 20 to 50 ms) an extraction line
composed of one small dipole and two quadrupoles is also foreseen This line will allow the measurement of
the beam properties after it has circulated in the ring The injectionextraction devices are symmetrically
implemented in the ring injection section
33 Storage ring
331 Optics and lattice
Thom-X must produce a high X-ray flux of 1011
divide1013
phs in a stable way The flux depends on the
characteristics of the electron bunch and of the laser pulse Assuming a 1 nC electron bunch charge at
50 MeV and a 20 to 30 mJ laser pulse ( 124 microm) the optics constraints at the interaction point (IP)
are as follows
- an rms size of the electron beam less than or equal to 70 microm in the transverse plane requiring
low beta functions of 01 m with an rms geometric emittance of 50 nmrad injected from the
linac (5 mmmrad normalized)
- a vanishing dispersion at the interaction point in order to avoid a beam size increase resulting
from an e- energy spread increase as well as to avoid potential synchro-betatron coupling
resonances
- a bunch length less than 20-30 ps to ensure a good overlap with the laser pulse (the crossing
angle will be 2deg) and long enough to reduce collective effects stemming from the pipe
impedance CSR bunch internal scattering etc
Besides these specific IP constraints there are two general layout constraints namely
- a ring circumference as short as possible with enough straight sections to accommodate the
interaction region the RF the injection and possible other equipment as RF and transverse
feedback systems
- the need for some flexibility as far as the optics and the working point are concerned
The ring layout and its optics are based on a two-fold symmetric lattice with four achromatic arcs and
four straight sections two short ones (02 m) and two long ones (16 m) This layout preserves the ring
compactness and provides an adequate interaction region in one of the two straight sections between
two adjacent dipoles (cf Fig 35) The main ring characteristics are summarized in Tables 32 and 33
lsquo
32
Figure 35 Ring layout
The laser pulse frequency is the harmonic 2 of the e- bunch revolution frequency in the ring ie 357
MHz This leaves open the possibility of storing two bunches in the ring in order to increase the
Compton flux but at present this operation mode is not taken into account because of injection
constraints The optical cavity mirrors are located outside the ring vacuum chamber thus enabling a
mirror substitution without affecting the ring In addition the extraction of the X-rays can be performed
closer to their source than in the case where the optical cavity is inserted in a long straight section
The two long free straight sections accommodate the injectionextraction devices for one and the RF
and feedbacks for the other
The ring is composed of 8 dipoles 24 quadrupoles and 12 sextupoles Its circumference is 168 m long
the revolution frequency is 1785 MHz The RF frequency is set at 500 MHz ie at the 28th
harmonic
of the ring revolution frequency The nominal optical functions have been optimized with the BETA
code [31] They are plotted in Fig 36 The dispersion vanishes in the long and in the short straight
sections (RF injection and interaction regions) The beta functions decrease down to 01 m at the IP to
provide a 70 microm focus assuming that the injected beam has an emittance of 50 nmrad (5 mmmrad
normalized) in both the vertical and the horizontal planes
RF and feedback section
Injection extraction section
IP 1
Interaction Point (IP)
lsquo
33
Ring parameters value unit
Circumference 1680 m
Nominal energy 50 MeV
Betatron tunes nx nz 317 174
Beta max xz 11 11 m
Dispersion max 08 m
Beta xz dispersion IP 01 01 0 m
Momentum Compaction Factor 0014
RF frequency 500 MHz
RF harmonic 28
RF voltage 300 kV
Synchrotron frequency 360 kHz
Period Revolution frequency 56 1784 nsMHz
Natural chromaticities -32 -82
Dipole nb family 8 1
Quadrupole nb families 24 6
Sextupole nb families 12 3
Corrector nb families 12 12
Table 32 Ring parameters
Max field 70 MeV Bore length Gap Diameter
Dipole 07 T 2765 mm
(R=352 mm)
40 mm
Quadrupole 5 Tm 150 mm 41 mm
Sextupole 15 Tm2 60 mm 42 mm
Correctors 510-4
Tm
Table 33 Main magnet characteristics
Taking into account the constraints imposed by the optical functions the chromaticity and the dynamic
aperture the operating point was chosen with tunes νx=317 and νz=174 There is still enough
flexibility to vary these tunes by plusmn 02 without spoiling the main ring performances The momentum
compaction factor can also be changed over a large range in order to scan the best working point
regarding the beam dynamics and the X-ray flux
Figure 36 Nominal beta functions and dispersion over one super-period
lsquo
34
The RF acceptance for the nominal momentum compaction factor (MCF) and a larger one including the
non-linear MCF and the vacuum pipe limit are plotted in Fig 37
Figure 37 RF acceptance Left panel nominal lattice Right panel for a MCF twice as large as the
nominal one VRF= 300 kV
RF acceptance nominal r56 = - 02 m r56 = - 04 m
Linear (RF bucket) 10 10
Non linear -32 +18 - 68 +4
Pipe limit 25 25
Table 34 RF acceptance for two different values of the momentum compaction factor
The two following plots (Fig 38) show (i) the vacuum chamber occupation by the beam at injection
when the emittance is 50 nmrad and the energy dispersion is 03 and (ii) at the end of the storage
cycle when the emittance reaches 300 nmrad and the energy dispersion reaches 06 (see section
438) The orbit components (misalignment errors) are not included since they are negligible once
corrected The choice of the pipe aperture (40 mm in H by 28 mm in V full aperture) takes into account
several constraints the beam pipe occupation the magnet bore radius the vacuum and the coherent
synchrotron shielding [32]
b a
lsquo
35
Figure 38 Transverse occupation (3 rms) of the vacuum chamber at injection (left) and at the end of
the storage cycle (right)
332 Non-linear dynamics
The natural horizontal and vertical chromaticities are respectively -32 and -68 Chromaticity
corrections are achieved by means of 12 sextupoles (3 families) located in the dispersive sections
Without performing a specific optimization the dynamic aperture is large enough about 50 times the
rms transverse size of the injected beam (see Fig 39) the quadrupole fringe fields being taken into
account Furthermore this dynamic aperture remains about the same for off-momentum particles (up to
plusmn3) compared to particles having the nominal energy The fringe field of the dipole magnets
introduces high order non-linear components Since the Thom-X dipoles are short with a large gap
their magnetic design (see section 34) has been carefully worked out in order to maintain the dynamic
aperture
Pole number 6 8 10 12 18 20 28 30
Inner dipoles +10 0 +3
Outer dipoles -16 -1 -6
Quadrupoles +10 +5 -5
Sextupoles +10 -10
Table 34 List of multipole tolerances for each kind of magnets expressed in 10-4
relative field variation
at 18 mm radial position
Figures 39 and 310 show the change of the dynamic aperture with and without considering the
multipole (the fringe field of the quadrupoles is also taken into account in these plots) These
Dynamical Apertures (DA) have been calculated with two different codes BETA and TRACYIII They
lsquo
36
both give very similar results In addition the TRACYIII code dedicated to long symplectic tracking
performs Frequency Map Analysis (FMA) giving more detailed information concerning the particle
diffusion process
Figure 39 On-momentum DA and FMA for the nominal ThomX lattice Beta code left and TracyIII
code (right) sextupoles only (top) and including magnet multipole tolerances (bottom)
lsquo
37
Figure 310 Off-momentum DA and FMA for the nominal ThomX parameters Beta code left and
TracyIII code (right) sextupoles only (top) and including magnet multipole tolerances (bottom)
333 Collective effects feedbacks
At low energy and thus low beam rigidity the electron bunch is sensitive to electromagnetic self-
perturbations that act revolution after revolution in the ring Several instability sources can be
identified
‐ Interaction with surrounding metallic items such as the resistive walls (RW) the RF cavity and
various discontinuities such as bellows BPMs and flanges HOMs etc
‐ Space charge forces
‐ Coherent synchrotron radiation (CSR)
These interactions affect the bunch energy and the bunch longitudinal time-profile leading to
distortions emittance growth bunch oscillations or even above some threshold instabilities and beam
losses Similar interactions may also affect the transverse beam dynamics but to a lower extent The
two relevant parameters for the instability threshold are the bunch charge and its length The strength of
the interaction is proportional to the bunch charge and varies inversely with the bunch length
The two plots in Fig 311 show the longitudinal wakefield over one revolution at 50 MeV generated by
a 1 nC Gaussian bunch with an rms length of 12 and 6 mm (4 and 20 ps) Five different contributions
are compared in this figure
‐ the shielded CSR following the Murphy and Skrinsky [32] formulation with a beam pipe full height
of 40 mm a dipole field of 05 T and a curvature of 0352 m
‐ the longitudinal space charge (LSC)
‐ the resistive wall (RW) following the K Banes and M Sands formulation [33] with a full beam
pipe height of 40 mm and an electrical conductivity of 14 106
‐ the vacuum vessel broadband component including 12 BPM 12 flanges and 12 bellows based on
the synchrotron SOLEIL type This potential has been calculated by means of the HFSS finite
elements code [34]
‐ the 500 MHz ELETTRA type cavity broadband component This potential has been calculated by
using the HFSS finite element code
They exhibit a very strong CSR wake field (up to few per mil) with the shorter bunch case
Figu
re
311
Lon
gitu
dina
l
wak
e
field
over
one
lsquo
38
turn generated by a 1 nC Gaussian bunch at 50 MeV for a bunch length of 4 ps (left panel) and 20 ps
(right panel)
The ThomX low energy operation (50 MeV) practically suppresses the natural damping effect of the
synchrotron radiation and complicates the bunch stabilization Each kind of perturbation grows
irreversibly implying that feedback systems in the three planes are essential to cope with the induced
oscillations of the bunch center-of-mass In addition to these collective effects the unavoidable
injection jitters have also to be taken into account To calculate these effects 6D tracking simulations
that include the longitudinal impedance as well as the large mismatch between the linac and the ring
optics in the longitudinal phase space have been performed (see Fig 312)
Large longitudinal oscillations
without feedback at injection
induced by collective effects
Longitudinal oscillations damped by the
RF phase feedback system operating at
F~360 kHz
Damping lt 50 micros
Figure 312 Tracking results (50 MeV
1 nC bunch) over the first 3000
revolutions just after injection without
and with the RF phase feedback system (VRF = 300 kV)
Large oscillations start very quickly after injection when the bunch is still short (4 ps) and are of the
same order of magnitude as the ones induced by the energy-phase jitter that affect the linac injection
This effect is the one to address in the design of the longitudinal feedback As far as the transverse plane
is concerned the growth times characterizing various instability sources are listed in Table 35 The
revolution-by-revolution feedback strength indicated for each instability source is the one needed to
suppress the oscillations
Source Type Growth time Revolution-by-
revolution kicker
strength
Beam pipe
geometry
TMCI
Head-Tail
-
160 micros
gt10 nrad
Resistive Wall
600 micros gt 2 nrad
Ions lt 100 micros gt20 nrad
Table 35 Transverse instability growth time and feedback kicker strength for a 1 nC bunch at 50 MeV
lsquo
39
In contrast with what prevails in the longitudinal phase space the main transverse oscillation source is
the injection position jitter Since the non-linear sextupole components are large this jitter may produce
a large emittance increase as a consequence of the phase space painting process (see Fig 313)
Figure 313 Sketch of the transverse phase space painting process
Simulations show that a very fast increase of the transverse emittance can result from steering errors at
injection in about 500 revolutions or 25 micros To overcome this effect a fast transverse feedback is
needed it has to damp the oscillation in about 100 revolutions or ~5 micros The strength of the revolution-
by-revolution feedback has to be on the order of 2 microrad This strength is far larger than the ones
estimated to cope with the transverse instabilities For this transverse feedback a dedicated strip-line
acting in both planes is foreseen in the RF section (see Fig 314)
Figure 314 Simulated transverse emittance increase induced by a 05 mm injection jitter with and
without feedback
After N
turns
lsquo
40
Phase space Specifications Device
Horizontal amp vertical Fr Frequency = 178 MHz (F0)
Strength = 2 microrad
Dedicated strip-line for
both planes
Longitudinal Frequency = 360 kHz (Fs)
Strength = 105 rads
Main RF phase loop
Table 36 Summary of the feedback specifications
Other collective effects such as the intra-beam scattering (IBS) which is important at low energy or the
Compton recoil effect play an important role during the 20 ms storage time Their dynamics differ from
an instability dynamics but they also increase the electron energy spread and the bunch emittance mdash
and thus reduce the X-ray flux
To summarize three different contributions to the e- bunch dynamics must be dealt with
‐ at injection jitters which may affect either the injected bunch energy or its trajectory or the precise
timing of its injection in the ring generate oscillations which may be large and which must be
quickly damped by feedbacks in all three dimensions (x y z)
‐ during a transition period corresponding to the first ~1000 revolutions strong collective effects
enhanced by a possible synchronization error between the linac and the ring may lead to a complex
turbulent regime in the longitudinal plane prior to ring matching (elliptical phase space density
profil)
‐ during the 20 ms storage period IBS and CBS produce a progressive increase of the bunch size in
all three directions This evolution cannot be stopped by a feedback system In contrast RF HOMs
and other perturbations may generate instabilities that should be controlled by feedbacks
To take into account collective effects a dedicated code has been developed including a collection of
wake field as well as non-linear dynamics It is based on a 6D multi-particle symplectic tracking
restricted to perfect hard edge drift quadrupoles and dipoles of finite length Other higher order
elements as sextupoles are treated in the thin length approximation The RF cavity is simply treated as
a longitudinal cosine kick The mapping through elements is given by the exact Hamiltonian solution
[34] for the drift sections and the dipoles (including their small curvature and dipole edge effects) The
quadrupole mapping is restricted to small radial angles It has been compared to various codes
(TRACYIII ELEGANT BETA and MADX) by performing simulations of the SOLEIL and the
ThomX rings with special care given to the dipole edge effects [35]
The transverse beam size at IP and the bunch length are plotted in Fig 315 over the first 10 000
revolutions or 056 ms with a 1 nC bunch charge This study shows that a small positive chromaticity in
the horizontal plane is necessary to stabilize the horizontal emittance During the very first revolutions
the tune dependent emittance behavior has not been fully investigated nor understood It appears that
this behavior follows from the bunch length mismatch between the linac and the ring The fact that the
linac produces very short bunches of 4-5 ps (rms) leads to a complex turbulent dynamics (see Fig
316) The effect of this mismatch disappears progressively over the first 1000 revolutions then the
bunch length reaches 20-25 ps (rms)
lsquo
41
Figure 315 Red RMS beam size and bunch length at IP Magenta bunch centroid position over the
first 10 000 revolutions (056 ms) for a 1 nC bunch The beam energy is 50 MeV
Figure 316 Turbulent dynamics in the longitudinal phase space that us taking place just at injection
These plots show the phase space density after 100 and 3000 turns
lsquo
42
334 CBS and IBS effects
The electron beam dynamics that prevails in a ring where Compton back-scattering is taking place is
similar to the one resulting from synchrotron radiation (SR) in a synchrotron light source CBS gives
rise to random excitation in the transverse and the longitudinal planes In the transverse plane the
emittance turns out to remain close to the one at injection [37] In contrast the CBS excitation strongly
affects the dynamics in the longitudinal phase space It leads to a substantial energy spread increase
over 50 ms storage time (see Fig 317a) resulting in a significant bunch lengthening
Based on the lattice parameters the calculation of the Touschek lifetime leads to a value on the order of
10 s this effect can thus be neglected
Figure 317 a) Energy spread increase and b) Radial (x xrsquo) relative emittance increase during a 100 ms
storage time Blue curves CBS effect only red curves IBS effect only black curves (almost on top of
the blue ones) CBS and IBS combined effect The IBS calculation is based on the Mtingwa formalism
[38]
In contrast the multiple Touschek effect (lsquoIBSrsquo) plays a significant role in the radial phase space (x xrsquo)
as shown in Fig 317b After 20 ms this horizontal emittance is increased by a factor ~ 4 from 50 to
200 nmrad and the relative energy spread by ~20 from 03 to 036 as can be seen in Fig 317a
Figure 317 shows that the Compton back-scattering process plays a role by far more important than the
IBS does It also shows that because of the large emittance increase produced by the CBS the bunch
density decreases by a large amount making the IBS much less effective (see Fig 317b where the
curve showing the combined effects lies below the curve illustrating the IBS effect by itself)
When the electron beam energy is increased up to 70 MeV the IBS effect on the horizontal emittance
becomes less important During a 20 ms storage cycle this effect is reduced by a factor 2 On the other
hand the energy spread degradation increases by more than a factor two compared to the 50 MeV case
This change is due to the higher energy of the back-scattered photons In contrast if the electron beam
energy is decreased down to 25 MeV the energy spread is reduced by a factor two while the horizontal
emittance increases by a factor seven compared to the 50 MeV case
Assuming an optimal overlap between the e- bunch and the laser pulse in the interaction region and
neglecting the e- bunch size variation across this region (the lsquohourglassrsquo effect) as well as the variation
of the laser beam envelope near its waist (characterized by the Rayleigh length) the X-ray flux is given
by the following formula (see Eq 11)
The nominal storage time is 20 ms only
lsquo
43
where σth is the Thomson cross section Nx is the number of photons per laser pulse f is the e- bunch
revolution frequency ϕ is half the collision angle σxyze σxyzγ are respectively the three dimensional
bunch rms sizes for the electron (e subscript) and for the laser pulse (γ subscript) In fact because of the
increase of the e- bunch size during the storage time the flux is reduced by roughly a factor two at the
end of a cycle as shown in Fig 318
Figure 318 X-ray flux as a function of the storage time These results take into account the energy
spread increase and the emittance degradation due to IBS only (blue curve) and to the combined CBS
and IBS effects (black curve)
To calculate more precisely the X-ray flux produced by Thom-X the code CAIN [39] (cf sect 12 above)
has been included in the previous 6D tracking code including the non linear effects of the lattice
magnets as well as collective effects The X-rays characteristics are computed at each ring revolution
The intra-beam scattering effect is calculated also at each e- bunch revolution based on the Bjorken and
Mtingwa formalism [38] To reduce the computing time the code extrapolates the IBS effect whenever
there is only a little change from one revolution to the next (this method reduces the computing time by
a factor 4) This code runs on linux OS Using the computer farm of the IN2P3 computing center in
Lyon it takes about 15 days to simulate a full Thom-X cycle (20 ms ie 400 000 revolutions) when the
e- bunch is modeled by 500 000 macro-particles
The way the total X-ray flux decreases during the first 10 000 revolutions is shown in Fig 319 This
decrease is due to the e-
beam quality degradation mainly due to the CBS effect Also shown in this
figure is the X-ray flux evolution as a function of the storage time for three different lower cut-offs in
the X-ray energy spectrum Figure 320 shows a typical angular distribution of the X-ray emission
Figure 321 plots the bunch longitudinal phase space for the first revolutions The macro-particles in
green are those which have been hit by photons at the last crossing while those in blue have been hit
before the others are in red
( ) ( ) φσ+σ+φσ+σσ+σ
φfNNσ=N
2
γzez
2
γxex
2
γyey
γeth
x22222
sincos
2πcos
lsquo
44
Cross comparison and benchmarking of this code against other codes developed for ThomX (6D
tracking without Compton effect or Compton + IBS without tracking) are in progress and indicate a
good agreement between the codes
Figure 319 Left panel Number of X-rays produced as a function of the X-ray energy at the end of the first
10 000 first revolutions (in red) and during the last 10 000 revolutions before extraction (in blue) Right
panel Number of X-rays produced summed over 2500 revolutions as a function of the revolution number
Figure 320 X-ray intensity as a function of the emission angle in mrad (horizontal axis) and their
energy in eV (vertical axis) The color bar indicates the number of X-rays
lsquo
45
Figure 321 Upper nine panels Evolution of the e- beam longitudinal phase-space after injection The macro-
particle positions are plotted along the horizontal axis while their energy deviations are plotted along the
vertical one Green electrons which have collided with photons at the last crossing blue electrons which
have collided before red electrons which have not yet undergone any collision These plots reveal a turbulent
dynamics Lower nine panels Time evolution of the beam longitudinal phase space as a result of a turbulent
regime from the initial lsquobanana shapersquo at injection to a Gaussian distribution after a few ms (colors indicate
the electron number-density red corresponds to the most populated region)
lsquo
46
Figure 322 Evolution of the X-ray energy spectrum when the e- beam energy is increased from 475 to 505
MeV Left panel The X-ray energy spectrum is plotted vertically for seven e- beam energies Colors
characterize the X-ray intensity Right panel Each curve shows the total number of X-rays above some lower
energy cut-off Green cut-off set at 20 KeV blue cut-off at 30 KeV red cut-off at 40 KeV
335 Scattering on the residual gas
Coulomb scattering of the electrons on the residual gas molecules induces an increase of the circulating
beam emittance The amplitude of this effect is proportional to the average of the beta functions and to
the gas partial pressure Moreover it varies as the square of the atomic number of the residual gas
atoms and is inversely proportional to the electron energy With a low beam energy of 50 MeV it is
therefore essential to keep a low pressure to minimize the time-averaged emittance Fig 323 shows the
growth of the transverse emittance during the 20 ms storage time in the nitrogen case and in the
hydrogen one Assuming that one deals with nitrogen at a pressure of 8 10-7
Pa the emittance increases
by a factor of two (see Fig 323) while the degradation is negligible in the dihydrogen case We
conclude that a pressure lower than 3 10-7
Pa must be maintained to keep the emittance growth at an
acceptable level
Figure 323 Emittance increase due to Coulomb scattering assuming that the residual gas is composed
of a) 100 nitrogen b) 100 dihydrogen
lsquo
47
336 Ion effects
The beam-ion instability [310 39 312] originates in the ionization of the residual gas molecules Ions
can subsequently be trapped by the electron beam electrostatic potential We recall that the on-axis
potential is inversely proportional to the beam size
When the trapped ion number density reaches some threshold beam instability is triggered Besides this
effect trapped ions damage the beam quality by shifting the tunes increassing the emittance and
lowering the beam lifetime since the local pressure is increased
For the Thom-X ring the critical mass above which ions are trapped is on the order of one in units of
the proton mass Thus all ions whatever their mass can be trapped in our case
Once trapped ions oscillate transversally in the potential well Their oscillation frequency depends on
the electron beam size and on the ion species (see Fig 324 1c) Because of the e- beam bunching
when the ion oscillation frequency becomes larger than the beam betatron frequency a lsquofast ion
instabilityrsquo may develop the e- bunch is submitted to the electric field of the oscillating ions eventually
resulting in an exponential growth of the oscillations of all particles (electrons and ions) This instability
arises above some value of the neutralization factor which depends on the lattice
The neutralization factor and the growth time of either the vertical or the horizontal oscillations are
shown in Fig 324 as a function of the horizontal beam size for several ion species For a horizontal
beam size averaged over the ring of 05 to 1 mm (which is about what is expected) the growth time is
very short (less than one micros) but for this would happen for a neutralization factor much larger than one
obviously this is excluded To keep under control the other ion harmful effects mentioned above one
may prevent ion accumulation over many storage cycles by introducing gaps between short series of
injections or by clearing the ions by an electrostatic potential that cancels out the potential well
produced by the e- beam A simple solution consists in polarizing the beam position monitors (BPMs)
thus avoiding the insertion of electrodes along the ring This scheme should be effective since ions will
drift longitudinally as the kicks they receive at each e- bunch revolution has a longitudinal component
The amplitude of these kicks along the z direction is shown in Fig 325 where the BPMs are
represented by circles They are placed near accumulation points where the longitudinal kick vanishes
ie where ions will converge A difficulty may arise at s =125 m This point is opposite to the IP in the
lattice at this point the e- beam has small transverse dimensions and a vacuum pump is placed Indeed
with a small beam size the neutralization factor threshold is very low (10-3
) and the growth time
constant is on the order of one micros This potential difficulty can be overcome by adding an electrode in
the short section between the two dipoles to clear the ions at this accumulation point
Ion clearing with polarized BPMs or gaps introduced in the injection sequence should keep other ion
effects at an acceptable level (more work is needed to quantify this statement)
Defined as the total number of ions divided by the total number of electrons
lsquo
48
Figure 324 a) Threshold neutralization factor b) growth time and c) ion oscillation frequency for the
following fractional becirctatron tunes 1) 064 and 2) 017 for different ion species The dashed lines show
the e- bunch betatron frequency σx is the mean beam size
Figure 325 Longitudinal kick given to on-axis ions at each e- bunch revolution
337 Electron beam injection and extraction
Originally the linac beam was injected in the ring by using only a septum followed by a fast kicker
Given the high repetition rate (50 Hz) radiation safety issues and a possible noise in the X-ray
experimentation area lead to set up an e- beam extraction line The symmetric septum magnet located in
lsquo
49
the middle of one of the long straight sections is used in a double pass mode for injection and
extraction It is flanked by two identical kickers one downstream for the injection and one upstream for
the extraction (Fig 326)
Figure 326 Sketch (top panel) implementation (middle panel) and simulation (lower panel) of the
injectionextraction process
The main characteristics of the septum and the kicker magnets are listed in Table 37 Since in both
processes the beam runs through quadrupoles some margins with respect to the nominal deflection
values must be provided to cope with the fact that quadrupole fields are not uniform
Kicker Septum
Deflection 15 mrad max (11 nominal) 170 mrad (40 mTm 70 MeV)
Pulse duration lt 56 ns half sine 130 micros full sine
Effective length 200 mm
250 mm
Overall length 450 mm 500 mm
Table 37 Main characteristics of the pulsed devices
Septum center
x =14 mm
xp=9 mrad + 170 mrad
Kicker center
x =0 mm
xp=0+11 mrad
2 fast kickers
Inj Ext
1 slow septum
lsquo
50
338 Orbit corrections
A set of 12 beam position monitors (BPMs) and 12 correctors in both planes are implemented along the
lattice in order to control and correct the orbit An additional BPM located at the IP is fixed on the
table that supports the laser cavity The main error sources are listed below Uncorrected and corrected
orbits are illustrated in Fig 327
rms value dBB dL dX dZ dS tilt
Dipoles 10-4 01 mm 02 mm 01 mm 02 mrad
Quadrupoles - - 02 mm - -
Dipoles 10-4 01 mm 02 mm 01 mm 02 mrad
Quadrupoles - - 02 mm - -
Table 38 Main magnet misalignment errors
Uncorrected orbit with typical magnet alignment errors
rms value xmax (mm) zmax (mm)
Max 7 3
IP 05 03
rms value xʹmax (mrad) zʹmax (mrad)
Max 12 4
IP 6 3
lsquo
51
Corrected orbit
rms value xmax (microm) zmax (microm)
Max 200 140
IP 200 30
rms value xʹmax (microrad) zʹmax (microrad)
Max 600 300
IP 350 60
Figure 327 Uncorrected and corrected closed orbits Red lines horizontal projection blue lines
vertical projection
Quadrupoles and sextupoles are fixed on girders with a maximum error of 50 microm The magnetic field
center error relative to the magnet yoke being of the same order the expected total alignment errors are
~100 microm in each plane The girders supporting all the elements will be aligned as solid blocks Since
lsquo
52
dipole misalignments have a large effect on the orbit dedicated additional support tools will allow the
readjusting of the dipole positions relative to the girder
One needs a maximum corrector strength of 05 mrad when this strength is computed by assuming just
one sigma alignment errors to cope with alignment errors which would be twice as large corrector
strengths of 1 mrad are thus necessary
Figure 328 Orbit bumps (red horizontal blue vertical) that can be produced at the IP
In addition to this standard orbit correction procedure a dedicated orbit scan at the IP will be very
useful to ensure an optimal overlap between the e- bunch and the laser pulse in the interaction region
(see Fig 328) To provide a scan range of up to 1 mm or 1 mrad the corrector strength must be further
increased by 1 mrad Thus the corrector maximum strength will be 2 mrad
The orbit correction will be performed by a slow feedback operating around 01 The correctors are in
the sextupoles magnets (combined magets) The orbit feedback correction will also control the RF
frequency in order to keep the orbit length at the ring design value A specific synchronization scheme
that ensures the proper injection timing and that maximizes the electron-photon collision rate is
described in chapter 5
Position amplitude scan at IP location up to 1 mm
Angle amplitude scan at the IP
location up to 1 mrad
lsquo
53
34 Magnets
Table 39 describes the magnets that will equip the transfer line the ring and the extraction line
Magnet type Dipoles Dipoles for inj Quad Sext Corr
Number 4+8+2 2 7+24+2 12 4+12+1
PS number 3 2 33 3 34
Strength 0180 T m 5 Tm 40 Tmsup2 05 mTm
Length 2765 mm 150 mm 60 mm 30 mm
Table 39 ThomX magnet parameters
341 Dipole magnetic design
The dipole bending angle is 45deg A magnetic field of 05 T is needed at 50 MeV Since their radius of
curvature is very small (0352 m) special attention has been paid to the field profile between the magnet poles
and to the pole edge shape where large multipole components are present Lateral shims and pole edge
specific profiles have been optimized with the TOSCA3D code to minimize the impact of the multipoles on
the beam dynamics
Characteristics unit value
Gap mm 40
Field at centre T 063
Overall dimension mm3 350 x 400 x 400
Current A 263
Voltage V 177
Pole width mm 100
Pole height mm 100
Yoke length mm 2765
NI Aturns 13000
Coil area mmsup2 67x40
Current density Ammsup2 116
Number of turns turns 48
Transverse size of the conductor mmsup2 6x6
Cooling channel mm 4
Water pressure drop bar 7
Temperature increase degC 47
Flow rate (per dipole) lmn 86
Electric power (per dipole) W 3100
Table 310 ThomX dipole parameters
lsquo
54
Figure 329 Drawing of a ThomX dipole
342 Quadrupole magnetic design
The quadrupole design (see Fig 330) was also performed with the TOSCA software The magnetic field
is produced by four coils carrying a DC current The conductor is wrapped around four poles which
concentrate and define the quadrupole magnetic field map
Figure 330 Quadrupole magnetic design
A quadrupole can be manufactured in four identical parts each one comprising one pole and one quarter
of the yoke or the yoke and the four poles can be manufactured separately The quadrupole will be
equipped with special shims which will be adjusted during the magnetic measurement to ensure a good
alignment of the magnetic axis and low harmonic components The ThomX quadrupoles are air-cooled
to reduce the infrastructure cost This is possible since the maximum current density is 15 Ammsup2
Figure 331 shows the magnetic field intensity versus the radial distance from the quadrupole axis The
maximum gradient is 5 Tm Figure 332 shows the magnetic field intensity at a distance of -20 mm
from the magnet axis versus the longitudinal coordinate z The maximum magnetic field is 01 T
lsquo
55
Figure 331 Magnetic field (in T) versus the distance (in mm to the magnet axis
Figure 332 Magnetic field intensity (in T) versus z (the longitudinal coordinate in mm)
lsquo
56
The main parameters of the ThomX quadrupoles are given in Table 311 Each quadrupole is powered
by a 10 A 32 V DC power supply thus the released heat power remains below 32 W A dedicated
magnetic bench equipped with a rotating coil will be used for the quadrupole magnetic measurements
This bench has been developed for the magnetic measurements of the SOLEIL storage ring
quadrupoles
Parameters unit value
Bore mm 41
Gradient Tm 5
Current density Ammsup2 15
Overall dimensions cm3 19x19x18
Current A 10
Voltage V 35
NI Aturns 810
Pole width mm 20
Pole height mm 50
Coil dimensions mmsup2 15x36
Number of turns turns 81
Wire dimensions mmsup2 5x125
Yoke length mm 150
Yoke thickness mm 20
Chamfer mm 5 (not optimised yet)
Table 311 Main parameters of the ThomX quadrupoles
343 Sextupole magnetic design
The ThomX sextupole design (see Fig 333) was also performed with the TOSCA software The magnetic
field is produced by six coils that carry a DC current The coils are wrapped around six iron poles which
concentrate the field and define the sextupole field map These magnets are combined function and are also
equipped with additional coils to produce the corrector dipole fields
Figure 333 Sextupole magnetic design
lsquo
57
Each sextupole yoke is manufactured as a single piece and the six poles are screwed on this yoke Then a coil
is wound around each pole During the design phase a chamfer was calculated with TOSCA to guaranty a low
harmonic content The sextupoles will be air-cooled to reduce the infrastructure cost The maximum current
density is 15 Ammsup2
The magnetic field intensity versus the distance to the magnet axis is shown in Fig 334 The maximum
sextupolar strength is 40 Tmsup2
Figure 334 The ThomX sextupole magnetic field versus the distance to the magnet axis
Fig 335 shows the variation of the magnetic field at a distance of -20 mm from the sextupole axis versus the
longitudinal coordinate z The maximum magnetic field is 0016 T
Figure 335 ThomX sextupole magnetic field versus z (the longitudinal coordinate)
Table 312 gives the main parameters of the ThomX sextupoles They will be powered by a 10 A 05 V DC
power supply
lsquo
58
Parameters unit value
Bore diameter mm 42
Sextupolar Strenght Tm2 40
Field 20 mm T 0016
Current density Amm2 25
Overall dimension cm3 19x19x8
Current A 10
Tension V 05
AmpereTurn At 100
Pole width mm 20
Pole height mm 40
Coil dimension mm2 6x40
Number of turns Tours 10
Conductor dimension mm2 2x2
Yoke length mm 60
Yoke thickness mm 20
Chamfer mm 15
Table 312 Main characteristics of the ThomX sextupoles
A dedicated bench equipped with a rotating coil will be used for the magnetic measurements of the sextupoles
This bench has been developed for the magnetic measurements of the SOLEIL storage ring sextupoles
344 Corrector magnetic design
Simple window frame correctors based on an already existing SOLEIL design will be used for the orbit
corrections of the two transfer line
Figure 336 Magnetic design of the window frame correctors
lsquo
59
345 Injection dipole
The two injectionextraction dipoles compensate the septum deflection with a 170 mrad deflection (cf sect 337
see Fig 326) They are also air-cooled Their design and their technical characteristics are illustrated in Fig
336 and Table 313
Figure 336 Magnetic design of the injection dipole
Parameters unit value
Gap mm 40
Field at the magnet centre T 0224
Current A
10
Voltage V 6
Pole width mm 60
Pole heigth mm 70
Yoke length mm 150
AI (per coil) At 3600
Coil cross section area mmsup2 60x60
Current density Ammsup2 09
Number of turns tours 360
Conductor size mmsup2 10 x 1
Pressure drop of the cooling water bar NC
Temperature increase degC NC
Power (per dipole) W 120
Table 313 Parameters of the injection and extraction dipoles
lsquo
60
35 Power supplies
In order to provide a high operation flexibility ThomX will be equipped with the following power supplies
ndash One power supply for the ring dipoles (the latter will be connected in series) one for the TL dipoles
and one for the DL
ndash Individual power supplies for all the other magnets (quadrupoles sextupoles and correctors)
All the power supplies will have a 16 bit-resolution and a plusmn20 ppm reproducibilitystability over 24 hours The
technology is a switched-mode one in order to achieve good efficiencies (gt 85)
Magnet Nb PS Nb mag current
(A)
voltage
(V)
type load
R
(mOhm)
L
(mH)
Dip ring 1 8 300 170 Unipol 565 48
Dip TL 1 4 300 85 Unipol 280 24
Dip DL 1 2 300 45 Unipol 150 12
Dip inj 2 1 12 18 Unipol 1330 180
Quad 33 1 12 6 Unipol 500 10
Sext 12 1 12 3 Unipol 250 08
Corr ring 24 1 plusmn12 plusmn3 Bipol 250 08
Corr TL DL 10 1 plusmn12 plusmn3 Bipol 250 08
Table 314 Overview of the ThomX power supply equipment
351 Pulsed magnets
3511 Layout
As discussed above the e- beam injection in the storage ring and its extraction will take place in the
same straight section (see Fig 337)
Figure 337 Injectionextraction section layout
Eddy current septum magnet
(under vacuum) Fast Kicker Fast Kicker
lsquo
61
The first injection step involves a thin septum magnet It bends the incoming bunch trajectory to bring it
inside the ring vacuum chamber close to the stored beam orbit Then a fast kicker compensates for the
residual trajectory angle and brings the injected bunch on-axis
Extraction is realized by another fast kicker followed by the same septum magnet as the one used for
injection This kicker deflects the bunch trajectory so that it enters into the septum channel in turn the
thin septum bends the bunch trajectory which enters then into the extraction channel that brings the
bunch into a beam dump
In this configuration the same thin septum magnet is used for both the bunch injection and the bunch
extraction This requires a symmetrical design with two flanges one dedicated to the injection and the
other to the extraction The magnetic gap is wide enough to accommodate the two bent trajectories
When operating at a 50 Hz injection rate the septum current pulse consists of a slow full sine whose
same positive half sine will be used successively for the lsquoold bunchrsquo extraction and the for the lsquonew
bunchrsquo injection with a suitable delay between the two steps
The two fast kicker magnets are identical their length has been reduced as much as possible because of
the very limited space available The current pulse in the kickers will have a half-sine shape One must
achieve a rise time (from zero current to peak current) and a decay time (from peak current to zero)
each less than a ring revolution period Thus the current pulse can have a maximum duration of two ring
revolution periods Consequently their charging voltage reaches high values even though most of the
deflection is performed by the septum magnet
The design of these three devices ensures that they can operate with a beam energy of 50 MeV
(nominal) or 70 MeV (maximum) and at a 50 Hz repetition rate
3512 Slow septum magnet and its pulsed power supply
The main constraints on the septum design are as follows This septum must produce a large trajectory
deflection (160 mrad nominal peak value 170 mrad maximum peak value) to bring the injected beam
very close to the ring straight section axis with no significant disturbance of the stored beam The
magnet mechanics should preserve the ring vacuum chamber geometry to keep its impedance as low as
possible Furthermore its stray field must remain very low within the vacuum chamber where the stored
beam circulates as the septum excitation is done by a full sine pulse whose width (130 micros) is very long
compared to the ring revolution time (56 ns)
An eddy current septum magnet installed in vacuum will fulfil these constraints The effective septum
blade thickness can be as small as 3 mm thus ensuring the closeness of the stored beam orbit and the
desired bunch trajectory at the output of the septum yoke An internal chamber connected to the magnet
yoke extends the ring vacuum chamber and preserves the impedance budget The injected bunch
deflected by the septum magnet field penetrates through a small hole in this internal chamber Such a
septum magnet is very asymmetric in the transverse plane it is driven by a copper loop that wraps the
steel yoke
In order to contain the magnetic flux the whole yoke is enclosed in a copper box which carries the eddy
current On the side facing the stored beam orbit the copper box thickness is reduced so that it
constitutes the thin septum blade As a result of the electric shielding provided by the eddy current
circulating in the copper septum such a structure is very effective to avoid a large magnetic stray field
outside the magnet gap The fact that the excitation pulse is a full sine reduces the duration and the
lsquo
62
amplitude of the stray field (the slow delayed stray field tends to be the sum of the two opposite half
current arcs this feature brings about a reduction of the stray field)
An additional magnetic shielding is required to strongly reduce the residual strayfield This shield is
provided by a 05 mm thick sheet of high permeability mumetal that will completely surround and
enclose the internal chamber (thus shielding the stored beam orbit from the strayfield)
Such a design that combines an eddy current shield a full sine pulse excitation and the use of a high
permeability magnetic material reduces the stray field to a very low level (a few 10 ppm of the main
pulsed field) [313]
Assuming a septum opening of H x V = 30 x 12 mm2 and an active length of 250 mm (the length was
chosen as short as possible because of constraints coming from the septum implementation in the
straight section) the septum magnet inductance is estimated to be 16 microH Connexions and cables
increase this inductance up to 23 microH
This design leads to the following parameters for the septum magnet and its power supply (values for a
70 MeV ring operation)
Septum deflection field integral field peak current charging voltage
170 mrad 40 mTm 160 mT 1500 A 226 V
Table 315 Septum main parameters
The pulsed power supply will be based on the lsquoclassicalrsquo resonant discharge circuit scheme composed
of a storage energy capacitor (190 microF) a fast high-current thyristor switch and a transmission line to
the magnet comprising a few low impedance coaxial cables
A 300V-20A power supply is foreseen It will be connected to the storage energy capacitor through a
current-limiting power resistor
3513 Fast kicker magnet and its pulsed power supply
The two fast kicker magnets must be identical Because of the very limited space available their length
has been reduced as much as possible The final numbers are 200 mm and 450 mm for the active length
and for the overall length respectively They will be located outside the vacuum vessel with their
lsquowindow framersquo rectangular ferrite yokes surrounding the ceramic vacuum chamber (see Fig 338)
Figure 338 Front view of the kickers
The two ceramic sections of the vacuum chamber will have a flange-to-flange length of 450 mm Their
aperture will be the same as the one in the ring standard sections but with a racetrack transverse shape
Both extremities of each ceramic chamber include a smooth shape transition and a short bellow that
lsquo
63
allows for RF finger contacts These bellows can accommodate thermal dilatations especially during
bake-out The ceramic chambers will be coated with a metallic deposit that will carry the bunch mirror
current and thus avoid an increase of the chamber impedance This coating needs to be very thin It will
consists of a titanium layer with a thickness le0050 microm The thickness has been calculated so as to
avoid the deterioration of the fast pulse shape and to avoid the temperature rise due to the mirror current
[314]
Figure 339 Time profile of the kicker pulse versus the titanium layer thickness
The kicker dimensions follow from
- the aperture needed by the stored beam H x V = 40 x 28 mm2
- the required deflection kick nominal 11 mrad maximum 15 mrad
- the beam energy nominal 50MeV maximum 70MeV
- the short revolution period of the stored e- bunch Trev = 56 ns
- the very short length of the injected bunch from 4 to 25 ps (rms)
With a ceramic thickness of 6 mm the effective magnet aperture must be H x V = 55 x 55 mm2 These
parameters determine the current and voltage pulse characteristics
The kicker magnets will be powered by a circuit delivering half-sine pulses [315] In fact because of
the e- bunch short revolution period (56 ns) the field pulse duration has to be less than twice the ring
revolution period (less than 112 ns) and thus must a have very fast rise time and decay time (both less
than a ring revolution period) On the other hand with such a very short bunch length (4 or 25 ps (rms)
whether one deals with the incoming bunch or the extracted bunch one may assume that the pulses do
not have to present a marked flat top
These considerations lead to the following kicker parameters
00000E
-2
17000E
-10
34000E
-10
51000E
-10
68000E
-10
85000E
-10
10200E
-9
11900E
-9
0000E+00
2000E-01
4000E-01
6000E-01
8000E-01
1000E+00
1200E+00
Thom-X injection kicker pulse field attenuation relating to Titanium thickness
Bext(t)
Bint(t) 002microm
Bint(t) 004microm
Bint(t) 005microm
Bint(t) 006microm
Bint(t) 008microm
Bint(t) 01microm
Bint(t) 02microm
Time
Fie
ld a
mp
litu
de
lsquo
64
Kicker beam energy deflection field integral
Operation at Enominal 50 MeV 15 mrad 25 mTm
Operation at Emax 70 MeV 15 mrad 35 mTm
field peak current estimated voltage
92 mT 406A 10 kV
175 mT 771 A 18 kV
A specific RampD program is necessary to guarantee that these constraints will be fulfilled by the power
supply and by the magnet Preliminary studies were based on the standard circuit used for such a pulsed
power supply (see Fig 340) involving a high voltage (HV) capacitor to store the energy in series with
a HV switch and a load whose inductance is close to the one of the kicker magnet In this circuit the
HV switch is a very fast MOSFET switch followed by a HV fast recovery diode module
Figure 340 Power supply circuit for the ThomX kickers
These first studies lead to unsatisfactory preliminary results
- The BEHLKE Ultra-Fast 17 kV 480 A MOSFET module can switch ON in a very short time less
than 8 ns Then the pulse shape is normally determined by the passive component parameters in the
circuit provided that the negative current is quickly interrupted either by HV diodes in series or by
a turn Off control of the HV switch
- But the different types of HV diodes used are not able to switch OFF the pulsed current quickly
enough Even the lsquofast recovery diodesrsquo have a reverse recovery time too much long to stop cleanly
the pulse at its zero crossing Thus it was necessary to work on other topologies and to find better
suited components
Further simulations were performed to see how the circuit could be improved It turned out that
controlling the switch both at the closing and at the opening can improves the shape of the current
pulse A prototype using the same Behlke MOSFET modules and including a coaxial transmission line
was built and tested A plot of the current pulse in the kicker coil that was obtained is shown below
lsquo
65
This plot was obtained with the following prototype circuit
As one can see more work is still needed to improve the shape of the current in the coil (namely one
should reduce the level of reflections improve the current pulse shape and work on the peak current to
meet the 70 MeV requirements) We are also looking for an other supplier who could provide controlled
HV switches ensuringr both ON and OFF fast operation
Mechanical and magnetic studies of the kicker magnets are ongoing
36 Storage ring RF system
The choice of 500 MHz for the RF frequency is a compromise between several constraints the cavity
fundamental and high order mode (HOM) impedances space requirements the availability of RF power
sources and other components The required accelerating voltage of 500 kV can be provided by only
one 500 MHz single-cell cavity with about 40 kW of RF power As shown in the following the choice
of the cavity design is mainly dictated by the need for a strong damping of the HOM
lsquo
66
361 HOM impedance thresholds as regard to beam instabilities
When a cavity mode is excited in resonance it generates beam oscillations with a rise time
approximately given by one of the two following expressions
(1τ)l = Ib α fm Rl (2 Qs Ee) in the longitudinal case and
(1τ)t = Ib βt fo Rt (2 Ee) in the transverse case
where fm is the HOM resonant frequency Rl [Ω] and Rt [Ωm] are respectively the longitudinal and the
transverse HOM impedances Qs = fs fo is the synchrotron tune (fs and fo are the synchrotron and the
revolution frequencies) βt is the beta function amplitude at the cavity location α is the momentum
compaction factor Ib is the average beam current and Ee is the beam energy in eV
Assuming typical values for the cavity HOM impedances (Rl fm = 01 to 1 MΩ GHz Rt = 1 to
10 MΩm) and the machine parameters listed in Table 315 the instability growth time is around 10 micros
in both the longitudinal and the transverse cases
Orbit length C 168 m
Electron energy E 50 MeV
RF frequency fRF 500 MHz
Revolution frequency fo 1786 MHz
Harmonic number h 28
Momentum compaction factor α 0015
β-function amplitude at the cavity location βxz 3 m 4 m
Relative energy spread σE E 40 10-3
Peak acceleration voltage VRF 300 (500) kV
RMS bunch length σs 27 (21) ps
Synchrotron frequency fs 358 (462) kHz
Bunch charge Qb 1 nC
Number of stored bunches Nb 1
Maximum current Ib 1786 mA
Table 315 Ring parameters used to estimate the rise time of the HOM induced instabilities
In a low energy ring like ThomX radiation damping is so weak (the damping time is ~ 1 s) that a
stationary state is not reached during the beam storage time (20 ms) On the other hand to ensure a
good ring operation it is sufficient to make all instability rise times much longer than this storage
period This can be achieved by attenuating the HOM impedances very strongly typically by a factor
104
362 Cures to the HOM induced instabilities
There are essentially two ways of coping with such HOM induced instabilities either by performing a
strong de-Qing of the HOM resonances or by tuning their frequencies away from the beam spectral
lines in order to prevent resonant excitation
In practice HOM de-Qing is achieved by extracting the cavity HOM power and transferring it towards
external dissipative loads But in this approach it is difficult to reach attenuation factors higher than
102 - 10
3 over a large frequency range and this technique requires heavy and cumbersome equipment
close to the cavity (coaxial lines with coupling antennas or ferrite loaded waveguides) The DAMPY
lsquo
67
cavity designed at BESSY [316] and which is used in ALBA [317] and the PEP II cavity developed
at LBNL [318] are two variants of this technique
The other method which consists in controlling the HOM frequencies is better suited to a small
circumference machine like ThomX where the beam spectral linesrsquo spacing (~ 18 MHz) is very large
compared to the HOM bandwidth (this bandwidth is of a few tens of kHz for Qo equal to a few 104) As
long as the HOM density is not too high and the modes can easily be tuned far enough from the beam
spectral lines (δf gtgt fm Qo) it should be possible to reduce their effective impedances (the ones lsquoseenrsquo
by the beam) down to tolerable levels ie Reff asymp Rs (2 Qo δf fm) 2 ltlt Rs
We are investigating the potential performance of this technique by applying it to an ELETTRA-type
cavity (see Fig 344) [319] which combines three frequency tuning means The HOM frequencies can
be precisely controlled by an appropriate choice of the temperature of the water cooling the cavity while
the fundamental frequency is maintained constant by a mechanism which deforms the cavity
longitudinally In addition a movable plunger tuner also called HOM frequency shifter (HFS) located
in the cavity equatorial plane provides an additional degree of freedom [320]
Tables 316 a and b list the characteristics of such a cavity fundamental and HOM resonances They
have been calculated with the 3D GdfidL code [321] One finds 18 significant HOM (9 monopoles and
9 dipoles) with resonant frequencies lower than the cavity cut-off frequency Figures 345 a and b
represent the corresponding HOM spectra fm Rl(f) and Rt(f) folded over a single base band of
18 MHz The horizontal dashed line in the graph represents the impedance threshold corresponding to
an instability rise time of 20 ms the beam storage time Although they remain somewhat critical these
results tend to confirm that it should be possible to find acceptable operating conditions as long as the
HOM frequencies can be shifted by a few MHz by combining temperature and HFS tuning
lsquo
68
Figure 344 Drawing and photo of the ELETTRA cavity
lsquo
69
Figure 345 (a) Longitudinal and (b) transverse HOM spectra folded over a single base band of
18 MHz for the ELETTRA-type cavity
τL = 20 ms
lb
os
I
eEQ
τα
2
ttob
o
fI
eE
τβ
2
τt = 20 ms
lsquo
70
a) Longitudinal modes fm [MHz] Q0 Rs Q0
[Ω]
Δfm (T)
[MHz]
Δfm (HFS)
[MHz]
L0 (fundamental) 499132 44164 765 000 000
L1 958618 48158 336 037 -100
L2 102781 57861 036 080 369
L3 14087 52562 220 174 021
L4 151528 69088 783 112 086
L5 155129 66375 850 240 721
L6 186912 55785 097 142 -092
L7 19495 81752 008 183 143
L8 203985 62729 086 400
L9 206896 87300 530 445 667
Table 316 Main characteristics of the monopole (a) and dipole (b) modes computed with GdfidL for
an ELETTRA-type cavity Δfm (T) and Δfm (HFS) are the effective temperature and HFS tuning ranges
assuming a constant fundamental frequency
It is worthwhile mentioning that in any event fast longitudinal and transverse feedback systems are
required to damp the beam oscillations resulting from injection transients (see section 333) They will
contribute to the HOM damping as well
363 RF power source
An RF power of about 40 kW is required to generate the desired voltage of 500 kV in a single 500 MHz
ELETTRA-type cavity For this frequency there exists a wide choice of power sources based either on
vacuum tubes (klystron IOT diacrode) or on solid-state technology The low operating voltage and
high modularity of the solid-state version offer significant advantages compared to vacuum tubes
Experience acquired at SOLEIL after few years of operation has definitely demonstrated all the benefits
of this technology [322] Figure 346 shows the 35 kW - 352 MHz solid-state amplifier used for the
SOLEIL booster synchrotron It consists of a combination of 147 modules Each one has a power of 330
W a gain of 11 dB and an efficiency of 62 This amplifier was commissioned about 7 years ago
During a running period of more than 35 000 hours it tripped only once and this trip was caused by a
human mistake
b) Transverse modes fm
MHz]
Qo Rsperp Qo
[Ω]
Rsperp[MΩm] Δfm(T)[MHz] Δfm(HFS)
[MHz]
T1 73701 50267 335 2599 04 075
T2 74606 52566 92 7557 155 285
T3 115202 47196 264 3006 049 -312
T4 125565 55386 51 7428 068 725
T5 130249 71674 36 7039 097
T6 169981 69325 103 2542 188
T7 173650 91039 165 5463 gt 2
T8 177337 59409 32 7061 gt 2
T9 183617 77697 55 1643 gt 2
T10 206142 88484 59 2254 gt 2
T11 222475 99092 11 5079 gt 2
T12 225933 56641 13 3484 gt 2
lsquo
71
This technology has already been extended to 476 MHz in the framework of a SOLEIL-LNLS
collaboration aimed at realizing two 50 kW amplifiers for the LNLS storage ring [323] More recently
the SOLEIL team has developed modules which can deliver up to 650 W at 500 MHz with a gain of
17 dB and an efficiency of 62 while still reducing the thermal stress [322] About a hundred of such
modules will be combined to obtain the 50 kW required by ThomX
Figure 346 35 kW - 352 MHz solid-state amplifier of the SOLEIL Booster (147 amplifier modules
and DCDC converters bolded on 8 water-cooled dissipaters)
364 Low level RF system and longitudinal feedback
The proposed low level RF (LLRF) system is conventional As shown by the synoptic diagram of Fig
347 it consists of three relatively slow loops (frequency amplitude and phase) aimed at controlling the
cavity voltage with an accuracy of about plusmn 02 in amplitude plusmn 02deg in phase and a response time
around 1 ms This scheme is suitable for compensating the pressure and temperature drifts as well as
the power supply ripples
Such a system is not fast enough to deal with the beam phase oscillations at the synchrotron frequency
fs on the order of 400 kHz that originate in injection transients These oscillations can produce an
undue emittance blow up and a phase jitter at the interaction point Beam dynamics simulations show
that preserving the beam quality requires a damping of these oscillations within a few tens of a micros (see
section 333) As the radiation damping is negligible during the storage time a longitudinal feedback
600 W 280 Vdc 28 Vdc converter
Power combination
330 W amplifier module
lsquo
72
(LFB) must be implemented It can be performed by a dedicated broad-band longitudinal kicker that
acts on the beam Since ThomX is operating with a single bunch we propose to use the main cavity as a
longitudinal kicker despite its narrow bandwidth of 25 kHz ltlt fs This difficulty can be overcome by
making the cavity effective bandwidth much larger than this value in fact larger than fs We will use a
high gain direct RF feedback (RFFB) that allows the operation of a fast phase feedback (ϕFB)
Assuming a cavity-to-amplifier distance of about 10 m the loop delay will be less than 150 ns Then
one expects an RFFB gain Go larger than 50 which leads to an effective bandwidth larger than 1 MHz
Simulations show that a LFB that combines such a RFFB with a ϕFB (see Fig 348) can damp injection
phase errors of up to 10 degrees within a few micros ie faster than the above specification
Figure 347 Basic LLRF system scheme with lsquoslowrsquo frequency amplitude and phase loops
lsquo
73
Figure 348 Schematics of the longitudinal feedback system combining a phase feedback with a direct
RF feedback
365 RF equipment status
For the ThomX commissioning we will use an ELETTRA cavity The design of all the components of
the 50 kW solid-state amplifier (600 W modules combiners power supplieshellip) was validated in 2011
The next two milestones are first the validation of a 9 kW unit (combining 16 modules) and secondly
the commissioning of the complete amplifier on a dummy load
Several versions of the LLRF system including LFB are under investigation either fully analogue (see
Fig 349) or mixed analogue for the fast feedback loops and digital for the slower ones (see Fig 350)
or even fully digital based on the new generation of fast ADC DAC and FPGA
366 RF system conclusion
For the RF system of the ThomX storage ring we plan to use a 500 MHz single-cell cavity of the
ELETTRA-type Maintaining at a tolerable level the impact of the RF cavity HOMs on the beam is a
challenge However this goal should be achievable with a proper control of the HOM frequencies It
can be done by combining three different means of tuning the cavity temperature control longitudinal
elastic deformation of the cavity and motion of a plunger The fast longitudinal and transverse feedback
systems - which in any event are required to damp the beam oscillations resulting from injection
transients - will contribute to the damping of the HOM excitations as well
In order to provide the required accelerating voltage of 500 kV the cavity will be powered by a solid-
state amplifier of the lsquoSOLEILrsquo type able to deliver a power of up to 50 kW The cavity voltage will be
controlled with an accuracy of plusmn 02 and plusmn 02 degree by means of conventional amplitude and phase
loops (either analogue or digital) with a few kHz bandwidths In addition a fast phase feedback system
combined with a high gain direct RF feedback will provide the longitudinal damping which is required
to preserve the beam quality
lsquo
74
Figure 349 Complete LLRF system longitudinal feedback included (analogue version with phase and
amplitude modulation ndash the option with IQ modulation is also considered)
Figure 350 Complete LLRF system longitudinal feedback included (mixed analogdigital version)
37 Transverse feedback
371 Specifications
Table 317 summarizes the instability growth time constants and the corresponding kicker strength
requirements for the main types of instability It shows that for Thom-X the betatron oscillations due to
the injection jitter in position and angle are the most critical ones They require a feedback damping
time constant of 5 micros or less (cf chapter 3)
lsquo
75
Source type growth time constant kicker strength
Beam pipe
geometry
TMCI
Head-Tail
-
160 micros
gt10 nrad
Resistive Wall 600 micros gt2 nrad
Ions lt 100 micros gt20 nrad
Injection Jitter 5 micros 2 microrad
Table 317 Overview of expected instabilities (1 nC 25 ps rms at 50 MeV)
372 Electronics
To cope with these instabilities we will use a SPring-8 based high performance digital transverse
feedback system which consists of a wideband detector (a BPM) an RF front-end an FPGA based
processor a power amplifier and a stripline kicker (see Fig 351) This system is capable of detecting a
coherent transverse motion and applying a counter kick to damp it bunch-by-bunch and turn-by-turn
with one or two bunches stored in the ring
Figure 351 Spring-8 based digital feedback system
Two BPMs (one for each plane) will provide the beam position information The RF front-end will
extract the betatron oscillation information which will be sampled by ADCs From the sampled data the
FPGA will compute a suitable kick to damp the oscillation The signal processor output will be
amplified by a 10 W wideband amplifiers before being fed to the kicker
In the RF front-end (see Fig 352) a baseband down-converter will be implemented in order to amplify
the beam signal and to minimize the jitter effect for the same reason the digital kick will also be
extended over four RF periods
lsquo
76
Figure 352 Front-end of the Thom-X RF electronics
For the commissioning phase a digital system (rather than an analog one) will be used to benefit from
the tuning flexibility provided by the digital parameters of the FIR filter Once the machine tune will be
fixed a more suitable low cost analog system will be installed
373 Kicker
The kicker is under development It consists of a stripline composed of four electrodes connected to
vacuum feedthroughs on each side (see Fig 353) The amplifiers will be connected to the downstream
ports whereas upstream ports will be used for signal detection and they will be equipped with 50 Ω
loads The same kicker will be used to act on the beam in both planes
Electrode line impedances must be adapted (50 Ω) Electromagnetic 3-D simulations are being
performed to optimize the bandwidth and shunt impedance (ie to maximize the stripline efficiency)
and also to minimize the kicker impedance to wakefields
Figure 353 Schematics of the Thom-X stripline kicker
lsquo
77
38 Vacuum
381 Transfer line vacuum
Concerning the design of the transfer line vacuum system three main constraints have been taken into
account
- Pressure of 10-7
Pa at the acceleration section output
- Ion pump operating at 10-6
Pa
- Ring pressure of 10-8
Pa
Simulations have been performed assuming that two 03 m3 s
-1 triode pumps will be installed one at the end of
the acceleration section and the other at the septum The vacuum chamber is a DN40 The septum output
conductance is 018 m3 s
-1 Without baking the outgassing rate would be σ τ = 2x10
-8 Pa m s
-1 and the
pressure at the ring entrance would be about 10-7
Pa Thus the septum and its environment must be baked to
bring τ down to 5x10-10
Pa m s-1
and obtain a pressure of 10-8
Pa at the ring entrance
Pressure in the line of transfer
The vacuum equipment envisaged for the transfer line is the following one
‐ 3 ion pumps 150 l s-1
with their baking systems and power supplies
‐ 1 ion pump 500 l s-1
with their baking system and power supply
‐ 4 Pirani gauges
‐ 4 cold cathode gauges
‐ 2 power supplies gauges
‐ 2 gate valves electro pneumatically actuated DN 40 (acceleration section output ring entrance beam
dump)
‐ 1 turbo pump (DN 63 ~ 80 l s-1
)
‐ 1 rotary vane pump (~ 12 m3 h
-1)
‐ 4 angle valves manually actuated for pre-pumping DN 63
lsquo
78
382 Ring vacuum
Concerning the ultra-vacuum to be maintained in the ThomX ring one must take into account first the
pipe relatively low specific conductance c = 452x10-3
m3 m s
-1 and secondly the pipe specific
outgassing rate θ τ = 108x10-10
Pa m2 s
-1 Also the pumpsrsquo effective pumping speed is reduced by the
T port with the grid Seff = 19x10-2
m3 s
-1 In a first stage the ionic pumps chosen for the simulations
are 0040 m3 s
-1 diode pumps This corresponds to S = 0030 m
3 s
-1 at 10
-8 Pa and with a grid pumping
port whose conductance is 00362 m3 s
-1 to an effective pumping speed Seff = 0019 m
3s
-1 The pressure
has been estimated by modeling a quarter of the ring including half of the non-baked optical cavity ie
in the most unfavorable case
Figure 354 Pressure distribution in the vacuum chamber
The calculated average pressure is 306x10-8
Pa It can be improved by increasing the grid pumping port
conductance by replacing the DN35 with a DN63 Monte-Carlo simulations show an improvement of ~
16 in efficiency
The entire ring must be baked Geometrical and temperature constraints (respectively a distance of only
07 mm between the magnet coils and the vacuum chamber a baking temperature of 150 degC for the
chamber while keeping the coilsrsquo temperature below 80deg C) are strict An ultra-thin baking system has
been constructed tested and validated
Figure 355 The vacuum chamber ultrathin baking set-up
lsquo
79
The following is the vacuum equipment needed for one quarter of the ring
- 3 ion pumps (40 l s-1
) with their baking equipment and their power supplies
- 1 residual gas analyzer (RGA)
- 2 turbo-pumps (DN 63 ~ 80 l s-1
)
- 2 rotary vane pumps (~ 12 m3 h
-1)
- 2 Pirani gauges
- 2 cold cathode gauges
- 1 gauge power supply
- 1 DN 63 gate valve electro-pneumatically actuated with an HF feed-through (one on each side of
the RF cavity and similarly for the septum) and the baking equipment
- the vacuum chamber baking equipment
- 3 angle valves manually actuated for pre-pumping DN63
- 3 T with the grid
lsquo
80
Chapter 4 INJECTOR TRANSFER LINE AND RING DIAGNOSTICS
Several diagnostic devices will be implemented in the ring to measure the electron beam parameters
listed in Table 41
Parameter unit value
Energy MeV 50
Average current mA 1784
Bunch charge nC 1
Energy dispersion 03 at injection to 06 at storage end
Emittance x mrad 5 e-8 at injection to 30 e-8 at storage end
Normalized emittance x mrad 5 e-6 at injection to 30 e-6 at storage end
Emittance y mrad 5 e-8
Normalized emittance y mrad 5 e-6
Bunch length ps-rms revolutions 1 ~ 4
revolutions 100-200 ~ 25
revolutions 200-300000 ~ 50
RF frequency MHz 500
Revolution frequency MHz 1784
Period ns 56
Injection repetition rate Hz 50
Ring circumference m 1680
(a) Ring parameters
Magnetic field T 05
Radius of curvature m 0352
R51 m 0 to -025
R52 m 0 to -045
R56 m 07 to -07
Beta x m 06
Beta y m 6
Eta x m 02 - 03
Transverse dimension x mm (rms) 06
Transverse divergence x mrad 2
Transverse dimension y mm (rms) 054
Transverse divergence y mrad 2
(b) Parameter values at the location of the bending magnets
Table 41 ThomX beam parameters
41 Charge measurement
411 Current transformers
The beam charge will be measured non-destructively with commercial in-flange high resolution
current transformers (ICT) Three devices will be installed
- one in the linac (LIDGICT010) located before the accelerating sections
- one at the transfer line entrance (TLDGICT010)
lsquo
81
- one at the transfer line exit (TLDGICT020)
In-flange ICTs are directly mounted in the beam line They are UHV compatible can be baked up to
150deg C (185deg C on request) and they are available for many pipe diameters (also with an elliptical
aperture or any other aperture shape) We will use the CF6rdquo model its internal diameter is 604 mm and
it is 60 mm long This device includes a ceramic gap a shield and a wall current bypass Bellows are
not required
The signal from each integrating current transformer will be processed by a BCM (beam charge
monitor) A built-in calibration generator provides an on-line check of the measurement reliability An
Integrate-Hold-Reset (BCM-IHR) device is suited for pulse repetition rates ranging from 10 kHz down
to single pulses and for bunch charges up to 2 nC The acquisition will be performed by using a 100
kHz 12 bit ADC
412 Faraday cups
Interceptive absolute beam charge measurements will be performed by Faraday cups There will be two
of them placed inside the two beam dumps (see Fig 2 of the Introduction) They should provide a
better resolution for small electric charges
42 Transverse profile measurements
421 Diagnostics stations
Five diagnostics stations will be installed to measure the transverse beam profile as shown in Fig 41
Diagnostics station 1 (DS1 ndash LIDGSST010) will be located after the RF gun at the beginning of the
linac Diagnostics station 2 (DS2 ndash TLDGSST010) will be located between the end of the linac and
the linac beam dump Diagnostics station 3 (DS3 ndash TLDGSST020) will be placed in the middle of the
transfer line Diagnostics station 4 (DS4 ndash TLDGSST030) will be located at the end of the transfer
line and Diagnostics station 5 (DS5 ndash ELDGSST010) will be located before the ring dump
lsquo
82
Figure 41 Location of the diagnostics stations (see text for the nomenclature)
The ThomX station design draws from the station built by LAL for PITZ (itself drawn from a SOLEIL
design) It is illustrated in Fig 42
Figure 42 3D view of a ThomX diagnostic station
Each station will have a motorized arm on which several screens will be mounted A stepper motor will
move this arm and insert the screens laterally The following screens will be available
- 1 YAG(Ce) screen in all stations
- 1 optical transition radiation (OTR) screen in all stations
lsquo
83
- 1 target to allow an online calibration of the camera in all stations
- one Cerenkov screen for bunch length measurements in station DS2 (described in more detail
below)
The YAG(Ce) and OTR screens and the calibration target will be tilted at 45deg with respect to the beam
axis so that the emitted or reflected light will travel downward and exit the vacuum through a DN60 CF
fused silica window A CCD camera will be located below the station to image the screens The
Cerenkov screen will be titled so that its light exits at the top of the diagnostic station Shutters will be
used to close either the Cerenkov window or the YAG(Ce)OTR window when not in use to prevent
measurement noise due to background photons Several LEDs located just after the bottom window will
be used to illuminate the inside of the chamber when needed
At the beginning of the Linac at station DS1 the beam size will be about 05-07mm (rms) in both
planes at the end of the Linac at DS2 the beam size will be 15 mm (rms) in both planes In the middle
of the transfer line at DS3 the beam size is expected to reach 25 mm (rms) in x and slightly less than
1mm in y At the end of the transfer line at DS4 it will be about 1 mm in both planes After the
extraction at DS5 the beam size will be about 05-07mm (rms) in both planes Given these beam sizes
we chose a screen diameter of 25 mm for all stations
Screen photon yield
The YAG(Ce) screen will be 100 microm thick Such screens have a light yield of 8 photons per deposited
keV [41] The YAG radiation length is 35 mm [42] assuming the usual model for energy deposition in
matter [43] each electron will deposit about 143 keV in the screen this corresponds to the emission of
~1100 photons Thus the total number of photons produced by an e- bunch crossing the screen will be
71 1012
We recall that YAG(Ce) emission occurs in all directions
The second screen will be of the OTR type The screen support design allows either the use of a 100 microm
aluminized silicon wafer or of an ultra-thin aluminized mylar film The silicon wafer has the advantage
of giving a better image quality while the mylar film allows a beam injection through the screen without
significantly affecting its quality The OTR yield within a cone of half-aperture θ for photons whose
wavelengths range between λ1 and λ2 is given by [44]
Thus for a standard CCD camera whose sensitivity ranges between 400 nm and 800 nm and assuming
an acceptance cone of 003 radian this expression gives ~0002 photon per electron at 50 MeV (station
DS2-5) A similar yield is reached at 5 MeV with an acceptance of 03 radians (station DS1)
To image the screens a lens will be located 500 mm away from them A small telescope made of two
lenses will be used to focus the light on the CCD camera At least one of the telescope lenses will be
movable to provide the needed dynamic range A Gigabit Ethernet camera with at least 600 x 400 pixels
will record the images The camerarsquos square pixels are about 74 x 74 microm2
With such a configuration one expects that a 50 MeV bunch passing through the YAG(Ce) screen will
emit 71x1012
photons and assuming that the CCD chip has an average efficiency of 50 1010
counts
will be recorded by the camera taking into account the solid angle covered by the lens
lsquo
84
The same bunch passing through the OTR screen will emit 82 107 photons within the lens acceptance
Thus 3 106 counts will be recorded by the camera
For a 5 MeV bunch (station DS1) the YAG(Ce) screen will emit 2 1012
photons and 109 counts will be
recorded by the camera The OTR screen will emit 82 107 photons within the lens acceptance and 3 10
6
counts will be recorded by the camera
Assuming a transverse beam size of 15 mm one expects 8 106 counts per pixels with a YAG(Ce)
screen and 25 105 counts per pixel with an OTR
If the 50 MeV beam is defocused (for example during a quadrupole scan) to produce a 5 mm spot size
these numbers become 8 105 for the YAG(Ce) screen and 2 10
4 for the OTR
Number of OTR
photons emitted
Number of
OTR
photons
recorded by
the camera
Number of
YAG
photon
emitted
Number of
YAG
photons
recorded
DS1 82 107 3 10
6 2 10
12 10
9
DS2-5 82 107 3 10
6 71x10
12 10
10
Assuming a YAG Cerium dopant density of 018 we get a Cerium density of 25 1019
cm-3
[45] and
a 15 scintillation efficiency It follows that the scintillating site density is 37 1018
cm-3
Therefore the
100 microm YAG(Ce) screen will saturate when the photon density reaches 36 1014
mm-2
This last figure
translates into an electron density of 32 1011
mm-2
for an e- beam energy of 50 MeV In these
conditions the YAG(Ce) screen will saturate if the beam is focussed on a spot smaller than 20 microm This
is five times less than the smallest beam foreseen at ThomX
Additional targets will be used to calibrate the cameras when necessary They will be made of stainless
steel plates with etched grids Each square of the grid will be 2 by 2 mm2 In some of these squares
smaller lines of known thickness will be etched Images of these targets will allow one to check the
whole optics
To minimize a possible radiation deterioration of the cameras lead-glass windows will be inserted
between the vacuum windows and the focusing lenses
43 Emittance measurement
The beam emittance will be measured with the quadrupole scan technique In this technique a triplet
(set of three quadrupoles) is used to find the minimum transverse beam size in each plane A fit of the
beam size variation versus the quadrupole strength enables one to retrieve the projected transverse
emittance
The sets of triplets and the diagnostic stations implemented along the ring allow an emittance
measurement at three different locations at the exit of the linac with DS2 in the transfer line with DS4
and in the ring with DS5 by using the extraction kicker The beam emittance measurement with DS5
lsquo
85
will provide a check of the turn-by-turn emittance growth thus bringing precious information on the
transverse beam dynamics
Pepper-pot measurements are also foreseen The advantage of this technique with respect to a
quadrupole scan is that it consists in a single shot measurement independent of machine operation
fluctuations
A pepper-pot set-up comprises a plate with evenly distributed holes Such a plate is placed in the beam
path together with a scintillating screen located downstream and an imaging system A pepper-pot
mounted on a movable arm could be located at the end of the linac between the last dipole and the
diagnostic station DS2 and another one could be located just before station DS5
At station DS2 the beam size will be about 15 mm (rms) and the geometric emittance about 5 10-8
m
rad Therefore the maximum beam divergence will be about 50 microrad By adjusting the quadrupoles one
may focus the beam down to a 20 microm spot and increase its divergence up to 250 microrad To measure the
transverse emittance one would have to position this focal point two meters before DS2 (or DS5) and
to position a pepper-pot mask with 100 microm holes or slits one meter upstream of DS2 (or DS5)
44 Beam position measurement
The beam position will be measured by a set of BPMs placed all along the ring Their locations are
shown in Fig 43
lsquo
86
Figure 43 Location of the BPMs along the ring (note the BPM located at the IP has been suppressed
since this drawing was made)
441 Beam Position Monitors (BPM)
Two types of BPMs will be used short circuited striplines in the linac and the transfer line and buttons
in the storage ring
Compared to button BPMs striplines generate a higher signal but need more space and contribute more
to the wake-field generating impedance For these reasons striplines are preferred for the linac and the
transfer line where we need single pass measurements and where wakefield is not an issue In the
storage ring button BPMs will be chosen for their small size and small contribution to the ring
impedance The lower signal produced by the small electrodes will be compensated by the high
repetition rate of the stored beam (averaging is possible)
BPM electronics detection frequency on the storage ring is usually equal to the RF frequency as it is the
only common frequency component present in the BPM data whatever the filling pattern is Even
though only one bunch will be stored in ThomX the same approach is chosen Therefore the storage
ring BPM electronics detection frequency is 500 MHz
Since it is more convenient to adopt the same electronics for all the BPMs (linac transfer line and
storage ring) stripline BPMs on the Linac and transfer line are optimized to give a signal with the
highest possible component at 500 MHz Their electrode length is thus adapted to this frequency which
means that it has to be equal to λ4 (150 mm 500 MHz) where λ is the RF wavelength
4411 BPM for the linac
The linac BPM equipment will be as follows
- Number of BPM 1
- Type short circuited stripline
- Electrode length 100 mm
- Required resolution 100 microm for a single pass measurement and a 1 nC bunch charge
Figure 44 Linac BPM
lsquo
87
Due to lack of space in the linac section where a stripline must be installed the electrode length cannot
be 150 mm but must be reduced to 100 mm Transfer impedance calculations show that at 500 MHz
the signal amplitude will be reduced by 13 compared to the optimal length of 150 mm (see Fig 45)
This amplitude reduction is acceptable
Feedthroughs will be soldered on CF16 flanges
Figure 45 Stripline transfer impedance versus frequency for various electrode lengths With an
electrode length of 100 mm the signal is reduced by 13 compared to its maximum value for a
frequency of 500 MHz
4412 BPMs for the transfer line
The transfer line will be equipped as follows
- Number of BPM 4
- Type short circuited stripline
- Electrode length 150 mm
- Required resolution 100 microm for a single pass measurement and a 1 nC bunch charge
lsquo
88
Figure 46 Drawing of the BPMs that will equip the electron transfer line
Feedthroughs will be soldered to CF16 flanges
4413 BPMs for the storage ring
The specifications relevant to the ring BPMs are as follows
- Number of BPM 12
- Type button
- Button diameter 107mm
- Required resolution ~1 microm 50 Hz and for a bunch charge of 1 nC
- Absolute precision plusmn 250 microm
In order to provide additional BPM signals for other applications (see below) 4 BPM blocks will be
equipped with a double set of electrodes In that case only 4 electrodes (among 8) will be used for
position monitoring The button size and the beam pipe size remain unchanged
Figure 47 Simple and double BPM blocks for the ThomX storage ring
The button positions and dimensions have been determined by simulations with Poisson Superfish (an
electrostatic simulation software) An evaluation of the impedance due to the button discontinuities has
shown that their effect will be negligible
Feedthroughs will be soldered directly to the BPM block They are bakable Connectors will be SMA
males to prevent any deformation during baking
lsquo
89
Alignment
BPM blocks will have an external mechanical reference This reference will be used for BPM alignment
during the installation phase and to position precisely the block in a calibration bench
Calibration
Storage ring BPMs will be calibrated by using a dedicated bench An antenna inside the block will be
fed with a 500 MHz wave of constant amplitude Calibration is done by measuring the signal output by
the 4 buttons A precise position of the antenna is not needed as position errors can be eliminated by
repeating the measurement with the BPM block turned by 180deg and averaging the results
The second halves of the blocks equipped with two sets of buttons will be dedicated to
‐ longitudinal (beam phase) feedback (one set of BPMs)
‐ transverse feedback (one set of BPMs)
‐ ion clearing To suppress ions which may generate beam instabilities BPM buttons will be
polarized with a high voltage (max plusmn200V) This electric field will deflect the ion trajectories
toward the vacuum chamber
Electronics
Electronics will be the same for all BPMs (whether equipping the linac the transfer line or the storage
ring)
Data rate
Revolution-by-revolution beam position measurements would be best But because of the high
revolution frequency (178 MHz) it might be difficult to get electronic devices that handle the
corresponding data rate Then one may work with averages over two turns This data will be provided
on demand and the acquisition will be triggered The buffer size should accommodate data collected
during at least 20 ms to cover full storage cycles Revolution-by-revolution data will be used for
machine physics studies and tune measurement The position measurement resolution must be better
than 100 microm
Slow acquisition
Slow data acquisition will be used for slow orbit correction and slow orbit feedback It will be obtained
by averaging the fast data flow with a sampling rate of ~10 Hz A data rate of 50 Hz would provide just
one measurement per storage cycle The resolution aimed at for this measurement is one micron or
better
Acquisition must be synchronous with the machine revolution frequency Moreover modules must be
synchronized together
Connections and cables
The four cables that carry the signals of each BPM should be put together in a strand Their electrical
lengths will be matched so that phase differences are kept below 13deg 500 MHz The cable attenuation
should not differ much from one BPM to another If the cable lengths cannot be the same for all BPMs
cable sections will be chosen in order to ensure the same attenuation for all cables To minimize both
lsquo
90
attenuation and cost cable lengths below will be kept below 30 m The LMR240 cable is an example
of what could be used its attenuation is ~6 dBm 500 MHz over a distance of 33 m
As radiation close to the machine can damage the cable insulator short (~1 m) pigtails (that can be
easily replaced if needed) will be inserted between the BPMs and the long cables
442 Orbit feedback system
A feedback system will keep the beam orbit stable within plusmn1 microm It will use the position measurements
provided by the 12 ring BPMs The correction rate will be comprised between 01 Hz and 10 Hz
Correction signals will be applied to the 12 correctors located along the ring The orbit feedback system
will be a matlab based application
443 Tune measurement and feedback
The tune will be obtained from the revolution-by-revolution data provided by the BPMs There is no
need for any kicker to excite betatron oscillations as the beam transverse position will naturally oscillate
after each injection (ie every 20 ms) If necessary a tune feedback will be implemented to keep the
tunes constant
444 Energy measurement
In the middle of the transfer line ie downstream of the dipole located at the end of the linac the
dispersive function is non-zero Therefore the beam energy and its spread can be deduced from the
electron horizontal distribution shown by the YAG screen in DS3 The same measurement will be
performed with DS4 using the dispersion induced by the extraction magnet
45 Bunch length measurement
The bunch length will be measured at two different places at the end of the linac and in the ring
Depending on the observation point two different physical processes are used to produce visible light
pulses which are then transported by dedicated optical lines through the ThomX shielding wall to a
double-sweep streak camera located in the experimental hall
451 Cerenkov radiation
Cerenkov radiation (CR) produced in a target placed at the end of the linac will be used to measure the
e- bunch length Cerenkov radiation is emitted when a charged particle passes through a medium at
velocity greater than the speed of light in that medium It is emitted at an angle θcr ndash with respect to the
particle momentum ndash determined by n(λ) the medium index of refraction and β the particle velocity as
follows
Above 10 MeV the emission angle is almost independent of the particle velocity Ncr the number of
radiated photons whose wavelengths range between λ1 and λ2 summed over a path of length d along the
charged particle trajectory is
( )
minus
minus
λnβλλπαd=N
cr 22
21
11
112
( )
minus
minus
λnβλλπαd=N
cr 22
21
11
112
( )( )λβn
=θcr
1cos
lsquo
91
where α=73 10-3
is the fine structure constant
Two types of radiator are usually used to produce Cerenkov radiation an aerogel or a solid target
Aerogels dramatically spoil the vacuum and therefore have to be mounted in a separate vacuum vessel
to maintain ultra-high vacuum conditions Solid targets are UHV compatible This is why they are
preferred
At the end of the linac when the bending magnet is off the beam goes straight into a beam dump
Before the dump several types of targets can be inserted in the beam trajectory for diagnostic purposes
For the production of Cerenkov radiation a sapphire target is adequate This materialrsquos index of
refraction is 177 With a 50 MeV beam the Cerenkov angle is θcr=5563deg Assuming that the window
thickness is 025 mm 16 photonse- are created with a wavelength ranging between λ1 = 300 nm and λ2
= 800 nm Their spectrum is given in Fig 48 For a 1 nC electron bunch 1011
photons are created The
energy deposited in such a target is negligible
Figure 48 Spectrum of the Cerenkov radiation emitted by a 50 MeV e- beam passing through of
sapphire window 025 mm thick
For simplicity the sapphire target is oriented so that the Cerenkov radiation exits in the vertical
direction For a Cerenkov angle θcr = 5563deg the window angle with respect to the beam axis must be θP
= 11475deg (see Fig 49)
8x10-8
6
4
2
0
In
ten
sity
[au
]
800700600500400300200
Wavelength [nm]
lsquo
92
Figure 49 Orientation of the sapphire target used to observe the Cerenkov light emitted by the e- beam
at the linac end
At the target location the beam dimensions will be σxy= 14 mm (rms) It will produce a light spot
whose dimensions will be σx = 14 mm (rms) and σy = 154 mm (rms) in the sapphire To intercept the
whole light beam and to leave some margin for the alignment we will use a target whose diameter will
be equal to 2 x 6 x σy ~ 20 mm
The Cerenkov radiation will exit the vacuum chamber via a UHV viewport made of fused silica (FuSi)
with a 90 transmission over the 300-800 nm wavelength range The viewport is mounted on a CF63
flange that provides a 68 mm clear aperture At the exit of the viewport a FuSi lens is used to focus the
light This collimated beam is then transported via a set of planar mirrors across the shielded area and
then through the ThomX shield to a dedicated hutch in the experimental hall (see sect 453)
452 Synchrotron radiation
Most of the synchrotron radiation (SR) emitted when the electrons circulate in a bending magnet (BM)
will be in the far infrared but visible light will also be present The cut-off wavelength of this radiation
corresponds to ~077 eV Thus the X-ray content is negligible This last feature makes the extraction of
the visible part of the spectrum easy The spectral flux per unit bandwidth per unit surface ΦBM
[Photons01bwmmsup2] is shown in Figure 410
Figure 410 On-axis spectral flux 1 m downstream of a bending magnet Photon energy in eV The
visible spectral range corresponds to [400 nm - 800 nm] or [155 eV - 31 eV]
The radiation is extracted at the exit of the BM ndeg5 through a porthole which provides a 63 mrad x 32
mrad aperture at 15deg with respect to the 0deg incident angle in the dipole The maximum power density is
lsquo
93
4 microWmmsup2 in the horizontal plane of this extraction porthole thus no dedicated absorber is required
(But for the porthole region the radiated power will be dissipated in the vacuum chamber wall) The
radiation will exit the vacuum chamber through a UHV viewport made of MgF2 (this material offers
the highest transmission coefficient over the 200-800 nm range) The synchrotron radiation distribution
at 5 eV one meter downstream of the bending magnet is around 6 mm (FWHM) So the spectral flux
integrated over a collecting area of 6 mm x 63 mm is about (around 5 eV)
Φe [Phs] = ΦBM [Phs01bwmmsup2] x 6 mm x 63 mm x 1000
Since photons of lower energy thus with a higher divergence will also contribute to the total spectral
flux this value gives a low limit estimate of the real total spectral flux Figure 411 shows the Φe
spectral flux
As was the case for the Cerenkov radiation a FuSi lens will be used at the exit of the viewport to focus
the beam The collimated radiation will then be transported via a set of planar mirrors across the
shielded area and then through the ThomX shield to a dedicated hutch in the experimental hall (see next
section)
Figure 411 Spectral flux Φe in a 63 x 32 mrad2 aperture 1 m downstream of a bending magnet
453 Transport of the Cerenkov and synchrotron radiations to the diagnostics hutch
Both the Cerenkov light and the synchrotron radiation have to be transported to the diagnostics hutch
located in the experimental hall Starting from different locations both radiations will be transported via
a set of planar mirrors to a lsquomeeting pointrsquo inside the shielded area A flip mirror (ie a double-
orientation mirror) will enable the selection of one of the radiations The one selected will be further
transported again with a set of planar mirrors through the shielding wall to the diagnostics hutch
Inside the diagnostics hutch the last lens will focus the radiation on the streak camera entrance The
lens aperture and its focal length will be adapted to the selected incoming radiation All the line
components and their parameters are chosen to minimize spatial aberrations and to maximize the
number of photons entering the streak camera
454 Streak camera for the bunch length measurement
We plan to use a double-sweep streak camera to measure the bunch length and to visualize its evolution
over short time scales Since the bunch length may vary from 4 to 20 ps (rms) a 2 ps (FWHM)
resolution streak camera is required
lsquo
94
The time resolution of the bunch length measurement depends on the radiation type (CR or SR) which
is used and involves several contributions the electron transverse beam size at the light emission point
chromatic effects etc All these contributions will be minimized as much as possible to maintain the
effective resolution as close as possible to the one of the streak camera
46 Bunch imaging using synchrotron radiation
The synchrotron radiation transported to perform the bunch length measurement (see section 45) can
also be used to image the bunch while it circulates For this purpose a beam splitter could be installed
at the radiation exit port the main part of the radiation would be sent to the streak camera while a small
fraction would be directed to a CCD camera mounted nearby A focusing lens placed between the
viewport and the CCD would enable the imaging of the stored beam
In the orbit section where the bunch emits the synchrotron radiation used for the above measurements
the e- beam transverse size is about 05 mm (rms) Since the size of a CCD chip is typically 5 mm the
lens gain should be about 10 Such a set-up would enable one to follow the evolution of the beam size
in real time (we will use a CCD whose acquisition rate is about 10 Hz) It would be a non-destructive
and cheap diagnostic Similar setups are currently used on storage rings to measure the electron beam
emittance assuming that the beta function is known
47 Beam loss measurement
Particle losses will be measured by using optical fibers [46] When a charged particle crosses a glass
fiber Cerenkov radiation is emitted within the fiber and propagates along the fiber It can be detected
by a photomultiplier that collects the light at the fiber end The measurement eg with an oscilloscope
of the pulse production (in the fiber) time enables one to recover the particle loss location
Two fibers will be installed One will be used to monitor e- losses along the linac and the transfer line
The second fiber will be used to monitor particle losses along the ring Each one of these fibers will be
connected to a fast photomultiplier (PMT) The PMT time resolution will determine the accuracy with
which the losses can be localized
A fused silica fiber has a yield of 227 photons per cm of a relativistic electron track [47 48] Therefore
a 5 mm thick fiber will have a yield of ~100 photons per relativistic electron which crosses it Such a
signal should be easily detected by a PMT
As it may be difficult to place the fiber close to the beam pipe in some parts of the ring the fiber may
have to be located at a distance d away from the beam axis Then the number of lost particles which will
cross the fiber will be reduced by a factor dsup2
48 Beam dumps
The ThomX set-up will include two particle dumps to stop the e- beam in such conditions that the
radioactivity then generated is minimal The first dump will be located at the linac exit The second
dump will be located downstream of the injectionextraction section The latter dump can be used to
stop the beam either at the exit of the transfer line or after any number of bunch revolutions in the ring
Both dumps are based on the SOLEIL design and can handle the 35 W power deposition (1 nC 70
MeV 50 Hz repetition rate)
lsquo
95
Chapter 5 SYNCHRONIZATION
51 Overall scheme
An ultra-low noise 500 MHz synthesizer will be used as the main clock for ThomX The 10 MHz
output of this synthesizer will be used to drive a second synthesizer (the so-called lsquolinac synthesizerrsquo)
operating at 3 GHz The RF output of the latter synthesizer will be used for the linac operation It will
be split in two lines The first one will be used to synchronize the gun laser oscillator (after appropriate
down conversion of the frequency) The second line will be used to drive the gun and the linac RF
The RF output of the main synthesizer (500 MHz) will be used for the ring RF it will be split in three
lines The first line will drive the ring RF (after appropriate phase shifting) The frequency of the second
line will be divided by 14 this line will synchronize the Compton cavity As in the MightyLaser
experiment [51] the laser used to inject power in the Compton cavity will be synchronized with that
cavity by using the PDH method The frequency of the third line will be divided by 28 and will
therefore be synchronous with the electron revolution frequency This line will be used to synchronize
programmable delays delivering turn-by-turn triggers and injectionextraction triggers To ensure that
injection is always taking place at a zero crossing of the mains the programmable delays will also be
synchronized with the mains A counter will reduce the mains frequency to the operations frequency
(when lower than 50Hz) The gun trigger will be generated when the mains will be at a zero crossing
and the ring RF at a preset phase that takes into account the time it will take for the electrons to travel
from the linac gun to the ring accelerating cavity
To ensure that the e- bunches will always be injected at a triple zero (50 Hz 500 MHz and 3 GHz) we
need to specify the tolerance on the 50 Hz zero crossing We may take 1 (ie 02 ms) for this
tolerance this means that the coincidence between the ring RF (~500 MHz) and the linac RF (~3 GHz)
must occur at a frequency greater than 5 kHz Therefore by choosing a heterodyne frequency (ie the
frequency difference between the linac RF divided by 6 and the ring RF) greater than 5 kHz we ensure
that there will always be a common zero crossing between the ring RF and the linac RF within 1 of
the mains zero crossing This sets a constraint on the ring radius of 10-5
ie 168 micrometres on a
168 m long orbit (see Table 2)
The RF bucket must be positioned with an accuracy of 3 ps in the ring RF Thus the phase noise
between the linac RF and the ring RF must be lower than 3 ps when integrated between 50 Hz and
500 MHz Most good-quality synthesizers achieve such a performance
The information collected by the ring diagnostics will be used to adjust the exact value of the frequency
(around 500 MHz) generated by the main synthesizer so that it matches the exact ring length This
information will also be used to adjust the phase between the injection trigger and the ring
To reach the correct signal level each RF line may have to be amplified An isolator will be added to
avoid reflections that may degrade the overall performance of the system
To ensure long-term low frequency synchronization a Network Time Protocol (NTP) server will be
available to allow all computers working at ThomX to synchronize their internal clock
lsquo
96
52 Specifications
Synthesizer
The main synthesizer will be used to generate a 500 MHz adjustable signal Its phase noise jitter
integrated between 50 Hz and 20 MHz must be less than a few picoseconds It must support a frequency
modulation of at least 100 kHz Typical mid-range synthesizers meet such specifications
Measurements done at LAL in collaboration with SOLEIL have shown that the addition of an external
frequency reference does not necessarily improve the phase noise of these synthesizers in the frequency
range of interest
The linac synthesizer specifications are the same ones except that it must be able to generate a
frequency of 3 GHz
Programmable delay
Two low jitters units with 8 channels each will be used
Phase shifter
The phase shifters must be voltage controlled and cover 360deg (two 180deg units)
Figure 51 Synchronization layout
lsquo
97
Chapter 6 OPTICAL SYSTEM DESIGN
Since the writing of the ThomX CDR three directions have been pursued improvement of the feedback
system improvement of the laser amplification and mechanical design of the optical resonator The
results are described in the following three sections
In the first section the ThomX feedback system is described During the past year a strong RampD
program has been developed on the locking of a TiSapph picosecond laser beam on a two-mirror high
finesse cavity at OrsayLAL Very good results in operating for the first time a 30 000 finesse cavity
in the picosecond regime were obtained The feedback techniques used for this task are directly usable
for ThomX and are reported in this section A 3000 finesse four-mirror cavity at KEK was also installed
and operated and we succeeded in producing γ rays off the ATF electron bunches (Mightylaser project
[61]) Hardware damages were however caused by the Japanese 2011 earthquake At the time of
writing this document we are recovering all our equipment to reach our final goal locking a 30000
finesse four-mirror cavity to produce a high γ ray flux at ATF Thanks to a contact with the SYRTE
laboratory we have acquired a new laser oscillator with a 1030 nm wavelength that we are testing and
that we plan to use for ThomX With this oscillator by embedding an intra-cavity EOM we are able to
increase our feedback bandwidth by more than one order of magnitude
The second section illustrates the laser amplification developments done at CELIA First a non-all-
connected fiber amplifier for the Mightylaser project was set up that allows us to extract an average
power of 50 W This amplifier was used to produce γ rays at ATF A new setup has been designed and
built to reach both the reliability and the power level required by ThomX An important activity is
devoted to the construction of a fully all-connected amplification chain As mentioned above the new
laser oscillator and the new laser amplification chain are being integrated in order to fill a 30000 finesse
high finesse cavity to conclude the ANR Mightylaser project
In the third section the details of the implementation of the cavity in the electron ring and the cavity
vacuum system are described with the opto-mechanical system that will be used for a precise alignment
of the cavity mirrors
61 Feedback system and results on a Fabry-Perot cavity locking
To produce a high X-ray flux the ThomX facility produces high intensity laser pulses and collides them
with electrons stored in a ring Hence the locking system (see figure below) has a double goal
- Stabilize the synchronization of the Fabry-Perot cavity (FPC) photon pulse with the electron bunch
- Stabilize the synchronization of the laser pulses with the FPC to obtain amplification
For this last point as will be seen in section 62 firstly the laser beam will be amplified with a CPA
technique using optically pumped fibers and then it will be amplified again by accumulation during
many round trips into a high finesse FPC (the target is a gain of 10000 which leads to a finesse of
30000)
This section describes the following topics related to the stabilization of the laser pulses relative to the
FPC
- Parameters to be stabilized repetition rate and carrier-envelop phase (CEP)
- Error signals the Pound-Drever-Hall (PDH) measurement technique
lsquo
98
- Actuators dynamic range and bandwidth
- Behavior of the digital feedback system (DFS)
General scheme for the locking system Detection and actuation elements are shown in red
The results on the locking of a TiSapph laser oscillator on a 30000 finesse cavity will also be
illustrated
Figure 61 Locking scheme for the synchronization of the electron bunch and the FPC pulse for the
ATF experiment at KEK
611 Electron and FPC pulse synchronization
This kind of stabilization has already been achieved for the MightyLaser experiment at KEK (ATF) a
1785MHz repetition rate A digital feedback system was used (i) to acquire two clocks one from the
laser and one from the master oscillator of the electron ring (ii) to perform a digital phase detection
(iii) to build a digital integrator filter to remove phase drifts and (iv) to react on a PZT inside the FPC
to change its length according to the measured phase drift (see Fig 61)
lsquo
99
612 Laser parameters to be stabilized
We will choose a laser with a repetition rate corresponding to twice the e- bunch revolution frequency
in the ring (two laser pulses per electron bunch revolution) around 500MHz14 = 357 MHz A phase
noise measurement on a similar product operating 1785 MHz was already carried out to characterize
the repetition rate phase noise and the drift (see Fig 62 and Fig 63)
Figure 62 Repetition rate phase noise spectrum Figure 63 Repetition rate drift measured
X-axis offset frequency from the carrier in Hz during about one hour X-axis repetition
Y-axis single side band phase noise power rate drift in Hz Y-axis time duration in
density in dBcHz seconds
The global shape of the phase noise in 1f will permit to use three feedback actuarurs a high bandwidth
low dynamic one (EOM for repetition rate and AOM or pump current for CEP) a middle
bandwidthmiddle dynamic one (PZT for repetition rate) and a high dynamiclow bandwidth one
(mirror motor and isochronic wedge motor)
For this kind of laser the harmonic content (index m) is well known and can be represented either in the
time domain or in the spectral domain (see Fig 63 and Fig 64)
( )2
CEL REP CE REPf m m f f m f
φ
π
Δ = + = +
lsquo
100
Figure 64 Time domain and spectral representation of the laser pulses
Since one is dealing with two parameters (the repetition rate and the CEP) one needs two different
sources of information to measure them The standard method to measure these two parameters consists
in using the Pound-Drever-Hall (PDH) technique [62] which gives the position of the laser harmonics
relative to the FPC resonances
( )FPC CAVf m m f=
One of the parameters is deduced from the whole beam signal using a central photodiode the second
one is derived from a part of the spectrum selected by a grating and a slit and measured by the grating
photodiode
If the repetition rate and the FPC round trip frequency are close enough the error signal coming from
the central photodiode (index m0) is proportional to the difference between the laser harmonics and the
corresponding resonances of the FPC
1 0 0
2
CEREP CAVm f m f
φε
π
Δ prop + minus
Frequency
domain
lsquo
101
As the locking system cancels the error the central laser harmonic m0 is perfectly aligned on the FPC
resonance but all other laser harmonics (m=m0+Δm) are slightly detuned from the corresponding FPC
resonances This introduces a correlation between the CEP and the repetition rate
( )( )
1 0 0
2 0
0 0
02
1 12
CEREP CAV
CEREP CAV REP CE
m f m f
m mm m f f f f
m m
φε
π
φε
π
Δ = rArr + =
Δprop minus minus = minus = minus
As a result of these equations the signal received by the PDH system located after the grating (ε2) is
directly proportional to the offset frequency (fCE) and then to the CEP Thus if we change the CEP of
the laser we should clearly see on the grating photodiode PDH signal ndash which measures the laser-FPC
detuning on the border of the spectrum (for an m index far from m0) ndash a linear relation between the error
signal and the CEP
This method well known for femtosecond lasers has been tested in our laboratory in the picosecond
regime Figure 65 shows our results (submitted to Applied Optics) on the locking of a 76MHz
TiSapph picosecond laser oscillator on a 30000 finesse cavity
Figure 65 Top experimental measurement of the CEP value of a TiSapph laser oscillator Bottom
measurement of the corresponding grating photodiode signal
In Fig 65 one sees that when the CEP is measured close enough to zero the grating photodiode
exhibits a linear relation with the CEP otherwise the photodiode is saturated (due to the PDH signal
saturation) In our experiment the ratio between the cavity reflected power and the incident beam
power was ~20 which leads to a value of 80 for the coupling of the incident beam power to the
cavity
These results demonstrate for the first time that a picosecond laser oscillator can be locked to a 30000
finesse cavity (implying a power enhancement factor of 10000) with a power coupling of 80
613 PDH Measurement technique
Having shown that we are able to measure independently both parameters of interest (the repetition rate
and the CEP) we will now describe the apparatus to transform these measurements into error signals by
lsquo
102
the PDH technique (see Fig 66) Side-bands to the laser optical harmonics are produced by an electro-
optic modulator (EOM) modulated by a generator coming from the digital system Side-bands and laser
harmonics are reflected by the FPC the first ones without acquiring any phase the second ones shifted
in relation with the discrepancy between the laser and the FPC frequencies All the harmonics and the
side-bands of the reflected laser are combined into the photodiodes and the front-end electronics giving
an error signal at the modulation frequency
Figure 66 Pound-Drever-Hall scheme with digital phase detection
The photodiode front-end (see Fig 67) amplifies these signals at a level compatible with the digital
feedback system (DFS)
Figure 67 Electronic scheme and picture of the PDH front-end
Inside the DFS the PDH signals are filtered by a band-pass filter to remove unwanted frequencies and
multiplied by the same modulating signal with a phase shift to allow for the phase detection A low-pass
filter at the end of the chain removes the image signal at twice the modulation frequency Figure 68
shows an example of the input and the output signals
lsquo
103
Figure 68 Modulated input (blue) error signal (yellow) FPC transmitted signal (pink)
614 Actuators
In the previous sections we described how the two parameters (the repetition rate and the CEP) of a
pulsed laser have to be controlled in order that all the laser harmonics match the FPC resonances and
we also detailed how to measure the variations of these two parameters We will now describe the
devices inside or around the laser on which we need to react to correct the variations of the laser
harmonics relative to the FPC resonant frequencies
Figure 69 Typical scheme of a laser with all its actuators
The actuators will produce an action on the two parameters to be controlled with different dynamic
ranges and different bandwidths (see chart in Fig 610)
Figure 610 Dynamic ranges and bandwidths of the laser actuators
lsquo
104
All of the actuators are located inside the laser except the AOM (see Fig 610) They are the key items
to achieve a good stabilization
All the actuators measurements have been made on the 1785MHz laser version used at KEK
- Mirror motor dynamic range precision and bandwidth
The motor is a step motor with a precision of 140 nm on the round trip variation (equivalent to 15 Hz on
Frep) Figure 611 shows the linearity and the maximum dynamic range available with the motor stage
while Fig 612 shows a step response which indicates the maximum bandwidth we can obtain
Figure 611 Mirror motor range Figure 612 Time response of Frep for
1k motor steps X-axis repetition rate
offset (Hz) Y-axis repetition rate
The repetition rate range is 178033 - 179244 MHz (gt 1MHz)
- Piezoelectric sensitivity dynamic range and bandwidth
The PZT sensitivity is approximately 3Hz V 28 nmV (length variation to be compared to the
length of a round trip of the FPC pulse)
The PZT can be operated up to 150V providing a total dynamic range of 42 microm (round trip length
change) Its first peak resonance has been measured and found above 40 kHz
lsquo
105
Figure 613 Repetition rate change produced by a Figure 614 PZT frequency response X-
PZT voltage variation from 0 to 28 V frequency range 01-5 kHz Y-axis
Attenuation (dB) ndash arbitrary offset
Figures 613 and 614 show the PZT sensitivity and its transfer function
- EOM sensitivity measurement
Measuring the sensitivity of the EOM is not direct thus we chose to apply a 10 V amplitude ndash 10 Hz
frequency modulation ndash to the EOM and to measure the power of the harmonics (see Fig 616)
Assuming a pure frequency modulation the power of the harmonics fis a Bessel series that depends on
the sensitivity Thus taking the ratio of two consecutive harmonics gives directly the sensitivity
Figure 615 EOM modulation spectrum used for sensitivity measurement X-axis frequency centered
on the repetition rate Y-axis relative optical power around the repetition rate due to the modulation
The table on the left gives the frequency and the power of each harmonic
0 5 10 15 20 25 300
10
20
30
40
50
60
70
80
90
100
PZT voltage (V)
Fre
p v
ariation (
Hz)
Frep variation vs PZT voltage
AJ0(z)
AJ1(z)
AJ2(z)
lsquo
106
We now check that the sensitivity calculated from the ratio of the first harmonic to the fundamental and
the one obtained from the ratio of the second harmonic to the fundamental are the same
∆
∆
~1
2
∆
10~ minus 2909 rarr ∆~07
∆
∆
~1
8
∆
10~ minus 6398 rarr ∆~07
Since the ratio is 07 Hz for an applied sinusoidal signal of 10 V the sensitivity is around 007 HzV
- Isochronic wedge
It is made of two glass prisms with different refraction indexes chosen to maintain a constant group
delay in order to avoid repetition rate variation
We measured that the maximum variation of the repetition rate for a full CEP range (several times 2π)
is about 300 Hz for a repetition rate of 1785 MHz (see Fig 616 which shows the measured repetition
rate shift plotted along the X-axis versus the date of the measurement plotted along the Y-axis) Thus
the influence of this actuator on the repetition rate is about 210-6
Figure 616 Measurement of the influence of wedge displacement on Frep
- Acousto-Optic Modulator (AOM)
Such an actuator has already been used with a laser which is not the same as the one used for the other
actuators namely a 76 MHz laser locked on a two-mirror FPC
In Fig 617 we present the closed loop measurement The lowest frequency part (lt10 kHz) is
attenuated by the loop gain If the loop were open this part of the transfer function would be quite flat
To estimate the AOM bandwidth limits we can use the frequencies where the gain starts to drop
significantly this bandwidth is about 300 kHz
300 Hz
lsquo
107
Figure 67 Frequency response of the AOM in closed loop
615 The digital feedback system
The feedback system will deal with several coupled inputs and outputs To ensure flexibility we have
chosen a digital system Since we also want to work with a large bandwidth in closed loop (gt 1MHz) a
low latency system (lt 100 ns) is needed A digital system featuring such performance does not exist on
the shelf Thus we decided to design and build such a system at LAL (see Fig 618)
Figure 618 Scheme of the DFS and picture of the DAC daughterboard
This system will perform several functions (see Fig 619)
103
104
105
106
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
Frequency (Hz)
Magnitude (
dB
)
Bode plot
lsquo
108
Figure 619 Organization of the digital feedback functions
- The first important function is to produce the error signal from the modulated signal coming from the
photodiodes It is a phase detection block (Digital PDH DPDH) the main problem being to reduce the
latency as much as possible This will be achieved by optimizing the low-pass filter and the modulation
frequency (see Fig 620 for a digital implementation)
Figure 620 Digital phase detection (DPDH) to produce the PDH error signals
- The second important function is to provide Infinite Impulse Response (IIR) filters to build
Proportional-Integrator (PI) filters in order to cancel the system drifts
lsquo
109
Figure 621 Digital IIR filter implementation with adders and multipliers
These filters can be programmed by the user to change the filter coefficients and it can be cascaded to
realize complex filters They have been cut in Second Order Sections (SOS) to minimize the
propagation of fixed precision errors due to quantized data and coefficients (see Fig 621)
- The last important function integrated in the digital system is performed by an identification block
which works like a network analyzer by (i) sending a small harmonic signal (ii) looking at the system
response in closed loop at this frequency and (iii) sweeping this small harmonic signal frequency to
cover all the range to be measured (see Fog 622)
Figure 622 Identification system
lsquo
110
This kind of identification system can provide information on (i) the feedback system itself (ii) the
system to be stabilized or (iii) the open and closed loop transfer functions by producing Bode or
BlackNichols diagrams (see Fig 623) These informations are very useful for system analysis and for
tuning or designing the feedback filters
Figure 623 Typical Bode (top-left magnitude bottom-left phase) and Black-Nichols (right) diagrams
produced by the identification system
62 High average power laser system and its coupling to a high gain FP cavity
The all-fibered laser system will draw from the system developed in the MightyLaser project It will
consist in an oscillator associated with three amplifying stages involving single and double clad Yb-
doped fibers (see Fig 625) At present the amplifiers are based on commercial components to facilitate
a potential replacement of damaged parts The architecture is designed to provide the mandatory
diffraction-limited beam This relatively simple and compact architecture will implement the lsquoChirp
Pulse Amplificationrsquo (CPA) scheme In order to limit non-linearity effects in the fiber the signal is
initially stretched in time thus reducing its intensity in the waveguide then amplified in the fiber and
finally recompressed to its minimal duration The difference between the MightyLaser and the ThomX
repetition frequencies (respectively 1785 MHz and ~357 MHz imposed by the different repetition rates
of the two accelerators) requires an adjustment of the laser design
Figure 625 Sketch of the all-fiber laser system
lsquo
111
621 The MightyLaser oscillator
This oscillator is an all-normal dispersion mode-locked fiber laser based on the nonlinear polarization
evolution technique equipped with a fibered output It uses an ytterbium doped fiber which allows an
emission centered around 1030 nm with a full width at half maximum of 10 nm (see Fig 626) It
delivers short pulses at a repetition rate of 1785 MHz with a low average output power (~10 mW) in
order to minimize the noise level (see Fig 628) The main advantage of the all-normal dispersion
regime resides in the significant peak power reduction provided by the pulse stretching during the
propagation along the cavity Non-linear effects are thus reduced allowing a high-energy extraction
The output pulses are naturally chirped with an estimated duration of 2 ps
Figure 626 Emission spectrum of the oscillator
The oscillator is passively mode-locked by taking advantage of the non-linear evolution of the
polarization This technique is based on the optical Kerr effect The polarization of a pulse propagating
in a fiber evolves non-linearly as a function of intensity Thus by combining polarization controllers
and a polarizer one may generate losses that depend on the intensity (see Fig 627) Beyond the
polarizer all the pulse components are linearly polarized The wave plates transform the linear
polarization into an elliptic one The latter polarization state then evolves in a non-linear way while it
propagates along the fiber This evolution results from the combined effect of the self-phase and the
cross-phase modulations induced on the two polarization components The polarization rotation of the
more intense central part of the pulse is larger than the polarization rotation of the pulse wings Since
the pulse center crosses the analyzer with a reduced loss compared to the wingsrsquo loss the pulse duration
is shortened
Figure 627 Principle of mode locking based on the non-linear polarization evolution
lsquo
112
Figure 628 Spectrum of the oscillator phase noise noise level (dBcHz) versus offset frequency (Hz)
622 Amplification stages
The amplification chain is divided in three stages in order to distribute the gain and to saturate the
second and the third amplifying stages The first and the second stages are based on the same
architecture It is an all-fibered amplifier whose central component is a polarization maintaining single
mode using an Ytterbium doped fiber with a core diameter of 6 microm and a clad diameter of 125 microm (see
Fig 629) A wavelength division multiplexer (WDM) is spliced on the doped fiber to transfer the pump
power into the gain medium and to couple the signal of the picosecond oscillator to the amplifier The
first stage is pumped by a single mode diode centered at 976 nm and delivering 200 mW The second
one is pumped at a higher average power namely600 mW The two amplifying stages are separated by
an optical isolator to avoid any parasitic feedback from one amplifier to the other
Figure 629 Schematic of the first and the second amplification stages
The average power level at the input of the first amplifying stage is about 1 mW A limitation is
imposed on the output of this stage which should remain below the damage threshold of the isolator
fixed at 100 mW Consequently we adjust the pump level in order to obtain 80 mW at the output of the
ytterbium-doped fiber The 3 dB losses of the isolator allow one to inject 40 mW into the second
amplifying stage The evolution of the average output power versus the pump level is shown in Fig
630 For the maximum pump level of 600 mW the second amplifying stage delivers 330 mW
lsquo
113
Figure 630 Second stage output power plotted versus the pump power
Beyond the second isolator the third amplifying stage (see Fig 632) is built by using as its central
component a polarization maintaining double-clad Ytterbium-doped fiber with a core diameter of 25
microm and a clad diameter of 250 microm The amplifier is clad-pumped through a special combiner 6+11
(see Fig 631) In practice this component has an input signal fiber 6125 microm directly spliced to the
previous amplification stages an output signal fiber 25250 microm spliced to the double-clad Ytterbium
doped fiber and six multimode fibers of 105 microm to couple the pump diodes Here only three pump
ports are spliced to multimode diodes emitting up to 55 W at 976 nm corresponding to a pump power of
at most 165 W The development of a mode field adapter was necessary to splice the pump combiner to
the double-clad Ytterbium-doped fiber because their numerical apertures are different Losses entailed
by this component are compensated by the last amplification stage
Figure 631 Pump combiner Figure 632 Schematic of the third amplification stage
Figure 633 is a plot of the average output power versus the pump level For a pump power level of
76W the last amplification stage delivers 52W This was the first objective The fiber laser power
stability was measured during 1 12 hour at this power level (see Fig 634) The residual pump power
the pump and signal feedbacks into the pump diodes are also plotted in this figure
lsquo
114
Figure 633 Third stage output power Figure 634 Laser stability at 52 W
versus the pump power
New measurements were made after optimizing the central wavelength of the multi-mode pump diodes
The efficiency was increased to reach 787 for an output power of 24 W (see Fig 635) The fiber
laser power has been tested at this level during two hours (see Fig 636) The observed power variation
is less than 01
Figure 635 Output power versus pump power Figure 636 Laser stability at 24 W
after wavelength optimization
The all-fibered amplifier setup has been integrated in a 19rsquorsquo rack as shown in Fig 637 An FCPC
connector placed in the front panel allows the connection of the fibered oscillator to the master
amplifier module Several 955 optical fiber couplers have been inserted in our setup at strategic
locations to monitor the power level in real time by using photodiodes they may also signal potential
problems The output fiber comes out of the module through its front panel The back-side is dedicated
to all the electronic controls (diode pump photodiodes and modulators for noise management described
elsewhere)
lsquo
115
Figure 637 Pictures of the 52 W all-fibered setup integrated in a 19rsquorsquo rack
623 Stretcher and compressor
A chirp volume Bragg grating (CVBG) is used in our setup to stretch and compress the pulse with a
very good efficiency of 94 The CVBG transmission bandwidth is around 2 nm (see Fig 638) leading
to a pulse duration ranging from 200 to 250 ps depending on the grating dispersion It is possible to
recompress the pulse after amplification by using a second CVBG Then the pulse length ranges
between 20 and 87 ps depending on the second CVBG dispersion (see Fig 639)
Figure 638 Transmission bandwidth of the CVBG
1‐2 couple 3‐2 couple 3‐1 couple
Figure 639 Time profile of the recompressed pulses for different grating pairs
624 ThomX development
Some changes must be made to the all-fibered lsquoMightylaserrsquo design in order to use it in the ThomX
project The oscillator repetition rate needs to be scaled down to match the frequency imposed by the
accelerator As far as the laser design is concerned the stretchercompressor couple must be modified to
limit non-linear effects The increase of the output power from 50 to 150 W imposes an upgrade of the
second amplification stage and eventually of the third one
lsquo
116
6241 Oscillator
The oscillator for ThomX will be based on the same principle as the one used for Mightylaser The only
difference is the repetition frequency imposed by the accelerator (357 MHz for ThomX) The low
average output power around 5 mW permits to limit the phase noise A particular attention will be paid
to the noise level of such a long cavity as no commercial products have been tested at this repetition
rate
6242 Amplification stages
For the ThomX project the lower repetition rate and the higher average output power require a change
of the second amplification stage architecture in order to limit non-linear effects and to saturate the last
amplification stage Consequently the second pre-amplifier will be based on a polarization-maintaining
double-clad Ytterbium doped fiber with a core diameter of 15 microm and a clad diameter of 130 microm (see
Fig 640) The amplifier will be clad-pumped through a special combiner 2+11 with an input signal
fiber 6125 microm directly spliced to the previous amplification stages an output signal fiber of 15130
microm spliced to the double-clad Ytterbium doped fiber and two multimode fibers of 105 microm to couple
the 9 W pump diodes
Figure 640 Schematic of the second amplification stage of the ThomX laser
Concerning the final amplification stage various fibers are available (see below) Micro-structured
fibers can be engineered and tailored but suffer from the lack of splicing techniques Hence their use in
an all-fibered setup will require additional developments dedicated to splicing (see Fig 641) and
integration If splicing is not possible we still have the possibility of proceeding with air propagation
and injection However this approach requires precise mechanics control and advanced engineering
The lsquocoherent beam combining techniquersquo will also be considered at this point In such a configuration
amplification would be supported by two parallel amplification stages dividing by two the energy level
in the gain medium Yet a first version will be developed with commercially spliceable flexible fibers
and preliminary tests of cavity locking with high power from these fibers will be performed to validate
or invalidate operating with a high level of non-linear effects
lsquo
117
Fig641 A micro-structured fiber
6243 Stretcher and compressor
Because of the high average output power required by ThomX we must modify the stretcher size Two
techniques can be considered First we may continue to use a CVBG which can stretch the pulse up to
450 ps with a maximum size of 25 cm (see Fig 642) It would be possible to stretch the pulse up to
900 ps with an efficiency of 88 through a double pass in a CVBG or a single pass in cascaded CVBG
The second way involves a chirp fiber brag grating (CFBG) as depicted in Fig 643 When associated
with an optical circulator it allows a pulse stretching with up to 800 ps in an all-fibered setup The
efficiency is around 70 (92 for the CFBG and 75 for the circulator) in this configuration This
solution is less efficient than the CVBG but the all-fibered configuration ensures the set-up robustness
and stability and the losses can be easily recovered by the amplifier chain However dispersion issues
have been reported for these devices and a detailed investigation is needed prior to a definitive
integration
Fig 642 Examples of CVBG Fig643 Example of CFBG
The high output average power forces us to use a CVBG to compress the pulse one or two units
depending on the stretcher configuration The final pulse duration can be varied between 1 ps and 50 ps
thus leaving the possibility to optimize the X-ray production by adjusting the pulse duration
We also performed simulations to evaluate the influence of the couple stretchingfiber core diameter on
the non-linearity with 3 different fiber cores 25 microm 30 microm and 40 microm Our results are summarized in
Table 61
lsquo
118
Fiber
dimensions
mode field
diameter stretching B-Integral
25250 20 microm 250 ps 47 rad
25250 20 microm 500 ps 263 rad
25250 20 microm 1 ns 118 rad
30150 22 microm 1 ns 09 rad
40200 30 microm 1 ns 0525 rad
Table 61 Pulse stretching as a function of the fiber core diameter (simulations)
A first configuration will be developed with a stretching of 500 ps If the non-linearity effects remain
too high to allow for an efficient locking on the Fabry-Perot cavity we will modify the setup to induce
a larger stretching factor or insert a fiber with a larger core diameter in the last amplification stage or
both
63 Mechanical design
Collisions between the laser pulse and the stored electrons will take place between two dipoles (see Fig
643) The constraints on the optical cavity mechanical design are as follows
- High stability level (vibrations)
- Good accessibility to the cavity mirrors
- Weak impact on the electron beam dynamics
- Ultra high vacuum (UHV) system with baking capability
- Motorized granite table for micrometric scan oof the electron beam
lsquo
119
Figure 643 Implementation of the cavity in the electron ring
The cavity mirror system is mounted on a massive granite table which can be driven to perform a
micrometric electron beam scan (see Fig 643)
lsquo
120
Figure 644 Scheme of the Fabry-Perot cavity mounted on the optical table
The Fabry-Perot cavity is composed of two cylindrical UHV vessels linked by a dipole chamber
Bellows are placed between each element to absorb the dilatation due to the backing (see Fig 644)
e‐
e‐
X‐Ray
1rsquorsquoMirrors
IP
Bellows
1rsquorsquoMirrors
Vessel
Vessel
Dipole
chamber
lsquo
121
Figure 645 On the left detail of the standard mirror assembly and on the right detail of the piezo
electric mirror assembly
The Fabry-Perot cavity is composed of two spherical and one flat mirror mounted in a standard
assembly and one flat mirror mounted on a special assembly with a piezo electric actuator for very
small longitudinal displacements (see Fig 645)
Figure 646 Detail of a truck system
Mirrors
Piezo
electric
actuator
Mirror
Gimbal
Actuators
lsquo
122
Figure 647 Cut of an UHV actuator
All the mirrors are mounted on a truck system that porvides three degrees of freedom (two rotations and
one translation) driven by three micrometric actuators These actuators are specially designed to be
compatible with UHV and backing constraints (see Fig 646 and Fig 647)
Figure 648 Cut of a vacuum vessel
Trucks (two trucks per vacuum vessel) and plugs with special cables composed of ceramic connectors
oxygen- free copper cables and ceramic pearls and spacers for electrical isolation are placed in a UHV
vacuum vessel One of the vacuum vessels has a beryllium window to extract the X-ray beam (see Fig 648)
Micrometric Actuator
Capsule
Trucks
Beryllium
window
lsquo
123
631 Optical cavity vacuum
A simulation was carried out in the most unfavorable case when the whole optical vacuum chamber is not
baked The area of one half of the optical cavity is about 2 m2 the conductance of the rectangular chamber
connecting the two cavity halves is C =70810-1
m3s
-1 (value for one half of that rectangular chamber) The
conductances of the slits connecting the optical cavity to the ring are respectively C = 45210-2
m3s
-1 (9 x 48)
and C = 16010-2
m3s
-1 (6 x 30) The outgassing rate of one optical cavity half without any baking (σ = 210
-8
Pams-1
) is Q = 410-8
Pam3s
-1 The outgassing rate of one rectangular chamber half (σ=10
-9 Pams
-1) is Q =
7610-10
Pam3s
-1 The outgassing rates of the two slits (σ=10
-9 Pams
-1) are respectively Q = 22810
-13
Pam3s
-1 (9x48) and Q=14410
-13 Pam
3s
-1 (6x30) The diode ion pump used for the optical cavity has a
pumping speed of 05 m3s
-1 DN 150 S = 0320 m
3s
-1 at ~10
-10 mbar The conductance of the pumping port
being C = 136 m3s
-1 the effective pumping speed is Seff= 0278 m
3s
-1 The diode ion pump used for one half
of the rectangular chamber has a pumping speed of 015 m3s
-1 DN 100 S = 0090 m
3s
-1 at ~10
-10 mbar The
conductance of the pumping port being C = 0046 m3s
-1 the effective pumping speed is Seff= 0075 m
3s
-1
The outgassing rate of the optical cavity without baking is very high 2 x 410-8
Pam3s
-1 (which is
approximately 40 times the outgassing rate of the whole ring) With such a rate one would need a pumping
speed of 8 m3s
-1 to reach the required pressure level of 10
-8 Pa (the present pumping speed is 07 m
3s
-1) The
conductance of the rectangular chamber that connects the two halves of the optical cavity (710-1
m3s
-1) and
the conductance of the two slits between the optical cavity and the ring (610-2
m3s
-1) are high These figures
do not allow a differential pumping between the cavity and the ring Simulations of the ring vacuum level
show that it is mandatory to bake the whole optical cavity
Figure 649 Location of the cavity pumps
The vacuum equipment needed for the optical cavity is as follows
‐ 2 ion pumps 150 l s-1
with their baking equipment and their power supplies
‐ 2 ion pumps 500 l s-1
with their baking equipment and their power supplies
‐ 2 gate valves manually actuated (pipe diameter DN100) and their baking equipment
‐ 4 Pirani gauges
‐ 4 cold cathode gauges
‐ 2 power supplies for the gauges
‐ 4 residual gas analyzers (RGA)
‐ 1 baking equipment for the vacuum chamber
‐ 4 angle valves manually actuated for pre-pumping (pipe diameter DN63)
lsquo
124
Chapter 7 X-RAY BEAM LINE AND EXPERIMENTS
Introduction
The equipment of the X-ray beam line should firstly ensure a continuous detection and monitoring of
the beam (mainly in the first optical hutch) and secondly allow a complete characterization of the
beam (in the experimental hutch) This characterization consists in measuring the beam absolute
intensity its total energy range its spectral and angular distributions and its degree of transverse
coherence Time analyses of the bunched X-ray beam will also be performed This line also aims at
confirming the ThomX assets in such fields as cultural heritage biomedical and materials sciences by
performing demonstrative experiments
71 Available extracted X-ray beam
The X-ray beam is extracted through a 50 mm diameter beryllium window as schematized in Fig 71
Because of limitations due to the dimensions and the location of the Fabry-Peacuterot cavity mirrors the X-
ray beam available for characterization and experimentation is a cone with a half-aperture of 16 mrad
Figure 71 The X-ray beam extracted from ThomX through a beryllium window The beam fills a cone
whose total aperture is 2 x16 mrad
In the case of a lsquoperfectrsquo beam (ie without taking into account the electron beam emittance) the X-ray
spectral and energy distributions are shown in Fig 72 as a function of the scattering angle for two
electron beam energies 50 and 70 MeV and a laser wavelength of 106 microm In Fig 73 the energy
bandwidth and the flux per unit solid angle are shown for on-axis Compton photons produced by either
a 50 MeV or a 70 MeV electron beam
72 Global view of the X-ray lne
As can be seen in Fig 74 the positioning of the machine in the Igloo imposes the splitting of the X-ray
line into two sections a first optical table (3x2 m2) located just after the beryllium exit window and a
second table (4x2 m2) located outside the Igloo in hall D1 Along its path and until the optical table 2
the beam is transported in the primary vacuum inside stainless steel pipes with the exception of a short
upstream section of a few tens of cm between the beryllium window and the optical table this
insulating section will prevent the transmission of harmful vibrations to the Fabry-Peacuterot cavity This
primary zone is kept oxygen free by a nitrogen flow that prevents ozone production
lsquo
125
The beam diameter at the entry of the first optical table is 45 cm After this first optical table the beam
reaches the second table via a 13 cm diameter beam pipe inserted in a hole which was drilled long ago
in the Igloo wall About 8 mrad beam angular divergence are preserved at the entry of the second
optical table where the beam diameter is 13 cm
Figure 72 X-ray spectral distributions (left panel) and X-ray energy (right panel) versus the scattering
angle for an e- beam of 50 MeV (black) and 70 MeV (red) The laser wavelength is 106 microm Grey
areas correspond to emission angles larger than 16 mrad
Figure 73 X-ray flux (left panel) and X-ray energy bandwidth (right panel) of on-axis photons per unit
solid angle versus the emission angle for an e- beam of 50 MeV (black) and 70 MeV (red) The laser
wavelength is 106 microm
lsquo
126
Figure 74 Global view of the X-ray line in ThomX
73 Chraracterization of the X-ray beam
731 Optical table 1
A global view of the optical table 1 design is presented in Fig 75 Just upstream of this optical table a
60 mm diameter beam shutter activated when one needs to take background data will allow stopping the
beam A few millimeter thick tungsten is sufficient to stop the full beam (tungsten attenuation mass
factor microen ~ 4043 cm-1
for 100 KeV photons) This shutter does not have to be cooled since the ThomX
radiated power will not exceed 100 mW A drawing of the shutter mechanical design is shown in Figure
76 (a)
Figure 75 Global view of the optical table 1 showing the location of the main devices placed on it
lsquo
127
Beam monitoring
The optical table 1 equipment (see Fig 75) is mainly devoted to the detection and the continuous
monitoring of the X-ray beam The first device monitoring the beam presence is a fluorescent screen
mounted on a two-position motor (up or down) as shown in Fig 76 (b) It is inserted in the beam path
and viewed by a CCD camera Its screen absorbs almost the totality of the beam and thus has to be put
in the up position when not in use Downstream of the fluorescent screen a device composed of two Si
PIN diodes measures the beam scattered by a thin Kapton foil inserted in the beam path This setup
shown in Fig 76 (c) will monitor the beam flux variations and beam position displacements The
Kapton foil is fixed at an inclination angle of about 60o with respect to the beam path The sum and the
difference of the two diode currents will provide a relative measurement of the beam flux and of the
beam position respectively This set-up which is nearly transparent to 20-90 keV radiations is a non-
intrusive monitor and can be used continuously Then a high precision double slit [71] intercepts the
X-ray beam and allows one to let through only the desired part of the Compton cone (see Fig 76 (d))
The minimum dimensions of the slit aperture are 10 microm in the horizontal and the vertical directions
Beyond the double slit a wire monitor [72] shown in Fig 76 (e) is inserted in the beam This device
consists in two tungsten wires stretched horizontally and spaced by a few tens of a millimeter This
monitor will be driven vertically through the beam The difference of the two wire signals allows the
measurement of the absolute beam position
Figure 76 Drawings of the X-ray line devices located on the optical table 1 (a) Beam shutter
(b) Motorized two-position fluorescent screen (c) Kapton foil and Si diodes (d) Double slit (e) Wire
profiler
lsquo
128
Focusing device
Downstream of the beam monitoring instruments a focalizing device will be installed (see Fig 75) to
make the X-ray beam parallel andor to focus it on a sample It can be either a device based on
compound refractive lenses [73] (a so-called lsquotransfocatorrsquo) or two bendable mirrors in the Kirkpatrick-
Baez configuration (KB Optics) or a capillary optical device These three techniques will be
investigated to evaluate the advantages and disadvantages (aperture efficiency focusing strength etc)
of their eventual implementation in the ThomX facility Such a focusing apparatus has to be located as
close as possible to the X-ray source because of the rapid growth of the beam transverse dimension
Finally at the end of the optical table 1 some space is kept free for any specific study which might
concern optics detectors or samples
732 Beam transport
As shown schematically in Fig 77 beyond the optical table 1 the X-ray beam crosses the Igloo wall
inside a beam pipe that runs through a 15 cm diameter hole and reaches the D1 hall where the second
optical table is installed in the lsquoexperimental hutchrsquo At the entrance of the wall hole a second beam
shutter in lead around 15 cm in diameter and 25cm thickness is installed This shutter is connected to
an interlock system that ensures radioprotection It absorbs radiations coming from the electron
machine which propagate through the Igloo cylindrical hole and allows X-ray users to work in the
experimental hutch without stopping the electron machine
Figure 77 Schematic of the X-ray beam transport across the Igloo wall from the optical table 1 located
in the Igloo until the optical table 2 located in the D1 hall
733 Optical table 2
Several kinds of instruments will be placed on the optical table 2 The goal is to study in detail the X-
ray beam produced by ThomX firstly to determine how the beam characteristics depend upon the
machine operation parameters and secondly to prepare the demonstration experiments A global view
of the optical and detection instruments placed on that table is provided by Fig 78
lsquo
129
Figure 78 Global view of the optical table 2 indicating the main instruments that will be used to
characterize the beam
Absolute and spectral flux measurement
The measurement of the X-ray beam absolute flux and of its energy spectrum will be performed by
using two detectors a calibrated Si diode [74] and a cooled high-purity Ge solid-state detector (SSD)
[75] The Si diode will be bought already calibrated [76] A lsquogoodrsquo Ge SSD has an energy resolution
(ΔEE) around 1 at 20 keV 06 at 45 keV and 045 at 90 keV [77] The energy calibration of the
Ge SSD will be carried out in the X-ray beam line by using the Kα fluorescence lines of reference
materials (see Table 71)
Element Ge Mo Sn Cs Ce
EKα1 (keV) 9886 17479 25271 30973 34719
EKα2 (keV) 9855 17374 25044 30625 34279
Element Eu Er W Pt Pb
EKα1 (keV) 41542 49128 59318 66832 74969
EKα2 (keV) 40902 48221 75981 65112 72804
Table 71 Kα fluorescence lines of chemical elements used to calibrate the Ge SSD in the 10-90 keV
energy range
The two detectors will be mounted alternately on a (X Y) translation stand to measure a large number
of points of the emission spectrum
- the mean emission energy E (Ge SSD)
- the energy spread ΔE (Ge SSD)
- the absolute flux (photon numbers) (Si diode)
These beam characteristics will be measured as a function of the size of the selected X-ray sample the
selection being carried out with a double slit device (see Fig 76 (d)) Our goal is to check
lsquo
130
experimentally the theoretical angularspectral distributions shown in Fig 12 of the Introduction by
measuring the number of Compton photons per second per mrad2 and per 01 of energy bandwidth
Time analysis of the bunch structure
The X-ray source has a time structure which involves two main scales a 50 ns period which is the
interval between two successive collisions between the laser pulse and the electron bunch and the 20
ms e- storage cycle To perform a pulse by pulse analysis we will use a high resolution time detector
(~1-5 ns) with a high repetition frequency (~10-100 MHz) eg a micro-channel plate (MPC) detector
[78] or an avalanche photo-detector (APD) Such time measurements have already been performed at a
similar facility by Sakaue et al [79] MCP Hamamatsu devices can reach a 07 ns (FWHM) time
resolution which is good enough not only to resolve each one of the collisions between the laser pulse
and the electron bunch but also to measure their relative intensity accurately from the beginning of the
e- storage cycle to the end These measurements will be performed on the X-ray beam axis They will
have to be synchronized with the ThomX clock
Transverse coherence of the beam
Some simple diffraction and interference measurements will be performed in order to analyze the
source spatial coherence The diffraction of the X-ray beam by a slit of a known aperture or the
interference effect produced by the insertion of a phase-shifting object in the beam allows one to
quantify the transverse coherence of the beam [710] In these experiments the on-axis photon intensity
distribution will be recorded with a 2D high resolution detector (such as for instance an XPAD CdTe
camera with 130 microm pixels) This measurement will be repeated at several positions along the beam
The pattern analysis will inform us about the degree of coherence of the source A monochromator will
be inserted upstream of the diffractive slit or the phase-shifting object to select a well defined energy
Later on Young double-slit experiments could also be carried out [711] to measure the beam
coherence length beam
74 Demonstration experiments
In parallel with the complete characterization of the ThomX beam described in the previous
sections some simple experiments will be performed in order to demonstrate the potentialities
of the ThomX source In this phase we will focus on medical and materials science
applications An important aim of the ThomX project is to perform medical and materials
imaging exploiting the high and tunable energy of its beam A second aim of the ThomX
project is to show its capability in the structural investigation of materials using X-ray
diffractionscatteringspectroscopy techniques
741 Experiments using the full X-ry beam
Three experiments issued from the expertise of the ESRF-INSERM teams in medical applications of
synchrotron radiation have been chosen to be carried out as demonstrators on the ThomX prototype
The goal of setting up such experiments is to demonstrate (i) the suitability of ThomX of performing
both imaging and therapy experiments (ii) the tenability of the source adjustable at energies
corresponding to the K edge of high Z elements routinely used in medicine (Gd I) or of those of new
experimental radiotherapy adjuvants (Au Gd nanoparticles CisPt) (iii) suitability of the source for
phase contrast imaging (iv) exploitation of the advantage of using a 2D beam with respect to the typical
laminar synchrotron X-ray beam Some imaging and therapy experiments in the biomedical field
require a wide 2D beam Using the natural divergence of the ThomX beam a sample of a few
lsquo
131
centimeters in diameter located at 10-20 meters from the ThomX interaction region can be fully
illuminated by the a conic beam with no need of optical elements Calculations have been performed in
order to show the potential of using the ThomX source in comparison with a hospital X-ray generator
(fluoroscopy unit type) as well as in comparison with the beam from the biomedical line ID17 at ESRF
Results of these comparisons are given below for each medical experiment we intend to perform with
ThomX
An imaging experiment
For imaging experiments the energy range of the beam has to around 60-90 keV According to Fig 73
the corresponding half-aperture will be 5 mrad leading to a photon flux of 61012
phs When the beam
reaches the sample stand placed on Table 2 at 20 m from the ring interaction region it will have a
diameter of 20 cm This conical beam will then have a cross section of ~300 cm2 and permit
tomographic acquisitions of a full human head using a 2D detector This wide 2D beam is a major
advantage of the ThomX source for biomedical imaging with respect to the laminar synchrotron
radiation beam The dose rate (DR) in Gys can be expressed as
(71) DR = 16 10-13
Ф[phscm2] E[keV] (microenρ)[cm
2g]
where Ф is the photon flux per cm2 E is the mean energy of the beam and (microenρ) is the mass
attenuation coefficient For an X-ray beam of 75 keV (ie the average of our 60-90 keV energy range)
the mass attenuation coefficient microenρ for soft tissues is 0028 cm2g leading to a dose rate of 67 mGys
delivered to a 300 cm2 area The corresponding expected lsquodose area productrsquo (DAP) of the ThomX
source for such imaging experiments is DAP = 20 Gyscm2 For comparison at the ESRF ID17 line a
dose rate of 800 mGys is delivered to an area of 150x2 mm2 corresponding to a DAP of 24 Gyscm
2
As far as conventional sources are concerned typical laboratory devices used in 2D fluoroscopy deliver
a DAP of 06 Gyscm2 [712]
A feasibility experiment will be performed by using a standard human head phantom (see Fig 79)
which will be mounted on a rotating stand that allows a 360deg rotation It will be possible to translate this
stand in three directions (along the X-ray beam axis and in the two other perpendicular directions) to
optimize the sample position in the beam Acquisition will be performed using the 60-90 keV pink
beam before and after the insertion of a high-Z element solution (typically iodine or gadolinium) in a
slot available in the phantom head Several images will be acquired using different concentrations of
high-Z element chosen in the range of concentrations that can be typically reached into a tumor (in
humans the high Z element is injected intravenously) The study will also aim at finding for each high-
Z element the lowest concentration that can be measured with the ThomX imaging system
Figure 79 Typical phantom featuring a human skull from X-ray absorption point of view
lsquo
132
The detector will be placed downstream of the sample A typical detector that may be used is a large
field of view 2D detector equipped with Si or CdTe sensors with ~100-200 microm pixels [713] Finally a
standard computed tomography (CT) algorithm will be used to reconstruct the tomography images and
to calculate the concentrations (in mgml) of the given high Z element in the skull The results will be
compared to previous work done at the ESRF [714]
A radiotherapy experiment
For this experiment the targets will be rats implanted in their right brain hemisphere with rodent brain
tumors (F98 9L) at an appropriate tumor stage (typically at D10 or D14 after implantation) For the
irradiation we need a pink beam with a 10 keV energy bandwidth and a median energy above the K-
edge of the high-Z element to be injected as a radiosensitizer (ie in the 45-80 keV range for both
iodine and gadolinium) The beam size will be a few centimeters in diameter Figure 710 shows the X-
ray beam energy and beam flux generated by a 69 MeV electron beam versus the photon back-
scattering angle X-rays of energy 80 keV within a 10 keV bandwidth are contained in a cone of half-
aperture 27 mrad and their flux is ~31012
phs At the sample location on optical Table 2 at 9 m from
the ring interaction region the beam has a diameter of 5 cm ie a cross section of ~20 cm2
The center of the ratrsquos right hemisphere will be placed on the rotation axis of the irradiation system and
slits will limit the beam transverse extension in order to define a precise field of irradiation Assuming
an X-ray flux (Ф) of 31012
20 phscm2 with an average energy (E) of 80 keV a mass attenuation
coefficient of 0025 cm2g (soft tissue 80 keV X-rays) Eq (71) leads to a dose rate DRe of 48 mGys
close to the tissue region where the X-ray beam penetrates Before reaching the tumor the radiation is
attenuated by the bone (~ 02 cm thickness (microρ)att = 022 cm2g ρ=192 gcm
3) and by the skull healthy
tissues (~ 15 cm thickness (microρ)att = 018 cm2g ρ = 10 gcm
3) Thus the dose rate DRt delivered at
the tumor is DRt = TDRe (where T = exp (-02219202) exp (-0181015) = 070) and is
expected to be 337 mGys The dose will be delivered while the rat undergoes a 360deg rotation The total
dose to be delivered to a tumor ranges between 10 to 20 Gy Therefore according to the expected rate
of 337 mGys the irradiation will take typically from 5 to 10 minutes with the big advantage of not
having to move the rat A dosimetry protocol (such as the one used at ID17 [715]) will be set to check
the homogeneity of the irradiation and to measure the absolute dose with a 2 to 5 uncertainty
Figure 710 X-ray energy (left panel) and X-ray flux (right panel) versus the photon back-scattering
angle for stored electrons of 69 MeV The laser wavelength is 106 microm
For humans and a tumor placed at 8 cm into the skull the transmission coefficient becomes T= 019
Therefore the expected dose rate DRt delivered by the ThomX source at the tumor is 91 mGys One
may compare with the ESRF ID17 beam-line where 800 mGys can be delivered for irradiation on a
lsquo
133
surface of 2x50 mm2 ie with T=019 a dose rate DRt of 015 Gys on 2 mm in height at the tumor
which is equivalent to a rate of 6 mGys to cover 5 cm of height with a scanning rate of 50 mms
ThomX could therefore deliver doses for human skull irradiation larger than those available at
synchrotron radiation facilities and also perform the irradiation without having to move the irradiated
sample Concerning the conventional hospital devices they deliver low doses deposited by X-rays
whose energy spectrum is broad and moreover they are not made to operate continuously for such long
time irradiation
Also the tunability of the ThomX beam will allow testing several high Z drugs containing iodine
gadolinium platinum or gold proven to be promising radiosensitizers [716]
Conformal radiotherapy
A final step in this sequence is performing conformal radiotherapy experiments ie doing both imaging
and therapy at the same time In fact one major issue of radiotherapy is to spare as much as possible
healthy tissues located around the tumor and at its border In this regard it is a key issue to adapt the
field of irradiation to tumor geometry size and positioning by performing first a tomography
Phase contrast imaging
The absorption medical imaging technique is based on the fact that different parts of a sample absorb X-
rays differently But the fact that X-ray absorption by biological material is weak constitutes also the
limitation of this technique To enhance or replace absorption-based imaging refraction diffraction and
interference of X-rays can also be used A better visibility at the edge of different regions of a specimen
can therefore be obtained by using the information contained in the phase of an X-ray wave For this a
spatially coherent X-ray source is required and was available only since the development of modern
synchrotron sources Experimentally the simplest way to visualize phase-contrast imaging is provided
by the propagation-based imaging (PBI) method In this method a spatially quasi-coherent radiation
illuminates an object which gives rise to a spatially varying phase shift in areas with spatial variations
of the refractive index As the radiation propagates beyond the sample parts of the wavefront which
have experienced different deflections on both sides of an edge of the sample produce a serie of fringes
due to the classical Fresnel-edge diffraction phenomena [717]) This gives rise to a characteristic phase
contrast pattern which is then recorded by a 2D detector placed at a proper distance downstream of the
object (see Fig 711 left panel) The actual phase-contrast signal is convolved with the detector point
spread function which has to be sufficiently narrow to avoid the smearing of the interference pattern
Additionally the phase contrast signal can be visualized if and only if the transverse coherence is
sufficient ie if Lcoh = λRs is larger than the details to be imaged (here R is the source-sample distance
and s is the source size) In contrast the requirements in terms of longitudinal coherence
(monochromaticity) are rather weak even a beam with a bandwidth of several keV does not lead to a
significant smearing of the phase contrast signal [717]
Assuming s = 70 microm R = 15 m a sample-detector distance of 5 m and a detector point spread function
of 50 microm the phase contrast signal expected when the ThomX beam illuminates a nylon wire of 300
microm diameter ranges from 10 to 3 when the X-ray energy increases from 20 to 80 keV (see Fig
711 right panel)
lsquo
134
Figure 711 Left panel The set-up for propagation-based phase contrast imaging Right panel The
simulated signal produced by a nylon wire of 300 microm diameter illuminated by the ThomX source
(simulation)
The experiment with ThomX will comprise two parts a planar radiography and a computed
tomography For the radiography three kinds of samples will be used (i) a custom-made phantom
composed of nylon wires of different diameters (ii) the ACR (American College of Radiology)
accredited phantom used in mammography quality (iii) a 4 cm thick formalin-fixed human breast tissue
obtained from a hospital bank The X-ray energy will be chosen in the 20-80 keV range For each
sample and for several energy values images will be acquired at two different sample-detector
distances in the absorption regime for comparison purposes (the detector will be in contact with the
sample) and at 5 m A silicium detector with 50 microm pixels will be used Images will be acquired for
different doses (measured with a standard PTW dosimeter) in order to obtain the signal-to-noise ratio as
a function of the dose These measurements will allow one to qualify the phase contrast imaging set-up
at ThomX
Computed tomography will be applied to (i) a meacutethyl polymeacutethacrylate (PMMA) custom-made
cylindrical phantom containing insets of different compositions and densities (water ethanol paraffin
etc) (ii) a formalin-fixed human breast tissue (5 cm in diameter) Images will be acquired using the
same parameter values as the ones chosen for the radiography Samples will be installed on a rotating
stand positioned on a double cradle that will permit an alignment with the beam and with the detector
Images will be taken during the sample rotation over 360 degrees
The final step will consist in applying standard algorithms to the images in order to calculate the phase
and to extract quantitative information about the samples This data will be compared with equivalent
images that will be acquired at the ESRF
742 Experiments using the central part of the beam
Most specimens from cultural heritage and functional materials are heterogeneous these lsquorealrsquo
materials often exhibit complex hierarchical structures involving various levels of order and
heterogeneity To understand the properties of such materials better one needs at least a description of
their structure at the atomic scale and at the macroscopicmesoscopic scale which takes into account
weak structural signals (relating to disorder minor constituents microstructure hierarchical orderhellip)
lsquo
135
This requires much probing as already demonstrated with synchrotron radiation sources A major goal
of materials science is to link global properties to local atomic features (orientation strain structural
distortion grain size) of some components in relation with neighboring domains and to deduce from
that the macroscopic properties As the functionalities of these materials often stem from the association
of different phases or constituents or of specific inter-relation between them (microstructure) highly
selective structural probes need to be used to characterize them Also in the particular domain of
cultural heritage many specimens require non-destructive methods and probes In order to understand
how they were made and to preserve them from degradation over time their physical and chemical
properties must be determined There is an increasing demand for material probing with complementary
techniques Fluorescence emission techniques excel in revealing the elemental composition of the top-
most layers of samples Diffraction is suited for studying quantitatively the phase composition the
structural details of the constituent phases and the characteristic distribution of sizes and orientations of
crystallites Tomography techniques excel in revealing the 3D mapping of the chemical andor the
density heterogeneities within a sample This is why the cultural heritage community is today strongly
involved in the development of new instruments that may characterize patrimonial artifacts by mapping
not only the chemical composition of a variety of samples but also their physical phases
Experiments described in section 741 require a large 2D beam and exploit the natural divergence of
the ThomX beam In this section diffraction scattering and spectroscopy experiments require a
focalized or a parallel beam and it should also be monochromatic or lsquopinkrsquo The optics will therefore
select the central part of the beam ie its higher energy part Let us give an order of magnitude
assuming a perfect 50 MeV electron beam a collimator of 2 mm diameter located at 2 meter from the
IP will cut the X-ray conic beam at 05 mrad leading to a flux of ~8 1010
phs with an energy of 45 keV
and an energy bandwidth ΔEE on the order of 10-3
(see Fig 73) In practice this bandwidth will be
broadened and will reach a few percent because of the actual e- beam divergence and energy spread For
experiments requiring a narrower energy bandwidth an additional selection will be performed with a
monochromator For instance selecting an energy bandwidth of 10-4
would lead to a flux on the order
of 109 phs
Since the goal of the first series of demonstration experiments is to qualify ThomX as an X-ray source
these experiments will be performed with samples previously characterized with other sources such as
synchrotron radiation sources
Diffraction and scattering
Diffraction and scattering experiments are major ways of performing materials structural and micro-
structural analyses This concerns highly ordered samples as well as powders or ill-ordered samples
Complete scattering analyses are based not only on Bragg diffraction but also on diffuse scattering in
order to look beyond the average structure and to investigate the local structure or short-range order
The crystalline quality of powders depends on the preparation methods Grain sizes vary from about 10
nm to several hundreds of microm In the case of nano-materials Bragg peaks are strongly broadened and
X-ray diffraction performed with standard laboratory sources does not provide suitable data for
structural investigations As illustrated by Fig 712 laboratory X-ray sources do not allow the
identification of minor phases in a poorly crystallized powder Figure 713 shows an example of a
diffraction pattern obtained with an excellent resolution The sample used for this analysis is an
Egyptian cosmetic powder dated to 2000-1200 BC The patterns were obtained with the ESRF
synchrotron source (bottom panel) and with a classical laboratory source (top panel)
lsquo
136
Figure 712 Powder X-ray diffraction patterns obtained by using a laboratory X-ray source Top panel
Poorly crystallized Prussian blue sample Bottom panel Prussian blue crystals From [718]
Figure 713 Powder X-ray diffraction patterns obtained by using a laboratory X-ray source (top panel)
and the ESRF synchrotron source (bottom panel) Arrays indicate minor phases From [719]
We intend to use the ThomX source to record diffraction patterns and scattering plots in order to
characterize short-range ordering and identify minor phases in such heterogeneous samples
For the first demonstration diffractionscattering experiments we will study ill-ordered Prussian blue
and white lead samples Pigments used for paintings or frescos have various compositions Since their
usage followed a definite chronology such pigments can sign an era Pigment characterization can also
help the undertaking of a restoration while a thorough knowledge of pigment synthesis through
centuries can help to apprehend the degradation mechanisms better
lsquo
137
For ordered samples diffraction will be performed with a single crystal by using a monochromatic
beam but also by using a lsquopink beamrsquo as done in the Laue method For ill-ordered samples the whole
scattering pattern will be recorded and analyzed with the Pair Distribution Function (PDF) method This
analysis technique (often called the ldquototal scattering methodrdquo) can be used alone or associated with
diffraction For experiments using the lsquopink beamrsquo Laue method the beam will be focused directly on
the sample (by using convergent refractive lenses (CRLs) or a capillary optic system as mentioned in
Section 731) To determine powder crystalline parameters diffraction analyses require the
monochromatization of the beam in order to reach ΔEE ~ 10-4
In the optic hutch (optical Table 1) the
diverging beam will first be made parallel by the CRLs or the capillary optic method and then
monochromatized in the experimental hutch (optical Table 2) by a monochromator The set-up for these
measurements needs a goniometer to align the sample and a low noise 2D-detector sensitive to high
energy photons
Fluorescent spectroscopy
X-ray fluorescence spectroscopy experiments allow one to determine the chemical composition of
materials and the mass concentrations of elements Indeed depending on the atomic number of a given
chemical element energies of the fluorescence lines emitted after an X-ray excitation are a signature of
the element An illustration of this approach is presented in Figure 714 where the fluorescence
tomographic images of a Globorotalia inflata specimen are shown and compared with a classical
radiography image
Figure 714 Fluorescence tomographic imaging (a) Set-up (b) Radiography (c) Fluorescence
spectrum measured at a given angleposition of the sample (d) 3D tomography reconstruction From
[720]
We plan to carry out fluorescence experiments since the energy of the ThomX beam can reach the
excitation energy of most chemical elements usually involved in material science for instance the
critical excitation energy of lead regarded as a heavy element is 88 keV) For this experiment the
excellent energy resolution of cooled Ge detectors will be an asset to detect the fluorescence light (the
detectors will be oriented at 90o with respect to the incident beam)
lsquo
138
K-edge and XANES spectroscopy
In K-edge experiments two images of a given element present in some sample are performed one at an X-ray
energy just below the K edge of the chosen element and the other at an energy just above Then they are
subtracted (see Fig 715 left panel) The K-edge imaging technique is used in particular in the medical and
the cultural heritage domains In XANES (X-ray Absorption Near Edge Structure) the absorption by a
material is measured over an energy range of about 100 eV on both sides of the absorption edge This
technique provides information about the local structure prevailing close to a particular chemical element (see
Fig 715 right panel)
Figure 715 Left panel K-edge subtraction imaging Right panel Principle of the XANES method
The first XANES spectroscopy experiments with ThomX could exploit the K-edge of heavy elements
such as W Pt or Pb By first selecting the central part of the ThomX beam with an energy spread of a
few 10-2
a monochromator could then provide a beam with an energy spread of 10-4
Samples will be
aligned by using a goniometer and the transmitted intensity will be collected by a low noise 2D-
detector
Combined diffractionabsorptionfluorescence tomography
A major limitation of diffraction methods comes from the fact that only average properties can be
obtained while most lsquorealrsquo materials are heterogeneous and complex Thus structural studies of such
materials are difficult to carry out A new tool for structure mapping of materials by means of X-ray
diffractionabsorptionfluorescence combined with tomography allows one to perform non-destructive
3D structure selective mappings of heterogeneous samples This way one may identify minor phases
and determine crystallite orientation and grain size Furthermore the selectivity of the method allows
one to separate and locate the origin of the diffuse or diffracted patterns to identify amorphous phases
In archaeology such complementary micro-structural analyses could shed light on the mechanical
history of artifacts
A recent experiment carried out with a micrometer beam at synchrotron radiation sources have
established the validity of such a local probe by analyzing a mixture of chalcedony and iron pigments
containing haematite [721] This experiment illustrated in Fig 716 consists in (i) performing a
structure tomography mapping directly from the diffraction data (top panel) (ii) obtaining selective
structure patterns of unknown minor phases from these maps (bottom panel) This study was realized by
the Neacuteel institute and the C2RMF and was one of the first diffraction tomography experiments
performed with a micro-beam
With the ThomX source we will use poorly crystallized samples and combine in the same experiment
and simultaneously tomography diffraction with fluorescence and absorption measurements Diffraction
lsquo
139
tomography is beam-time consuming it will be performed to demonstrate its feasibility at ThomX The
experimental set-up will be similar to the one used for multimodal analyses (cf the top panel of Fig
716) The sample will rotate and this rotation will be combined with a translation to provide a 2Θ scan
of 2D diffraction patterns obtained with a 2D detector sensitive to high energy photons The diffracted
2D fluorescence patterns will be recorded with our Ge detector while the sample absorption will be
recorded with our calibrated Si diode
Figure 716 Top panel Experimental set-up for multimodal spectroscopydiffractiontomography
analysis Insets Images of the chalcedonyndashFe mixed-pigment sample are reconstructed by combining
the various signals X-ray absorption (bottom) Fe X-ray fluorescence (top left) and chalcedony
diffraction (top right) Bottom left panel Reconstructed diffraction image based on the whole
diffraction diagram (black) compared with the calculated pattern for pure chalcedony (red) Bottom
right panel Reconstructed image that only uses the iron pigment scattering contribution (black)
compared with the calculated pattern for hematite and siderite grains (red) From [721]
75 Summary
As shown in this chapter two types of beams will be available at the ThomX compact X-ray source
(i) wide beams well adapted for imaging (conventional or phase contrast imaging) and therapy in the
biomedical field and (ii) almost monochromatic beams that can be either focused or made parallel for
diffraction and spectroscopy experiments in the field of materials science
The X-ray line is designed
- to ensure a continuous monitoring of the X-ray beam
- to measure the beam brightness and its transverse coherence
- to demonstrate the potential of this new type of compact source and identify the benefits it can
bring to the currently available X-ray sources
lsquo
140
Chapter 8 CONTROL SYSTEM
Introduction
The ThomX control system combines those hardware and software elements involved in the
management of all the facility components But more than a system dedicated to carrying out operating
processes (processes launched from the X-ray beam control room in order to undertake some specific
task such as a performing an imaging experiment or a material chemical analysis) it must also be an
advanced tool for various studies (data processing new equipment integration operating mode
evolution) and scientific experiments which will be performed with the X-ray beam line Hence the
control system has to be a versatile tool easily scalable by various users (scientists operators IT
technicians)
In the following a lsquoSub-systemrsquo is a physically connected set of hardware items sharing a global logical
function The ThomX sub-systems are as follows the linac (LINAC) the transfer line (TL) the ring
cell 1 (RI-C1) the ring celle 2 (RI-C2) the optical cavity (OC) the extraction line (EL) the section
of the X-ray line located in the Igloo (XLI) and the section of the X-ray line located in the X-ray hall
(XLH) A lsquoDomainrsquo is a set of hardware items related by a common purpose The ThomX domains are
as follows alignmentmetrology (AM) control-command (CC) diagnostics (DG) infrastruture (IF)
magnetic elements (ME)n optics (OP) pulsed elements (PE) power supplies (PS) radio-frequency
(RF) safety (SA) synchronization (SY) and vacuum (VA)
81 General principles
Some general principles drive the design of such a control system In broad terms the following criteria
will serve as guidelines
- Since monitoring is a major role of a control system comprehensive monitoring interfaces must be
provided to all users Moreover the monitoring system must take into account sub-systems specificities
dedicated GUIs must be designed for every need for instance in synoptic form to give a global system
status overview or as charts to show the time evolution of some specific parameters of a particular
device (eg pressure readings on TPG300)
- Operating processes management a control system also aims at the prioritization and sequencing
of processes It has to provide high-level functions in order to remotely launch complex operations (eg
machine startstop interlockshellip) Computing and data analysis tools which could launch such particular
procedures will be integrated or developed to maximize the automation of the machine
- Default management the facility control system must be able to manage operation defaults It will
be necessary to design graphical tools to quickly identify defaults when they occur Color display of
warnings and errors according to a criticality-based convention will be implemented in addition to the
recording of running process logs
- Safety for obvious reasons the ThomX radioprotection and safety system must standalone It should
not use any data generated by the control system Nevertheless each end-user must be able to make
sure that the ThomX facility does not present any danger whenever the accelerator is running This
information must be available in every control room through a continuous display of all the safety
equipment status (radioprotection sensors door openingshellip) A full historical record of safety-related
lsquo
141
events will be made available for analysis and a procedure of detailed acknowledgement (operatorrsquos
name action to close an incident) will be defined in order to guarantee safe operations
- Ease of use The ThomX control system is intended for various usages therefore it will concern
various users including operators who are not familiar with software development Hence it is crucial
to make it modular through relevant to sub-systems GUIs with various visualization interfaces control
buttons graphs sub-system overviews in synopsis or detailed display of subset status The definition of
new interfaces will be possible through visual development frameworks to fit new needs
82 Technical considerations
In order to guarantee a proper accelerator operation to make its use as easy as possible and to limit
costs some technical points have to be taken into account
- Distributed architecture an accelerator is a complex structure which results from various designs in
different fields The hardware part of the control system relies on dedicated IT devices (industrial PCs
programmable logic controllers) whose role is to drive sub-systems Each of these nodes will be fully
and remotely controlled by the system software from the control rooms (or from any authorized user
terminal in the ThomX network) independently of possible other issues All of these components must
communicate with each other when required by the operation needs Moreover the supervision part of
the control system must be available in different places (main control room X-ray line control room
perhaps others) The chosen system must meet these requirements and must run on several terminal
types
- Standardization of the hardware items and of the software codes is necessary in order to simplify
their integration in the control system (eg same brandmodel for all the stepper motors) Then it will be
possible to mutualize high-level controlling tools whether pre-installed or homemade Additional
benefits of this approach concern the overall maintenance the deployment of new releases and the
replacement of hardware parts
- Reliability The service continuity of the control system must be guaranteed A spare strategy must be
set up to make sure that the replacement of defective parts will be done on short notice In particular
storage memory redundancy will guarantee that none of the machine parameters or of the collected data
will be lost Periodic backup of main software systems (device databaseshellip) will be performed right
from the start Uninterruptible power supplies are foreseen to protect major elements of the control
system from electrical failures
- Upgradability Special attention will be paid to communication features when choosing hardware
devices Ethernet is now one of the major protocols and must be preferred when possible For the PLC
driven devices the fieldbus Profibus [81] is recommended since one single master interface can
manage more than 100 slave devices without data losses This product is also valuable as far as cabling
cost is concerned
- Safety Since several users may wish to perform various machine-related tasks (integration
maintenance measurementshellip) a policy must be established to control the access to the ThomX
network and its control system The management of these access permissions will be provided It will
take into account the role and the field of expertise of each project member
lsquo
142
83 The TANGO control system software
Tango [82] is the software chosen to be the backbone of the ThomX control system It is an open-
source software initially developed at the ESRF It is now used in several Institutes to control huge
accelerator facilities like the SOLEIL synchrotron In fact the SOLEIL staff is playing a major role in
maintaining and developing new Tango features
831 Software bus
An accelerator equipment is usually highly heterogeneous Thus its control system must enable various
devices to communicate with each other even if they use different protocols or communication
standards like Ethernet RS232 USB PCI DAQ Cardshellip
The first asset of Tango is the communication standardization that it provides between any type of
devices Tango implements a CORBA-based communication protocol so that a common network is
virtually created for the whole hardware If needed the parameters of a given device are thus available
to every other element of the control system This is achieved by adding an extra Tango-compliant
software layer above the hardware low-level libraries This layer is specific to a device brand and model
and is a simple executable file which can be instantiated as many times as required by the number of
devices This key element of the Tango system is called a lsquoDevice serverrsquo
832 A control system for accelerators
For basic reasons the Tango core and many of the tools developed around this software bus are open
source tools It is therefore an accessible system fully customizable for specific ThomX needs and a
system which can be maintained Since Tango is Linux and Windows compliant issues originating in
driver availability for the Linux OS can be avoided while Tango enables the use of scientific
applications which are often developed for Linux
Furthermore Tango is developed and maintained by a community which involves several accelerator
Institutes Even if Tango is not an accelerator-dedicated tool a number of developments have been
carried out or are being carried out that deal with devices currently used at such facilities Thus a large
Tango library of device servers is presently available to any new user
Finally more than a simple tool which provides a network access to a set of devices Tango is also a
framework that provides high level operational services such as configuration tool logging archiving
services and alarms permission management For developers specific tools are provided for the
generation of codes that fit in the common structure required by all device servers Other tools allow
easy GUI development As a result the integration of new devices or the upgrade of control and
supervision windows do not need in-depth coding As far as the compatibility with commercial software
is concerned dedicated bindings can be freely used eg applications developped for Matlab or
Labview can be used to perform a variety of tasks involving the ThomX devices
lsquo
143
84 Control system architecture
841 Equipment to control
On a complex machine like ThomX a wide range of devices and control signals must be taken into
account motors cameras scopes diodes or valves will deal with analog and digital inputsoutputs
complex signals transmitted through GPIB USB Ethernet or profibus interfaces
The inventory of devices to be controlled is presented in the following table Safety equipment is not
mentioned as it is a fully standalone sub-system
LINAC TL Ring OC XL
Power Supplies
Coils 2
Steerers 2 4 24 2
Dipoles 5 4 8 3
Dipole chicane 1
Quadrupoles 7 24 3
Sextupoles 12
Pulsed elements
Septum 1
Kickers 2
Synchronization
3GHz synthesizer 1
500 MHz synthesizer 1
Phase shifter 5
Programmable delays 2
Counter 1
Vacuum
Ion pump 8 4 17 4
Gauge 8 8 22 8 1
Valve 4 4 4 2
Primary pump 1
Diagnostics
BPM 1 4 12
Low frequency Signal
generator 2
ICT 1 2
Stepper motors 1 3
Camera CCD 2 3 1
RF
Klystron coil 1
Modulator 1
Amplifier 1
Canon 1
Feedback 2
Source laser
Photodiode 1
Camera CCD 1
lsquo
144
Motor 1
Mirror 2
Laser ndash FP cavity
Camera 4
CCTV Camera 10
Motor 16
Thermal control unit 2
Laser equipment 3
Scopes 3
FPGA 1
X‐ray Line characterisation
Xpad camera 1
Si PIN diode 3
Ge diode 1
Motor 13
Stepper motor 25
Others
Translation table 1 1
Slot 1
Table 81 Devices to control (TL=Transfer line OC = Optical cavity and XL = X-ray line)
842 Inputsoutputs
All devices listed above communicate with each other by digital inputsoutputs (DI DO) ie one bit
signals analog inputsoutputs (AI AO) or more complex signals as n-bit digital signals (n = 8 12 or
16) Several communication protocols mentioned in sect832 above will be used
They are detailed in Table 82
PLC InputsOutputs Bus Ethernet
connections
DAQ (ADCampDAC)
Vacuum 26 DI 13 DO 26 profibus
Diagnostics 2 gpib 30 3 x ADC 12 bits 100 kHz
Linac 2 DI 2 DO 5 AI 3 ADC 8 bits 1 GHz
Magnets 35 profibus
Feedbacks RF (x2) 10 DI 10 DO 10 AI
10 AO
USB
Ampli RF 64 DI 8 DO 32 AI 16
AO
RS232 2 4 channels ADC 12 bits
1MHz
Modulator RF 23AI 4 AO 69 DI 5 DI 5 DO
8 channels ADC 8 bits
Pulsed Elements (x3) 12 DI 3DO profibus ADC 12 bits
Laser ndash FP cavity RS232 22
Timing 2 gpib 14 15 channels DAC 16 bits
X Line 26 DI 26 DO 7
Table 82 Equipment IO to control (not complete at the time of writing)
lsquo
145
843 Front-end machine architecture PLCs and industrial PCs
The front-end architecture will comprise equipments individually dedicated to sub-systems These
machines are controlling nodes which will be located in sub-system server racks as close as possible to
the equipment to be controlled (when required)
Two kinds of front-end machines will be used
- PLCs programmable logic controllers are automation OS-based machines able to manage digital and
analog IO and to control processes carried out by devices connected to a field bus (eg Profibus) or
specialized devices They are reliable devices designed to operate in an industrial environment A
proprietary operating system runs on these machines which are powerful tools (eg automats) to
implement logic appropriate to experimentation Interconnection with the other Tango network
equipment is made through an Ethernet adapter interface
- Compact PCIs (industrial PCs) these crates can embed complex operating systems like a Linux
distribution or some Microsoft Windows Operating Systems hard drives Ethernet connections and
USB adapters Such crates are based on a PCI bus backplane to interconnect specific cards (DAC
ADC counters optoelectronics convertershellip) cPCI is a standard widely used in an industrial
environment it is compatible with numerous card suppliers These front-end machines will also allow
the running of Tango processes or specific computing tasks locally installed in a sub-system
The ThomX distributed control system will be build on these controlling nodes This structure will
allow the bandwidth optimization of the network main links and will provide cabling simplifications
The control equipment may be placed in the accelerator hall as well as in the technical rooms where
most of the electronics that must be controlled will be clustered in 19rsquorsquo standard server racks
‐ Some devices need not be controlled through these front-end machines They will be directly
connected to the control network (eg ethernet cameras or electronics for motor control) Three
ways to interconnect such devices are shown in Fig 81
‐
Figure 81 Control architecture for one sub-system
lsquo
146
These acquisition and controlling units can be Profibus PLC or a cPCI crate Each driven element will
be controlled through one of these acquisition and controlling unit
The overall control system will be composed of more than 25 front-end machines as described in Table
83
PLC cPCI Crate
Linac (wout vacuum diags modulator
magnets) 2 1
Vacuum 2
Diagnostics 2
Feedbacks 2 2
RF amplifiercavity 1 1
RF modulator 1 1
Pulsed elements 3 3
Fabry-Peacuterot cavity 1 1
X-ray line 1
Synchronization 1
Magnets 2
Total 13 14
Table 83 List of front-end machines (not complete at the time of writing)
85 IT infrastructure
- Network The ThomX local network will be organized around an IP backbone It will be based on the
Ethernet TCPIP standard in order to link user terminals front-end machines specialized servers etc
Because of the large number of devices and the requirements bearing on the bandwidth (eg for image
storage) 1 Gbitss compliant equipment will be chosen (LAN adapters switcheshellip) Depending on the
actual number of cameras an image-dedicated network could be added On the cabling side shielded
copper cables will be used as interconnection wires to limit electromagnetic interferences as much as
possible
- Data storage During the ThomX commissioning or for the operation monitoring or for the X-ray
experiments it will be crucial to store a large amount of data It is foreseen to implement the three
storage processes provided by Tango historical (long term storage at a slow rate) temporary (short
term storage at a fast rate) and snapshot (storage of parameters on operator demand or event triggered)
It will also be possible to meet the X-ray line needs for the storage of large size images during short
periods (at most for several days) This specific storage will not be necessarily on local servers it could
be convenient to move these data and store them on servers close to the final users possibly outside the
ThomX network
Two storage-dedicated servers will be installed for analyses replay of accelerator operation scenarios
and for some specific needs of the X-ray line These storage units will be specific machines which
prevent data outages or losses by redundant power supplies stored data redundancy (RAID) hard disk
hot swap etc The first of these two storage units will be a 2 To server equipped with a disk array
dedicated to the accelerator data management The second one will have a 10 To capacity it will be
available for the large size images of the X-ray line
lsquo
147
- Equipment of the control rooms Users will work in control rooms on system administration
accelerator control development and integration (device servers GUIs high level computing software
safety analysis tasks) Administration and end-user terminals should be provided in sufficient number
(see below)
The main control room will be equipped with
- 5 PCs 1 for the Tango core system 2 for operations 1 for development 1 for safety (dedicated to
visualization storage and analysis of safety related events)
- 1 server to run high-level applications and device servers
- 2 screens one displaying the ThomX status with synopses and the other one for the safety
supervision
- camera monitors for some direct controls
- 1 UPS (uninterruptible power supply)
The X-ray line control room equipment will consist of
- 2 PCs for the operation equipment and for macro development
- 1 server to run high-level applications and device servers
- 2 screens dedicated to the monitoring of the machine status to safety synopses and to the display of
safety related events
- Software needs
- Both the Linux and Windows OS must be installed in the control room PCs in order to run Tango and
many other operating processes (see the Introduction above) for development purposes and to use
configuration software Together with the Tango core and specific tools high-level software will be
installed to meet usersrsquo needs Labview and Matlab for science applications SPEC to control the
motors and for the analysis of the X-ray line data
- Data bases will run on the MySQL open-source database managing system
- An open-source http server Apache will allow the implementation of WEB interfaces for the control
system authorized users this way these users will be able to remotely visualize experiment data or
equipment status
- Finally all end-user stations will provide standard office tools
The following Figure gives an overview of the control room equipment
lsquo
148
Figure 82 Schematics of the equipment foreseen for the main control room and the X-ray line control
room
Overall the ThomX control system ndash designed to ensure harmonization and standardization and based
on open source tools ndash should be cost effective The investment cost is estimated around 200 keuro
including IT small equipment not described in this document cables a gateway to provide internet
access extra screens to display the ThomX status in the laboratory etc
lsquo
149
Chapter 9 SAFETY AND RADIOPROTECTION
91 Introduction
The ThomX facility will be sited in an existing building called lsquoIgloorsquo where a source of hard X-rays was
operated until 2003 by the Laboratoire pour lrsquoUtilisation du Rayonnement Synchrotron The ThomX linear
accelerator and storage ring will be built inside this building while the facility control room and the one of the
X-ray line will be located respectively in the D1 and D2 experimental halls The D4 hall will be devoted to
experimental tests
The ThomX operation will produce high-energy electrons and other particles such as X-rays and neutrons
which generate ionizing radiations through their interactions with matter An adequate protection against these
radiations must be ensured for people working at the facility The safety logic adopted is the so-called
lsquopositive safetyrsquo which means that any system dysfunction will immediately cause a complete stop of the
accelerator Furthermore redundant safety devices will be implemented allowing various actions that will
guarantee the staff safety such as
- Shining the beam into an area shall be submitted to prior people evacuation and to the interlocking of this
arearsquos doorways
- Access to a sensitive area shall be submitted to the prior stop of the upstream beam by using at least two
different means independent of each other
Complementary to these two procedures safety measures will be applied involving hard-wired logic safety
PLCs shielding beam dumps shutters personnel exclusion interlocks radiation monitoring of the
experimentation areas staff dosimeters and radiation hazard training of the personnel In case of dysfunction
radiological monitoring hard-wired logic or safety PLCs will act through interlocks that will break
connections to the mains voltage (Pre-Amplifier modulator or electron gun depending on the situation) in
order to stop the accelerator
92 PLC safety
The safety hard-wired logic works in association with an automaton PLC system via two completely separate
circuits A Safety Programmable Logic Controller (PLC) is a special purpose machine which is used to
provide a critical control and safety applications to automaton users This controller is an integral part of
lsquosafety instrumented systemsrsquo (SIS) which are used to detect situations involving potentially dangerous
processes When such a situation occurs the SIS is programmed to bring the process to a safe state
automatically
A safety PLC is specifically designed to accomplish two important tasks (ensured by a diversitary redundant
architrecture)
- lsquodo not fail but if this cannot be avoidedrsquo
- lsquofail only in a predictable safe wayrsquo
Hard-wired logic is a safety circuit fully independent of automatons It takes in charge in particular the
access door interlocking feedback contacts which provide a large number of checks bearing on the magnetic
field the high voltage the pre-amplifier status etc and the running control panel
The PLC safety level must be as high as the one of a positive safety hard-wired logic It manages access doors
emergency stop buttons safety rounds and reports issue authorizations to operate from a command-control
system
lsquo
150
93 Radiation monitoring
Concerning the radiation protection the ThomX facility design satisfies the following principles
- No one will receive a dose of radiation in excess of the regulatory dose limits as a result of normal
operation
- Professionnal occupation exposures will be kept As Low As Reasonably Achievable (ALARA)
- All practical steps will be taken to prevent accidents with radiological consequences and to minimize the
radiological consequences of any accident
Arrangements will be made for the monitoring of working areas such as the Igloo the bunker and the Optical
hutch For this purpose several monitoring modes will be available such as thermo-luminescent dosimeters
measurements made by the qualified person in charge of radiation protection (Competent Radiological
Protection Person) and a modular system for continuous monitoring A continuous monitoring of gamma rays
and neutrons will provide radiation dose rates and will manage alarms its measured values will be transmitted
to a central data acquisition unit Placed in the Compton room a special treatment unit will also be
implemented that will provide a local display of the dose rate signage and will manage alarms
94 Radiation protection zoning
The D1 D2 and D4 halls are not classified lsquosupervised arearsquo therefore their access is free to people who
satisfy access requirements In contrast the Igloo hall is classified lsquosupervised arearsquo
The D1 hall contains the X-ray line and technical equipment
- The X-ray line is not a classified area with the exception of the Optical hutch
- The Optical hutch is alternately classified lsquoprohibited arearsquo and lsquonot classifiedrsquo area depending on the
permission conditions issued by functional elements important to safety (shutter position permanent
radiological level)
The D2 hall contains the ThomX control room and a room which contains the laser equipment
The D4 hall is devoted to tests and mechanical assembly
The Igloo hall contains the electron accelerator located inside a bunker The Igloo is classified lsquosupervised
arearsquo therefore only workers subject to individual dosimetry are admitted However in exceptional
circumstances public access might be authorized as long as regulations and procedures are respected
The Igloo bunker on the one hand and on the other hand the basement area devoted to the facility are
alternately lsquoprohibited arearsquo during the accelerator operation and respectively lsquosupervised arearsquo and lsquonot
classified arearsquo when the facility operation has stopped
The accelerator energy is high enough to expect nuclear reactions of photons and neutrons with matter
leading to the activation of structural materials This is why it is particularly important to use appropriate low
activation structural materials or structural materials that are activated for short periods of time only
Furthermore one must ensure a nuclear waste management which will require a specific room to store and
monitor activated materials This room should be a lsquosupervised arearsquo located in the Igloo
95 Access control
951 General statement
It is a matter of absolute necessity to have appropriate procedures and devices in place to identify and screen
all persons entering the ThomX facility Consequently the LAL access control should be extended to the D1
D2 and D4 halls and to the Igloo
lsquo
151
952 Particular requirements concerning the lsquorestricted areasrsquo
The validation of door contacts as well as the validation of the position of protective shields and real-time
radiation measurement are prerequisites to access the restricted areas (ie Optical hutch and the accelerator
area) As it applies to all functional elements important to safety signage and door statements (openedclosed
lockedunlocked) from classified areas must be transmitted to the control room Local indications are
displayed alternately with the status messages The measured values of the dose rate and alarm symbols are
shown permanently
953 Access to the accelerator area and to the Igloo basement
The accelerator and the storage ring will be located in a concrete enclosure that will provide an efficient
shielding to protect people against ionizing radiations inside the igloo in line with regulations Prior to the
validation the floor load-bearing capacity must be checked by experts
The concrete containment structure will have a removable roof made of staggered concrete slabs Civil
engineering will be needed for the anchorage and supports to be used to adapt the bunker concrete blockds to
the igloo circular wall which will be part of the enclosure and to make sure that the roof of the whole
lsquoprohibited arearsquo provides a complete shielding Since doors will be made of standard steel the bunker access
will be indirect ie through (short) mazes to ensure protection against ionizing radiations
Access to the bunker and to the basement area which is part of the ThomX facility will be determined by the
equipment that guarantees peoplersquos safety in accordance with the facility operation As mentioned above
these devices ensure redundancy by using wired logic safety PLCs and also special safety door locks which
are simple but effective devices A mechanical system called lsquotrapped key lockrsquo will be implemented the key
is trapped in the lock when the door is open and it can be recovered only once the door is properly locked
Turning on the beam in lsquoprohibited areasrsquo is allowed only if the safety systems have been unlocked which
implies that the collection of access keys is complete as will be checked by the control board To meet the
safety requirements new access doors must be installed and some existing doors must be modified
954 Access to the X-ray line and to the Compton room
The D1 hall will not be a lsquoclassified arearsquo and will be open to authorized people The X-ray line sections will
be in open access during normal operation with the exception of the Optical hutch To meet the radiation
protection requirements the Optical hutch partition and its roof will be made of thick metal sheets A thick
lead shield will be installed at the end of the X-ray beam axis
The Optical hutch will be alternately classified lsquoprohibited arearsquo and lsquono classifiedrsquo area (under the worst-case
conditions it might become a lsquoregulated arearsquo) depending on permission conditions issued by safety functional
items External and internal light-signaling warning will indicate the zoning status Thus access to the Optical
hutch is firstly subordinated to the removable shutter position and secondly to the local permanent
radiological measurement values The shutter is dimensioned to protect people from high radiation rates
effectively A fail-safe closer device triggers the shutter closing automatically in case of electrical power
failure too (compressed air power supply) When a door is open the shutter command is prohibited
A local radiation detector located in the Compton room will check he radiation exposure and the dose levels
down to as low as reasonably achievable values Measured high dose rates will automatically shut down the
accelerator and activate the evacuation alarm The validation of these safety devices is mandatory to grant
access permission
lsquo
152
96 Beam dumps
The ThomX facility will comprise tow beam dumps made of concrete that will contain high energy particles
They are very important to ensure an adequate protection of people from the ionizing radiations that would be
produced if they were not used One will be aligned with the linac axis and the other one will equip the
extraction area These shields will be installed as close as possible to the radiation source to absorb ndash or at
least to moderate ndash as much as possible of the produced radiations Their size and thickness will be
sufficiently large to reduce efficiently all secondary radiation (X-rays and neutrons) produced by the stopping
of the e- beam They will be made of a lined lead shield designed to moderate back-scattered radiation placed
in a cavity made of concrete blocks or high-density polyethylene Ne peut-on pas ecirctre plus preacutecis donner des
nombres sur les dimensions et preacuteciser les mateacuteriaux utiliseacutes avec une figure
97 Interlocks
Depending on the dysfunction origin the safety system will operate and break command circuits such as the
pre-amplifier trigger the modulator high voltage the electron gun trigger or its high voltage Feedback
information from the control systems may be gradually requested according to the anomaly
98 Accelerator emergency shutdown
Accelerator Emergency Shutdown (AES) are signal and alarm devices located in specific areas A sufficient
number of AES should be installed in prohibited zones namely the accelerator area the bunker basement and
the Optical hutch For the users to spot them easily AES are devices equipped with flashing lights which are
turned on whenever the local zone is closed
If people are still present in a dangerous zone when a machine run is about to start (ie when the access is
going to be forbidden after a short while) they should signal their presence by pushing an AES This action
prevents the accelerator operation The system reactivation must follow a specific procedure
99 Control console
Placed in the control room the control console collects all access keys The accelerator may be turned on if
and only if all keys are inserted in that console
The safety circuit validation (PLC safety hard wired logic) allows one to launch the accelerator running
procedure and triggers audible warning devices for 30 seconds before operation becomes possible
910 Evacuation of dangerous areas
A specific procedure must be followed for the evacuation of the bunker and its basement Operators should be
trained in methodical evacuations with the validation of all checkpoints (roundsman function) Non
compliance with the procedure will lead to the non validation of the facility operation
A sufficient number of roundsman functions should be located in the accelerator area Also an exit path
should be marked leading to those areas that provide a possibility to regress
911 Audio system and horns
Loud-speakers and horns will be installed in the Igloo basement (in the so-called lsquorotundarsquo) and pits and
inside the bunker The sound system must be of good quality speeches and broadcast messages must be
intelligible Horns must be loud enough 80 dBA at least
lsquo
153
912 Safety notices
Local notices Posters should be put up in lsquosupervisedrsquo and lsquocontrolled areasrsquo to remind people of the
regulation requirements Such posts maintain the workerrsquos awareness whenever they are in a radiological
environment Access doors to zones with multiple classifications should be equipped with highly visible
luminous warning panels Also measured radiation levels and messages could be flashed regularly Regarding
the radiation measured level in the Optical hutch an alarm symbol must be permanently displayed that
informs of this level
Signal boards at zone entrances and within the zones will specify the real-time safety conditions display
warnings (such as presence of live voltage ongoing operation mode etc) and the access permissions (such as
open access prohibited zone etc)
913 Radiological monitoring
The control of the radiological environment will comply with the ALARA principle It will be performed by
- using bimonthly exposure dosimeters based on radio-photo-luminescence (RPL) and neutron sensitive
dosimeters
- PCR controls with live measurements
- a centralized modular system which will continuously monitor exposures feeding a data acquisition system
that can handle up to 10 sensors
This centralized system will perform dose rate and integrated dose measurements manage alarms test
measurement channels check settings and calibrations It will allow a special device placed in the Optical
hutch to display in real time local safety information
The ALARA principle is a commitment to keep radiation exposures and doses at a level as low as reasonably
achievable (social and economic factors being taken into account)
lsquo
154
Chapter 10 MANAGEMENT OF THE THOMX PROJECT
Introduction
The management of the ThomX project will have a simple structure that takes into account the three
project phases 1) The construction 2) The exploitation 3) The technology transfer Three boards will
govern the ThomX activity covering all project aspects the machine design the purchasing and the
assembly the commissioning the usersrsquo access the communication and the relationships with the
industries This management board will comprise
1) A steering committee that will supervise the project activity and define the collaboration scientific
policy
2) A scientific committee composed of the scientific coordinators of the collaboration partners It will
assess the usersrsquo projects and prioritize the beam-time allocation This committee must also define the
scientific policy of the machine exploitation and RampD of the ThomX project
3) A partnership and valorisation office This body will evaluate the cultural and the economic
potential of the ThomX innovations It will stimulate transfer initiatives and check their outcome It will
work in close contact with the steering and the scientific committees
The composition of these three bodies is given in Fig 101
Figure 101 The ThomX project committees
In parallel the communication group (see Table 101) will be in charge of all activity concerning
communication and reach-out in the ThomX project It will organise events realise the publications
relating to the project status and progressrsquo and coordinate the official announcements of the
collaboration
LAL Orsay
Achille Stocchi
CELIA Bordeaux
Philippe Balcou
UMPC
Philippe Walter
Institut Neel
Alain Schuhl
INSERM Grenoble
Franccedilois Esteve
Synchrotron SOLEIL
Amor Nadji
ESRF
Jean Susini
THALES
Yves Trinquet
Alessandro Variola
LAL Orsay
Alberto Bravin
ESRF
Geraldine Le Duc
ESRF
Jean-Louis Hazemann
Institut Neel
Philippe Walter
UMPC
Alexandre Loulergue
Synchrotron SOLEIL
Francois Esteve
INSERM Grenoble
Eric Cormier
CELIA Bordeaux
Alessandro Variola
LAL Orsay
Pierre Bordet
Institut Neel
Philippe Deblay
Synchrotron SOLEIL
Yves Trinquet
THALES
Annabelle Alves
CNRS DR4
Edward Mitchell
ESRF
Fabian Zomer
LAL Orsay
lsquo
155
The project management is organized according to a matrix structure The ThomX facility has been
divided in five sub-systems Linac Ring and transfer lines X-ray line Laser and Fabry-Peacuterot cavity
There is a coordinator responsible for the design installation and operation of each sub-system All
these sub-systems involve various technologies which have been grouped in expertise domains
Mechanics and integration RF systems Magnets Diagnostics Power supplies and Pulsed elements
Vacuum Synchronisation Controls Beam Physics Alignment Cabling Nomenclature and
Infrastructures There is a coordinator for each one of these domains These coordinators will be in
charge of the activity harmonization within each technical field and of the maintenance during the
facility exploitation (or ldquoscientific phaserdquo)
The project leader his deputy and a project engineer are responsible for the project overall
coordination Fig 102 describes the project structure
Figure 102 The project structure
Project and Quality Engineer Technical
coordinator
AVermegraves Francois Wicek
LAL Orsay
Deputy Project leader
A Loulergue
Synchrotron SOLEIL
Project Leader
A Variola
LAL Orsay
Systems
LINAC RRouxLAL Orsay
Ring and Transfer Lines A Loulergue Synchrotron SOLEIL
X line M Jacquet LAL Orsay
Laser E Cormier CELIA Bordeaux
Fabry-Perot Cavity F Zomer LAL Orsay
Groups
Mechanics and Integration MLacroix LAL Orsay
Beam Physics ALoulergue Synchrotron SOLEIL
Alignment ALestrade Synchrotron SOLEIL
RF systems P Marchand Synchrotron SOLEIL
Magnets ME Couprie Synchrotron SOLEIL
Diagnostics L Cassinari Synchrotron SOLEIL
Infrastructures BPieyre MLanglet LAL Orsay
Control PGauron LAL Orsay
Vacuum C Preacutevost LAL Orsay
Power Supplies - Pulsed Magnets P Lebasque Synchrotron SOLEIL
Synchronisation N Delerue LAL Orsay
Cabling JN Cayla LAL Orsay
Nomenclature PCornebise LAL Orsay
lsquo
156
In the following Table the support structures are presented
Support Structures
Administration Communication Quality
Administration Brigitte Renard LAL
Orsay
Procurements Anngaeumllle Mardam
Beck LAL Orsay
EDMS Support Agnes Vermes LAL
Orsay
Secretary Chloe SevestreLAL Orsay
DR04 Support Edouard Dorion
Dominique Bony LAL Orsay
Marie-Pauline Gacoin
Synchrotron SOLEIL
Claus Habfast ESRF
Sandrine Agnes Vermes
LAL Orsay
Table 101
The ThomX collaboration needs an important support from all administrative services For this an
administrative cell has been set up which is headed by the person responsible for the administration the
purchasing office the project database management the secretariat and the contact with the regional
CNRS delegation support Last but not least a quality engineer plays a major role in the project
101 Partnersrsquo complementarity
The ThomX project requires a multidisciplinary environment given the complexity of the facility and of
the set-up operation Each partner contributes its competences to reach the final goal The ThomX
collaboration was organized so as to meet the requirements of a multicultural scientific environment
where different communities are called upon to share their know-how and their knowledge to develop a
common scientific program This was already the case ndash and still is the case ndash for the MightyLaser
project In this project LAL and CELIA teams have developed a fully integrated set-up composed of a
fibre laser and a high gain optical resonator which is presently installed at ATF (KEK Japan) There the
complementarity between the two teams was crucial LAL has a solid experience in developing optical
resonators in an accelerator environment while CELIA is among the leading laboratories in fibre
amplification technology This program aims at a progressive increase of the light beam power stored in
an optical cavity in order to maximize the Compton back-scattering rate The final goal of this RampD is
to demonstrate the feasibility of a γ factory based on Compton backscattering It started as a
demonstrator for Compton-based polarized positron sources needed for the next generation linear
colliders The interest shown by other scientific communities in this technology made it clear that a low
energy Compton source would be very useful in various research fields This conclusion led to the
launch of the ThomX collaboration
The ThomX CDR has already been published and the present TDR is about to be widely circulated For
the TDR writing it was mandatory to develop close connections between the various communities
lsquo
157
participating in the ThomX project in order to merge technological developments and usersrsquo
requirements in a single document All partners have contributed to the scientific case elaboration the
study of the facility feasibility and the definition of its main components
- LAL and SOLEIL contributed the description of the Compton backscattering characteristics and
carried out the study of the accelerators and the Fabry-Peacuterot resonator
- CELIA Bordeaux Lyon provided their expertise for the laser dimensioning
- THALES provided its experience in industrial components and sub-systems
- UMPC and ESRF enriched the scientific case
This task sharing results from the multidisciplinary approach to the ThomX project where half a dozen
institutes play an important role
The writing of this TDR provided a unique opportunity of collaborating for laboratories that belong to
different fields LAL specialized in fundamental high energy physics SOLEIL and CELIA in advanced
technology in laser and accelerators LC2RMF in social sciences and compact detectors ESRF in
medical sciences In the future the diversity of this set of teams will still increase with the participation
of the Neel Institute and the GIN-INSERM in Grenoble
After a prototyping phase ThomX will operate as a mini-synchrotron whose beam line can be accessed
not only by the facility partners but also by other communities Moreover the aim of the
industrialization phase is to provide the high technology scientific and industrial with compact and
reliable sources that can be operated with little or no accelerator and laser expertise This should open
the way to a large community of CBS source users that would benefit from the complementarity
between techniques developed at synchrotron sources and Compton back-scattering facilities Imaging
techniques biological tests on cells and animals metallurgy fast chemistry lithography are examples
of future possible applications of these unique instruments
102 ThomX larger framework
ThomX is one of the EQUIPEX projects submitted for approval to the French government by the P2IO
Consortium (Physique des deux Infinis et des Origines) of the Paris-Saclay Campus This Consortium is
composed of eight laboratories or institutes (CSNSM IAS IMNC IPNO IRFU LAL LLR LPT)
some of which are among the largest French laboratories All together their staffs comprise 2000
scientists engineers and technicians working in particle physics nuclear physics accelerator science
astroparticle physics astrophysics and their interfaces P2IO represents between 20 and 90 of the
national potential in the various sub-domains Although obviously not all of the eight laboratories
participate in each EQUIPEX project each project ndash including ThomX ndash is fully supported by P2IO and
will contribute to the very ambitious common technological platform that P2IO wants to create in the
coming years in the framework of the Paris-Saclay Campus initiative Once equipped with these
EQUIPEX facilities this campus will represent a unique opportunity for French researchers and
students
lsquo
158
References
11 Geoffrey A Krafft Gerd Priebe Compton Sources of Electromagnetic RadiationlowastReviews of Accelerator
Science and Technology Vol 3 (2010) 147ndash163
12 VBesson et al NIMA 608 (2009) S75-S77
13 T Akagi et al 2012 JINST 7 P01021
14 CDR ThomX LAL RT 0928 SOLEILSOU-RA-2678 editors AVariola FZomer ALoulergue
15 PRussbueldt et al Optics Express Vol 17 Issue 15 pp12230-12245 (2009)
16 IPupeza et al OPTICS LETTERS Vol 35 No 12 June 15 2010
17 M Y Shverdin et al 2009 OSAIPNRANLOSL 2009
18 CVaccarezza et al IPAC Conference proceedings New Orleans USA2012 TUOBB01
19 AVariola IPAC Conference proceedings San Sebastian Spain 2011 WEOAA01 1903-1905
21 S Doumlbert ldquoIntegration of the PHIN RF Gun into the CLIC Test Facilityrdquo Proceedings of EPAC 2006
Edinburgh Scotland 2006 MOPLS129
22 R Roux S Cavalier M Bernard G Bienvenu MJoreacuteBLeblondBMercier BMoutonCPrevost A
Variola ldquoPHIL a Test Beamline at LALrdquo Proceedings of EPAC08 Genoa Italy 2008 WEPP078
31 J Payet Beta Code CEA SACLAY
32 JB Murphy et al Longitudinal Wakefield for an Electron Moving on a Circular Orbit Vol 57
Part Acc 1997
33 K Bane M Sand The Short-Range Resistive Wall Wakefields SLAC-PUB 95-7074
34 E Forest Phys Tech Part Photon Beam
35 J Zang et al lsquoDipole fringe field effects in the Thom-X ringrsquo Proc IPAC13
36 R Nagaoka et al Recent Studies of Geometric impedance At SOLEIL PAC2006
37 Z Huang RD Ruth Phys Rev Lett 80976 (1998)
38 S K Mtingwa African Physical Review 2 (2008) 001
39 P Chen G Horton-Smith T Ohgaki AW Weidemann K Yokoya
NuclInstrumMethA355107-1101995
310 K Ohmi Phys Rev E 55 7550 (1997)
311 S Haifets D Teytelman Phys Rev Spec Top Acc and beams 8 064402 (2005)
312 E Weihreter Compact synchrotron light sources Series On Synchrotron Radiation Techniques
And Applications - Vol 3 World Scientific
313 P Lebasque et al Synchrotron SOLEIL lsquoEddy current septum magnets for booster injection and
extraction and storage ring injection at Synchrotron SOLEILrsquo EPAC 2006 Edinburgh Scotland
314 P Lebasque et al Synchrotron SOLEIL lsquoOptimisation of the coating thickness on the ceramic vacuum
chambers of SOLEIL storage ringrsquo EPAC 2006 Edinburgh Scotland
315 P Lebasque et al Synchrotron SOLEIL lsquoFour matched kicker systems for the SOLEIL storage ring
injection a full solid-state solution of pulsed power supplies working at high currentrsquo EPAC 2006
Edinburgh Scotland
316 F Marhauser E Weihreter (BESSY) ldquoImpedance Measurements of the HOM-Damped 500 MHz
Metrology Light Source Cavityrdquo BESSY Gmbh Germany 08 January 2007
317 M Langlois P Sanchez M Cornelis F Perez (ALBA) ldquoMeasurements on the RF cavity for the ALBA
storage ringrdquo Proceedings of EPAC08 Genoa Italy
318 R A Rimmer (Lawrence Bekeley Lab) et al ldquoDevelopment of a high-power RF cavity for the PEP-II B
factoryrdquo DE-AC03-76SF00098 (LBL)
lsquo
159
319 P Fernandes R Massarino R Parodi ATarditi (INFN Italy) A Massarotti (Sincrotrone Trieste Italy)
rdquoThe Design of the RF Cavities for ELETTRArdquo IEEE 1989
320 M Svandrlik A Fabris C Pasotti (Sincrotrone Trieste) rdquoImprovements in curing coupled bunch
instabilities at ELETTRA by mode shifting after the installation of the adjustable higher order mode frequency
shifter (HOMFS)rdquo IEEE 1998
321 W Bruns web site httpwwwgdfidlde
322 P Marchand et al Development of high RF power solid state amplifiers at SOLEIL Proceedings of the
IPAC2011 San Sebastian Spain p 376
323 C Pardine et al Status of the 476 MHz 50 kW solid state amplifier Proceedings of the IPAC2010
Kyoto Japan p 3972
41 YAG(CE) data sheet Saint Gobain Crystals httpwwwdetectorssaint-gobaincomYAG28Ce29aspx
42 SPI Suppliesreg YAGYAP Comparison Table
httpwww2spicomcatalogscintillsintillator_properties_comparison_tablehtml
43 K Nakamura et al (Particle Data Group) J Phys G 37 075021 (2010)
44 Wartski et al Detection of optical transition radiation and its application to beam diagnostics IEEE
TransNuclSci20544-5481973
45]Saturation of YAG Beam Profile Monitors Measured at ATF A Murokh J Rosenzweig V Yakimenko X
J Wang
46 Radiation Detection by Cerenkov Emission in Optical Fibers at TTF E Janata M Koumlrfer Tesla-Report
2000-27
47 CERN-2009-005 p273
48 CERN-2009-005 p272
51 J Bonis et al 2012 JINST 7 P01017 doi1010881748-0221701P01017
61 J Bonis et al Non-planar four-mirror optical cavity for high intensity g ray flux production by pulsed laser
beam Compton scattering off GeV-electrons JINST SISSA Vol 7 January 2012
62 R V Pound lsquolsquoElectronic Frequency Stabilization of Microwave Oscillatorsrsquorsquo Rev Sci Instrum 17 490ndash
505 (1946)
71 httpwwwjjxraydkhome
72 RW Alkire et al Nucl Inst and Meth A 350 (1994) 13-16
73 B Lengeler et al J Synchrotron Rad 6 (1999) 1153-1167
74 RL Owen et al J Synchrotron Rad 16 (2009) 143-151
75 MS Nogueira et al Radiation Protection Dosimetry 111 (2004) 105-110
76 M Krumrey et al AIP Conf Proc 705 (2004) 861-865
M Gerlach et al Metrologia 45 (2008) 577-585
77 httpwwwcanberracompdfProductsUltra-LEGe-SS-M3843pdf
78 K Sakaue et al Rev Sci Instrum 80 (2009) 123304
79 httpsaleshamamatsucomassetspdfparts_FF9892-31_-32_TMCP1039E03pdf
710 O Chubar et al Proc DIPAC (2001) CT09
711 D Paterson et al Opt Com 195 (2001) 79-84
712 M Mahesh RadioGraphics 21 (2001) 1033-1045
713 K Medjoubi et al JINST 6 C01080 (2011)
httpwwwvariancomusxrayproductsdigital_radiographypaxscan_productshtml
714 H Elleaume et al Physics in Medicine and Biology 47 (2002) 3369-3385
lsquo
160
715 Y Prezado et al Medical Physics 38 (2011) 1709
716 I Martiacutenez-Rovira and Y Prezadoa Medical Physics 38 (2011) 4430
717 L Samain Thesis Liege university (ULG) 2012
718 P Martinetto Thesis Joseph Fourier university (UJF) 2000
719 P Bleuet et al Analytical Chemistry 29 (2010) 518-527
720 P Bleuet et al Nature Materials 7 (2008) 468-472
81 httpwwwprofibuscomncdownloadspecifications-standardsdownloadsprofibus-standard-dp-
specificationdisplay
82 httpwwwtango-controlsorgDocumentstango-kernel
lsquo
161
Table of content
INTRODUCTION 2
1 The ThomX project 2
2 Scheme and options 3
3 General layout of the machine 4
4 Machine parameters 6
5 Summary of the TDR main technical points 8
Chapter 1 THE THOMX INTERACTION REGION 9
11 Electron beam and laser pulse characteristics in the interaction region 9
12 Expected flux and its characteristics 10
13 Position stability 11
14 Timing stability 12
15 Energy stability 13
Chapter 2 INJECTOR 14
21 Injector specifications 14
22 Technical description of the linac components 16
221 Photo‐injector 16
222 Solenoids 17
223 Laser 18
23 Acceleration section 20
231 The RF source 21
232 RF network 22
24 Beam dynamics 23
241 Nominal operation 23
242 Alternative operation modes 24
Chapter 3 TRANSFER LINE and RING 28
31 Transfer line 28
32 Extraction line 31
33 Storage ring 31
331 Optics and lattice 31
332 Non‐linear dynamics 35
333 Collective effects feedbacks 37
334 CBS and IBS effects 42
335 Scattering on the residual gas 46
336 Ion effects 47
337 Electron beam injection and extraction 48
338 Orbit corrections 50
341 Dipole magnetic design 53
342 Quadrupole magnetic design 54
343 Sextupole magnetic design 56
344 Corrector magnetic design 58
345 Injection dipole 59
lsquo
162
35 Power supplies 60
351 Pulsed magnets 60
36 Storage ring RF system 65
361 HOM impedance thresholds as regard to beam instabilities 66
362 Cures to the HOM induced instabilities 66
363 RF power source 70
364 Low level RF system and longitudinal feedback 71
365 RF equipment status 73
366 RF system conclusion 73
37 Transverse feedback 74
371 Specifications 74
372 Electronics 75
373 Kicker 76
38 Vacuum 77
381 Transfer line vacuum 77
382 Ring vacuum 78
Chapter 4 INJECTOR TRANSFER LINE AND RING DIAGNOSTICS 80
41 Charge measurement 80
411 Current transformers 80
412 Faraday cups 81
42 Transverse profile measurements 81
421 Diagnostics stations 81
43 Emittance measurement 84
44 Beam position measurement 85
441 Beam Position Monitors (BPM) 86
442 Orbit feedback system 90
443 Tune measurement and feedback 90
444 Energy measurement 90
45 Bunch length measurement 90
451 Cerenkov radiation 90
452 Synchrotron radiation 92
453 Transport of the Cerenkov and synchrotron radiations to the diagnostics hutch 93
454 Streak camera for the bunch length measurement 93
46 Bunch imaging using synchrotron radiation 94
47 Beam loss measurement 94
48 Beam dumps 94
Chapter 5 SYNCHRONIZATION 95
51 Overall scheme 95
52 Specifications 96
Chapter 6 OPTICAL SYSTEM DESIGN 97
61 Feedback system and results on a Fabry‐Perot cavity locking 97
611 Electron and FPC pulse synchronization 98
612 Laser parameters to be stabilized 99
lsquo
163
613 PDH Measurement technique 101
614 Actuators 103
615 The digital feedback system 107
62 High average power laser system and its coupling to a high gain FP cavity 110
621 The MightyLaser oscillator 111
623 Stretcher and compressor 115
624 ThomX development 115
63 Mechanical design 118
631 Optical cavity vacuum 123
Chapter 7 X‐RAY BEAM LINE AND EXPERIMENTS 124
Introduction 124
71 Available extracted X‐ray beam 124
72 Global view of the X‐ray lne 124
73 Chraracterization of the X‐ray beaugrave 126
731 Optical table 1 126
732 Beam transport 128
733 Optical table 2 128
74 Demonstration experiments 130
741 Experiments using the full X‐ry beam 130
742 Experiments using the central part of the beam 134
75 Summary 139
Chapter 8 CONTROL SYSTEM 140
Introduction 140
81 General principles 140
82 Technical considerations 141
83 The TANGO control system software 142
831 Software bus 142
832 A control system for accelerators 142
84 Control system architecture 143
841 Equipment to control 143
842 Inputsoutputs 144
843 Front‐end machine architecture PLCs and industrial PCs 145
85 IT infrastructure 146
Chapter 9 SAFETY AND RADIOPROTECTION 149
91 Introduction 149
92 PLC safety 149
93 Radiation monitoring 150
94 Radiation protection zoning 150
95 Access control 150
951 General statement 150
952 Particular requirements concerning the lsquorestricted areasrsquo 151
953 Access to the accelerator area and to the Igloo basement 151
954 Access to the X‐ray line and to the Compton room 151
lsquo
164
96 Beam dumps 152
97 Interlocks 152
98 Accelerator emergency shutdown 152
99 Control console 152
910 Evacuation of dangerous areas 152
911 Audio system and horns 152
912 Safety notices 153
913 Radiological monitoring 153
Chapter 10 MANAGEMENT OF THE THOMX PROJECT 154
Introduction 154
101 Partnersrsquo complementarity 156
102 ThomX larger framework 157
lsquo
2
INTRODUCTION
1 The ThomX project
The fast performance evolution of laser and particle accelerator systems has opened the way to compact
radiation sources based on Compton backscattering (CBS) [11] While CBS-based sources do not
compete with the more classical synchrotron sources as far as the photon total flux and beam brightness
are concerned the CBS scheme has several attractive characteristics it provides the most efficient
photon energy boost so that hard X rays can be produced with a comparatively low energy electron
beam resulting in very compact and low cost devices This is extremely important in view of the
integration of such a radiation source in a lsquonon-laboratoryrsquo environment like a hospital or a museum
Another attractive feature of CBS sources resides in the energy versus emission angle correlation This
correlation makes it possible to use a simple setup with a diaphragm to obtain a quasi-monochromatic
beam with a bandwidth on the order of one percent Moreover the CBS scheme allows the tuning of the
photon energy range in several ways by varying the laser wavelength the beam energy or the collision
angle Since the laser polarization is conserved in the electron-photon collision a CBS-based machine
may provide polarized photons
The ThomX project is taking advantage of the preeminent French technology in accelerator and laser
fields The goal is to design and build a demonstrator with cutting edge performances compared to
similar projects either in operation or planned A flux between 1011
-1013
γs in the hard X-ray range is
expected and the photon energy tunability will provide a Compton edge that can be set between 50 and
90 keV
This project is a direct outcome of the sustained effort made by several laboratories to achieve a high
amplification of laser pulses by stacking them in a passive optical resonator These studies resulted in a
record amplification efficiency in the picosecond regime obtained in the PLIC experiment [12] They
are presently pursued within the MightyLaser program at the ATF accelerator facility at KEK (Tsukuba
Japan) where a vacuum-compatible four-mirror cavity is installed [13]
The ThomX collaboration gathers experts in accelerator physics coming from SOLEIL and LAL
experts in laser systems from CELIA (Bordeaux) in optical resonators from LAL and in X-ray line
instrumentation from the Neel Institute in Grenoble The ThomX community includes representatives
from the medicine field and from the cultural heritage field The ESRF and INSERM (Grenoble)
together with the CNRS C2RMF will design and perform a series of demonstration experiments to
check the assets of such a radiation source Furthermore the ThomX overall characteristics give to this
machine a strong industrial potential the latter will be investigated by an industrial partner THALES
Electron Devices which will play an active role in the ThomX collaboration
As a first step in this program a CDR has been written [14] It describes the basics of the Compton
backscattering effect and gives a first estimate of the ThomX performances based on simulations The
main technological solutions are also described in this document The machine lattice and the entire
machine layout are provided together with the design of the optical resonator and its integration in the
collisions region The main items which need RampD were identified A global integration plan and a
machine cost evaluation were also given
The present TDR provides a detailed description of all the technical sub-systems together with the
machine integration scheme It also provides the results of complete beam dynamics simulations which
take into account the ring characteristics and all the effects resulting from the collisions between the
lsquo
3
electrons stored in the ring and the photons stored in the optical cavity These simulations lead to
precise predictions concerning the X-ray yield The last part of the TDR deals with the project structure
and its management Finally the updated cost estimate is presented
2 Scheme and options
X-ray sources based on Compton backscattering can be designed with various configurations
depending on the main performance goals (average flux peak flux brilliance monochromaticityhellip)
Once this choice is made one may envisage either a high or a low electron-photon collision frequency
In the first case the electron beam is recirculated in a storage ring or in an ERL in CW mode and
continuously interacts with the laser Another option is to operate directly a high duty cycle or CW
super-conducting Linac Correlatively the laser system has to provide a high average power in the CW
locked mode [15] In this case to amplify the laser pulse energy one may envisage the use of a passive
optical resonator with a high gain [16] If a high peak instantaneous flux in single bunch or multi-
bunch pulsed mode is required the laser system will be a high power laser with a low repetition
frequency Recirculation of the laser beam is possible for a few passages by means of passive or
regenerative cavities [17] As far as the accelerator technology is concerned the warm Linac
technology can be made to work in burst mode [18] This allows the production of short low emittance
bunches to increase the source brilliance Because of the Compton energy-angle correlation this
scheme can also provide an excellent performance as far as monochromaticity is concerned by using a
diaphragm which selects a fraction of the X-ray beam
The ThomX source is designed to maximize the average X-ray flux Another goal of this project is to
provide a compact reliable and tunable source which can be operated in hospitals or in museums in a
user-friendly way
These constraints imposed the choice of a high collision rate scheme and of the warm RF technology
Thus the ThomX accelerator system is based on a 50 Hz warm S band (2998 MHz) linac whose energy
is tunable up to 70 MeV an injection line and a compact electron storage ring whose RF cavity will be
warm and will operate at 500 MHz The ring revolution frequency is 178 MHz The laser is a fully
integrated fibre laser which provides a high average energy Its power is amplified by stacking the
pulses in a high gain four-mirror Fabry-Peacuterot resonator with a 30000 finesse The laser and the optical
cavity repetition frequency is twice the ring revolution frequency ensuring an electron-photon collision
every e- bunch revolution
The basic operating scheme of ThomX is illustrated in Fig 1 A 1 nC electron bunch is produced by an
RF gun and then accelerated up to the ring injection energy by an S band section A transport line
ensures the bunch transfer from the linac exit to the injection section of the storage ring After injection
the electron bunch is recirculated for 20 ms in the ring The ring optics and the RF peak value are such
that the beam size is very small (in all directions) at the interaction point (IP)
In parallel a high power fibre laser produces light pulses of ~ 14divide28 microJ energy at ~ 12 eV These
pulses are stacked in a four mirror optical resonator at twice the ring revolution frequency The
expected energy gain per pulse is 103 divide10
4 corresponding to a beam power of ~ 70divide700 kW in the
optical cavity
The synchronization system allows a near head-on collision (the crossing angle is two degrees) between
the electrons and the laser pulse every revolution of the electron bunch These collisions produce hard
X rays which are backward emitted in a cone whose angular opening is ~1γ (as a result of a relativistic
effect)
lsquo
4
Fig1 Schematic diagram of the ThomX source
At the end of a storage period of 20 ms the electron bunch is extracted from the ring and dumped in a
dedicated beam dump to avoid the background noise that would be produced by uncontrolled particle
losses At the same time a new lsquofresh beamrsquo is injected in the machine
3 General layout of the machine
As previously mentioned the ThomX design takes into account a strong constraint coming from the
goal of a small size machine Nevertheless this design allows for the possible integration of two
collision regions [19] Figure 2 shows the general layout of the machine The linac includes an electron
RF photogun (1) followed by a section dedicated to diagnostics (2) the acceleration structure (3) and a
quadrupole triplet (4) to allow for a three-gradient measurement in the direct line dump (5) where a
diagnostic chamber will be located The injection line consists of four 45 degree dipoles to steer the
beam four quadrupoles and two chambers (6 7) for energy spread measurements and beam
characterization before injection The injection is performed by a septum (8) and a fast kicker (9)
Another kicker (10) is inserted in the same line to provide together with the septum a fast extraction
Once extracted the beam is transported towards a dedicated beam dump (11)
lsquo
5
Figure 2 ThomX layout
The ring design ensures a four-fold symmetry based on a double bend achromat (DBA) optics It
includes 8 dipoles 24 quadrupoles and 12 sextupoles for chromatic corrections The bunch storage
time is too short (20 ms) and the beam energy is too low for synchrotron damping to play a role in
regard to the Compton back-scattering recoil effect nor in regard to collective effects such as CSR IBS
and ion instabilities For this reason an lsquoELETTRArsquo type RF cavity (12) will be used not to restore
energy but to bunch the electrons and to perform a longitudinal feedback Transverse position
diagnostics and feedback will be provided by a strip-line and by BPMs A synchrotron radiation
chamber will be used for transverse and longitudinal optical diagnostics The required ultra-vacuum
will be maintained by ion pumps distributed along the ring and by two dedicated pumps in the Fabry-
Peacuterot resonator
In the ring lattice two of the achromatic lines have been shortened by removing all the quadrupoles (see
Fig 3) so that the Fabry-Peacuterot cavity (13) can be easily integrated between two dipoles By minimizing
the beta functions it was possible to locate the collision region (14) in such a short section
Furthermore this design provides the possibility of implementing a second interaction region in the
future
lsquo
6
Fig 3 Layout of the interaction region and of the optical cavity integration
4 Machine parameters
The ThomX machine is composed of four main systems the injector the storage ring the laser and the
Fabry-Peacuterot resonator As mentioned previously we foresee some flexibility in the machine operation
so that the X-ray energy can be varied For this all four systems are dimensioned to withstand an
operating energy up to 70 MeV Nevertheless to characterize the machine a baseline configuration has
been chosen at 50 MeV In Tables 1 2 3 and 4 the parameter values which are given below correspond
to this baseline configuration
Table 1
Injector
Charge 1 nC
Laser wavelength and pulse energy 266 nm 100 microJ
Gun Q and Rs 14400 49 MWm
Gun accelerating gradient 80 MVm 5 MW
Normalized rms emittance 44 π mm mrad
Energy spread 04
Bunch length 43
lsquo
7
Table 2
Table 3
Laser and FP cavity
Laser wavelength 1030 nm
Laser and FP cavity Frep 3568 MHz
Laser power 50 ndash 100 W
Laser pulse energy 14 ndash 28 microJ
Fabry-Perot pulse energy 28 mJ
Fabry-Perot pulse length (rms) 5 ps
FP cavity finesseGain 3000-30000 1000- 10000
FP waist 70 microm
Power circulating in the FP cavity ~ 007 ndash 07 MW
Ring
Energy 50 MeV (70 MeV possible)
Circumference 168 m
Crossing-Angle (full) 2 degrees
βxy IP 01 m
εxy just after injection 5 10-8
m
Bunch length just after injection (rms) 4 ps
Bunch length at the end of a 20 ms storage
cycle
50 ps (rms)
Beam current 1784 mA
RF frequency 500 MHz
Transverselongitudinal damping time 1 s 05 s
RF Voltage 300 kV
Revolution frequency 1784 MHz
σx IP (just after injection) 70 microm
Tune xy 317 174
Momentum compaction factor αc 00136
InitialFinal relative energy spread (with IBS
and Compton back-scattering)
0406
lsquo
8
Table 4
Source
Photon energy cut-off 46 keV (50 MeV) 90 keV ( 70 MeV)
Total Flux 1011
-1013
photons
Bandwidth (with diaphragm) 1 - 10
Divergence 10 mrad (1γ) without diaphragm 50 MeV
5 Summary of the TDR main technical points
After the publication of the CDR much work has been put in simulations design and technical choices
The goal of this TDR is to provide a reference document in view of the machine construction as well as
a more precise planning and a more precise costing
The first chapter deals with the machine performances and the tolerances to be met so that the collisions
between the electron bunch and the laser pulse do take place in the conditions specified in the CDR
Then the main systems are described and our technical choices are presented This last section is
subdivided as follows
1 Linac
2 Beam transfer and injection section ring
3 Laser and Fabry-Peacuterot cavity
4 X-ray user line
A special attention is given to the integration of each system and to the global integration of the ThomX
machine The lsquoOptical systemrsquo chapter includes a short report on recent results obtained in the laser and
Fabry-Peacuterot cavity RampD program both in the laboratory and on a beam line at KEK-ATF (Japan)
Special chapters are dedicated to diagnostics the control of the main systems and their synchronization
safety and radioprotection The last chapter presents the project management
lsquo
9
Chapter 1 THE THOMX INTERACTION REGION
11 Electron beam and laser pulse characteristics in the interaction region
The ThomX injection frequency has been fixed at 50 Hz (20 ms period) by taking into account the
Compton scattering recoil effect which continuously deteriorates the beam longitudinal emittance This
effect is all the more important as synchrotron damping is strongly suppressed since the beam energy is
low Because of this lack of damping the Compton scattering recoil effect and eventual beam
instabilities must be controlled by longitudinal and transverse feedback systems
We stress the fact that in ThomX the beam does not reach a permanent state during a storage cycle its
properties are continuously varying To describe the Compton interaction dynamics we define a set of
nominal beam characteristics which are those prevailing just after the injection of a fresh e- bunch
Simulations have been carried out to characterize the X-rays emitted during a single crossing of the
electron bunch and the photon pulse (stored in the optical cavity) as a function of the collision
geometric parameters
These nominal collision parameters are as follows
Parameter value unit
Crossing angle 2 degree
Electrons energy 50 MeV
Electrons energy spread 03
Number of electrons 625 109
Fabry Perot pulse energy 28 mJ
Repetition rate 1784 MHz
Laser pulse transverse dimension (rms) 35 microm rms
Electron bunch transverse dimension (rms) 70 microm rms
Laser pulse duration (rms) 5 ps rms
Electron bunch length at injection 4 ps rms
Table 11 Nominal collision parameters
To extend these results to other geometric configurations one has to take into account the parametric
dependence of the Compton collision rate The X-ray flux is the product of the Compton cross-section
and the luminosity It is proportional to the electron bunch intensity and to the laser pulse power The
geometry dependence is given by the luminosity formula which involves the geometric 6D size and the
crossing angle Assuming Gaussian bunches and neglecting the hourglass effect the ring luminosity is
given by the following expression
Eq 11
L = NeNγ f cosφ
2π
1
σ ye
2 +σ yγ2 σ xγ
2 +σ xe
2( ) cos2φ + σ ze
2 +σ zγ2( ) sin2φ
lsquo
10
where σeγxyz represent respectively the two transverse dimensions and the longitudinal one of the
electron bunch and of the laser pulse f is the collision frequency Ne and Nγ are the number of
electrons per bunch and photons per pulse Φ is half the crossing angle
The X-ray energy cut-off is higher for head-on collisions and for photons which are exactly back-
scattered In this case the photon energy gain is 4 γ2
This calculation provides a useful reference to assess the efficiency of other configurations but it
remains an approximation since exact results must take into account the beam evolution during a
storage cycle and should be obtained from a full revolution by revolution simulation
12 Expected flux and its characteristics
In nominal conditions the expected flux is 78 1012
X-rayss whether one uses Eq 11 or the monte-
carlo simulation code CAIN Figure 11a shows the energy spectrum of the scattered photons and its
cut-off at 45 keV As shown in Fig 12b most photons are emitted very close to the interaction region
axis within a small cone with a 2 mrad opening angle They may be selected by using a small
diaphragm The energy spectrum of the X-rays emitted in a cone of 225 mrad opening angle is shown
in Fig 12a with a FWHM energy spread of 25 keV
Figure 11 Results of simulations performed with CAIN using the parameters listed above a)
Energy spectrum b) Transverse size of the source c) Angular emission distribution d) Horizontal (x-
xrsquo) phase space of the scattered X-rays
lsquo
11
The spot size at the source is dominated by the electron bunch of 7 10-5
m as illustrated in Fig 11b
The dimansions are assumed to be the same in the horizontal and vertical plane The total divergence
of the source is around 15 mrad given by the electron beam energy (ie the 1γ parameter) with an rms
size of 4 mrad (Fig 11c) The emittance of the photon source shown in Fig 11d is 03 mm mrad The
energy-angle correlation and the collimation effect are illustrated in Fig 12 As expected the high-
energy X-ray flux is peaked with a 1γ opening angle The diaphragm which is assumed for this
simulation selects emission angles and thus photon energies resulting in a ~ 10 monochromatic
photon beam
Figure 12 Results of simulations performed with CAIN using the parameters listed above
Left panel Energy spectrum of photons emitted in a cone with an opening angle of 225 mrad
Right panel Number of photonsmradsup2s versus energy and angle (the color bar indicates the photon
number)
13 Position stability
To optimize the X-ray flux a precise transverse alignment must be acieved in the interaction region It
can be performed either by steering the electron beam or by adjusting the optical table position These
procedures will be monitored by means of a double BPM which is integrated in the Fabry-Peacuterot cavity
vacuum chamber exactly at the interaction point The resolution of this technique is a few microns A
simulation has been performed to calculate the flux reduction resulting from a possible mismatch
between the electron bunch orbit and the laser pulse trajectory Figure 13 shows the number of X-rays
produced per bunch crossing and per keV as a function of a transverse displacement of the optical
cavity axis with respect to the electron orbit from zero to 140 microns
lsquo
12
Fig13 Number of back-scattered photons and their energy as a function of a transverse alignment
error This figure clearly shows that the losses entailed by a displacement comparable to the BPM
sensitivity are negligible The color bar indicates the number of X-rays produced at each crossing
14 Timing stability
A precise synchronization between the electron bunch and the laser pulse at the interaction point is
another important requirement A timing mismatch introduces a longitudinal shift between the two
bunches at the IP This can be due to an error in the location of the Fabry-Perot cavity with respect to
the e- storage ring or to a jitter in the synchronization system Fig 14 shows the variation of the X-ray
flux as a function of a longitudinal displacement
The flux is not so much reduced as long as the jitter is less than 10 ps This number fixes the
synchronization tolerance and thus the performance of the longitudinal feedback
Fig 14 Flux and energy spectrum of back-scattered photons as a function of the relative
synchronization of the beams (longitudinal displacement)
lsquo
13
15 Energy stability
The nominal ThomX injection energy is 50 MeV Phase rotation in the RF source or phase mismatch
will generate a jitter in the injection energy Such a jitter entails synchrotron oscillations which will be
damped by the longitudinal feedback However before this feedback has stabilized the longitudinal
oscillations the collision conditions will not be nominal This situation has been simulated the results
are illustrated in Fig15 One can see that it is in the higher part of the X-ray energy spectrum that the
loss is more important
Fig 15 Flux and spectrum of the back-scattered photons as a function of the e- energy
lsquo
14
Chapter 2 INJECTOR
The most important components of the linac injector are the electron gun and the acceleration section
which brings the beam to the desired energy Other components are the laser used for the e- gun the
magnetic elements used to transport the beam the diagnostics the vacuum system the waveguides and
the radio-frequency (RF) source Each component is detailed in the following First of all the injector
specifications are given since they lead to the choice of some of the linac components
21 Injector specifications
The project goal namely to produce a high flux of X-rays of energy 50 keV leads to the following
specifications for the linac
Energy 50 MeV
Charge 1 nC
Number of bunches per RF pulse 1
Emittance (rms normalised) lt5 mm mrad
Energy spread rms lt1
Bunch length rms lt5 ps
Average current 50 nA
Repetition frequency 50 Hz
Table 21 Nominal linac parameters
First of all one notices that the low average current 50 nA does not justify the use of cold technology
(ie of a superconductor) for the acceleration structure But there is still a large choice of possibilities
for an RF structure operating at room temperature For economic reasons and because of the long
experience accumulated in the field we have chosen an RF structure operating at 3 GHz This
frequency is a European standard
Furthermore the above specifications led us to choose a photo-injector for the electron source since a
low emittance beam is required Again to minimize risks we chose to build the same gun as the one we
constructed for the CLIC lsquoTest Facility 3rsquo at CERN [21] This gun has been running successfully for 4
years To increase the current per bunch we will use a metallic magnesium photocathode which can
deliver more than 1 nC with a laser pulse energy of a few tens of a microJ at a wavelength of 260 nm The
gun is made of 2 12 copper cells magnetically coupled to the waveguide To get 1 nC with an
emittance lower than 5 mmmrad an accelerating field of 80 MVm is required which means an RF
power of 5 MW in a 3 micros pulse The electron beam energy output by the gun is around 5 MeV
We will use an industrial laser to extract the electrons from the cathode by photoelectric effect The
main specifications of this laser are the following ones Wavelength 260 nm energy per pulse 100 microJ
pulse duration around 5 ps It must be synchronized with the RF frequency and triggered at 50 Hz This
laser will be installed in an optical hutch located outside the concrete shielding which will enclose the
accelerator Therefore its beam must be transported with dielectric mirrors and lenses over a distance of
about 20 meters
Concerning the acceleration section SOLEIL a partner of the project will lend us a lsquoLILrsquo section This
section was made by LAL for the pre-injector of LEP at CERN It has been kept as a spare for the
SOLEIL linac The ThomX project will provide an other section in case SOLEIL needs one The
section length is about 46 m To reach a final energy of 50 MeV the energy gain in the section must be
lsquo
15
45 MeV The latter value implies an electrical field of 125 MVm and an RF power of about 10 MW
However the accelerator should reach higher energies in order to produce X-rays beyond 50 keV Since
the maximum targeted X-ray energy is 90 keV the linac design should allow a beam energy of 70
MeV corresponding to an RF power of 20 MW Thus taking into account the RF system efficiency the
specification for the total RF power is set at 35 MW
This specification lead us to choose a klystron producing 35 MW in a 45 micros pulse To feed the
klystron a modulator is needed that provides high voltage pulses of typically 240 kV through a high
voltage transformer soaking in an oil tank The two technologies which can be used for this component
are discussed in detail below
For the electron beam steering two coils and two magnetic correctors are required The coils are placed
around the gun Opposite sign currents circulate in these coils (-310 A 336 A) so that the magnetic field
vanishes at the cathode location Its amplitude is 023 T roughly 7 cm downstream from the cathode
Concerning the magnetic horizontalvertical correctors once a good mechanical alignment is achieved
experience has shown that beam trajectory errors are not larger than a few mm between the gun and the
acceleration section Hence the correctors must produce a beam deflection of up to 10 mrad For the
model operating at LAL [22] two bipolar power supplies of 10 A per corrector are adequate
For the diagnostics three different instruments will be installed between the RF gun and the
acceleration section An Integated Current Transformer (ICT) will measure the charge per bunch
Beyond this device a beam position monitor (BPM) will measure the transverse positions (x y) of the
electron beam We will use a BPM based on striplines which was designed for SOLEIL The third item
comprises a vacuum chamber equipped with an arm which can be translated by compressed air in order
to introduce a YAG screen to intercept the beam The YAG crystal fluoresces when ionized by the e-
beam The emitted light shows the beam transverse shape A short optical transport line and a CCD
camera allow the measurement of the transverse dimensions LAL designed and constructed the
diagnostics chamber It has already been successfully tested in the PHIL facility A drawing showing
the linac layout is shown in Fig 21
lsquo
16
Figure 21 Linac layout In the upper figure the different components of the LINAC are illustrated
starting form the gun and subsequently passing trough the stripline the ICT for current measurements
the laser introduction chamber the diagnostics station the steerers and the accelerating section
22 Technical description of the linac components
221 Photo-injector
It is a copy of a gun built by LAL for the CLIC Test Facility 3 of CERN the so-called lsquoprobe beam
gunrsquo[22] Simulations have shown that the performances of this gun fulfill the ThomX specifications
We are thus benefiting from all the technical drawings which already exist and all the fabrication
processes are mastered by LAL from the machining to the brazing This RF gun is comprised of 2 12
cells The output cell is magnetically coupled and the other end is short-circuited The main parameters
are given in Table 22
Frequency 299855 MHz
Rs 50 MΩm
Q 14500
β 1
E 80 MVm
PRF 5 MW
Table 22 RF characteristics of the photo-injector at 30degC in vacuum
The photo-cathode will be made of magnesium It has the big advantage of producing the specified
charge without requiring an in-situ photo-cathode preparation chamber which is a heavy and expensive
lsquo
17
installation Preliminary tests performed at LAL have demonstrated the possibility to obtain good
quantum efficiency with such cathodes
The main difficulty expected in operating this device is the laser cleaning of the photocathode surface
when polluted by an exposure to air To perform this cleaning a laser intensity of 160 microJmm2 is
required together with an RF electric field of at least 50 MVm The magnesium disk holder is not
designed yet it could be press fitted screwed or brazed
222 Solenoids
The gun is equipped with two solenoids one is the focusing coil while the other is the bucking coil
which cancels the magnetic field at the photo-cathode location They have been modeled by usning
POISSON A schematic is shown in Fig 22
Figure 22 Schematic of the gun with its coils
Dimensions
For both solenoids the inner diameter is 160 mm and the outer one is 500 mm The copper part is 30
mm long for the bucking coil and 56 mm for the focusing coil
The conductor used for these magnets is a standard one with a 6x6 mm2 square shape It is cooled by
water circulating in a 4 mm diameter channel One can easily calculate the current the resistance the
dissipated power and the temperature rise assuming that the cooling water is supplied at 20degC and 8
bars Values are provided for each lsquopancakersquo since the conductors are wound by columns of two The
bucking coil Bbc has two pancakes and the focusing coil Bfoc four
Bbc Bfoc units
Number of coils 84 168 MHz
Number of pancakes 2 4
Current -274 326 A
P 23 33 kW
R 306 31 mΩ
Flow 164 164 lmn
ΔT +20 +28 degC
Table 23 Solenoid electric and cooling parameters lsquoNumber of coilsrsquo refers to one pancake
Two power supplies are required one for Bbc (400A 30 V) and one for Bfoc (400A 60 V)
lsquo
18
223 Laser
The laser extracts a bunch of electrons from the photo-cathode surface by photo-electric effect Table
24 specifies its characteristics
Parameter value unit
l 260‐270 nm
Emax 100 plusmn 05 microJ
Beam transverse quality M2 M2 lt 11
Gaussian Pulse width 5 ps
Transverse profile square
Pointing stability 10 of beam diameter
Oscillator frequency 74963750 externally triggered with jitter lt 1 ps MHz
Amplifier repletion rate 50 externally triggered Hz
Table 24 Laser specifications for the RF gun photocathode
The laser architecture is shown in Figure 23
Figure 23 Architecture of the laser proposed by industry
An oscillator provides a signal at the specified frequency It can be either a fiber or a TiSaphire laser
Then the pulse is stretched amplified (with one or two amplifiers) recompressed and converted in the
UV range
The laser will be installed on an optical table in a hutch whose temperature and hygrometry will be
regulated
224 Laser injection chamber
The laser injection chamber is located at a distance of 8011 mm downstream of the photo-cathode It
contains a metallic aluminum mirror placed 30 mm away from the accelerator axis It provides an 80
reflectivity The angle of incidence of the laser beam on the photocathode is 214deg This mirror is fixed
To adjust the laser beam position on the photo-cathode and its incident angle we will use a motorized
mirror with a remote control placed on an optical table just beside the laser injection chamber
The size of this optical table will be 1x1 m2 On it there will be
Oscillator Stretcher Preamplifier Amplifier Compressor
nJ microJ
mJ
Frequency
shifter
Clock
Syncro
Line syncro
Shaper
lsquo
19
- 2 dielectric mirrors
- 2 beam splitters
- 1 UV photo-diode to monitor the laser intensity
- 1 CCD camera to observe the laser spot on a screen (in paper or lanex) facing the photo-cathode
- An optical beam expander to adjust the laser spot size on the photo-cathode
This is the minimum number of optical items that will be put on the table some changes may be made
depending on the very final geometry of the accelerator and laser transport lines The set-up design can
be finalized during the last stage of the accelerator construction since it involves standard components
only
225 Beam steering coils
The e- beam trajectory will be adjusted by sets of two steering coils which produce dipolar magnetic
fields over short distances along the X and the Y axes One such device will be located at the gun exit
and the other about 800 mm downstream Only very small position and angle errors must be corrected
Figure 24 shows the LAL design for such a coil Experience with the PHIL photo-injector at LAL [22]
showed that an integral of Bmiddotdl of 52 G m is sufficient The latter figure has been validated by
calculations made by a private company With a current of 5 A a Bmiddotdl of 58 G m was obtained that
results in a 100 mrad deviation of a 5 MeV beam Thus a 100 mm lateral displacement of the beam can
be produced at the entrance of the acceleration section
Figure 24 Schematic of the beam steering coils Dimensions are in mm
All together the steering coils need 4 bipolar DC power supplies Their maximum current is 10 A
lsquo
20
23 Acceleration section
The acceleration section is shown in Fig 25 Its characteristics are given in Table 25
Figure 25 Picture of the SOLEIL LIL acceleration section
Length 45 m
Number of cells mode 135 2π3
Frequency 299855 at 30degC MHz
Rs 63divide74 MΩ
Q 14800
Attenuation 74 dB
Energy gain 15 MW 60 MeV
τ 135 micros
Ez max 182 MVm
Table 25 Characteristics of the acceleration section
In a second stage of the ThomX project a call of tender will be issued for a new acceleration section
that should be shorter in order to make the machine more compact
lsquo
21
231 The RF source
Figure 26 shows the RF source schematic
Figure 26 Schematic of the RF source
The RF timing and its synchronization are described in chapter 5 A clock signal is sent to the linac
which uses it to trigger a 3 GHz synthesizer Its output is fed to a preamplifier which brings its power to
350 W The preamplifier is triggered at 50 Hz Then the 3 GHz signal is amplified by a klystron whose
gain can reach 53 dB to produce a 35 MW RF wave as required by ThomX This power is distributed
via a waveguide network which is described below
To drive the klystron one needs a modulator delivering a pulsed high voltage (HV) whose
specifications are given in Table 26 The klystron cathode must be heated by a power supply that
provides a current rising slowly from 0 to 25 A and a voltage rising from 0 to 30 A in 20 minutes A
precision of 1 on the current value is required The klystron is generally equipped with focusing coils
and their power supply whose standard parameters are 40 A 120 V
Repetition rate 50 Hz
Plateau length 99 gt 3 micros
Plateau ripples lt plusmn 05
Pulse duration at 50 6 micros
Rise time 10 divide90 lt 05 micros
Fall time 90 divide10 lt 1 micros
Pulse to pulse stability lt 05
Voltage range 0divide300 kV
Peak current 285 kV 300 Hz
Table 26 Modulator specifications
The classical and well-proven modulator technology relies on the high voltage (40 kV) loading of a
delay line made of inductances and capacitances The delay line is switched by a thyratron A new
technology is presently emerging based on solid-state amplifiers Each amplifier produces a 100 V
lsquo
22
pulse Thus to produce a 40 kV pulse roughly 400 amplifiers are put in a rack and their output pulses
are added The performances of both technologies are similar
232 RF network
The RF power is transported and distributed to the electron gun and the acceleration section by a
waveguide network shown in Fig 27
Figure 27 Schematic of the RF waveguide network
The WR 284 waveguide network is pressurized at two bars with SF6 between ceramic windows at the
klystron output and at the acceleration structure entrance A 5 dB bidirectional coupler splits the power
in two channels In the branch which runs toward the electron gun there is a 10 dB attenuator and a
circulator to protect the klystron from the RF power reflected by the electron gun In the branch running
toward the acceleration structure there is a 10 dB attenuator and a phase shifter to adjust the relative
phase between the gun and the acceleration section Various RF loads (five in total) ensure the system
safety by absorbing of RF power of the reflected waves
lsquo
23
The network also includes three couplers to measure the incident and the reflected RF power One is
placed at the klystron power exit one is placed at the gun entrance and the last one is placed at the
acceleration section entrance The RF gun is equipped with a loop that measures the stored RF energy
All of these loops used for measurements are weakly coupled at -70 dB The signals are measured with
crystals or C boxes
24 Beam dynamics
241 Nominal operation
The beam dynamics was simulated with the PAMELA tracking code for the ThomX nominal
parameters ie a bunch charge of 1 nC at 50 MeV The 50 MeV can be obtained with an electric field
of 80 MVm in the RF gun and 125 MVm in the acceleration section It is clear that a different set of
field values can lead to the total of 50 MeV In particular one may wish to operate the gun at a higher
field in order to improve the beam quality We will operate the linac with this possible improvement in
view
The beam will be accelerated on crest Thus the coil magnetic fields are the only parameters savailable
to minimize the transveres and the longitudinal emittances Simulations have shown that for precise
magnetic field values (see Table 27) the e- beam emittances are optimized as illustrated in Fig 28
20
15
10
05
00
σxy
(m
m)
6005004003002001000
z (cm)
20
15
10
5
0
em
it (
mm
mra
d)
6005004003002001000
z (cm)
20
15
10
05
00
ΔE
E (
)
6005004003002001000
z (cm)
46
44
42
40
38
36
34
32
30
σz (p
s)
6005004003002001000z (cm)
lsquo
24
Figure 28 Beam characteristics (rms values) as a function of the distance along the accelerator (z
coordinate) Upper left panel Transverse size upper right panel normalized emittance lower left
panel Energy spread lower right panel Bunch length
The beam parameter values at the linac end are given in Table 27
Beam parameters value unit
Energy 50 MeV
Beam size 17 mm
Normalised Emittance 44 π mm mrad
Energy spread 04
Bunch length 43 ps
Table 27 Beam parameter rms values at the linac end
The accelerator parameters which should ensure the above beam characteristics are given in Table 28
RF structure parameters value unit
Ez in the gun 80 MVm
PRF in the gun 5 MW
Ez in the section 142 MVm
PRF in the section 14 MW
Solenoid parameters value unit
Bz max 2500 G
Ibc -274 A
Ifoc 326 A
Laser parameters value unit
Energy impinging on a Mg photocathode 10 microJ
Square transverse profile diameter 2 mm
Variation of the intensity from axis to border 14
Table 28 Summary of the linac parameters
242 Alternative operation modes
Concerning the injector other operation modes can be envisaged One at least will occur namely an
operation at 70 MeV in order to extend the X-ray energy range up to 90 keV Such an option has
attractive applications In this mode the electron beam performances should remain exactly the same as
the ones prevailing at 50 MeV (cf Table 27) For the accelerator parameters the only change is the RF
power required for the acceleration section it must reach 195 MW instead of 14 MW at 50 MeV
The operator can also be tempted to change the RF phase The one used in the simulations gives the
minimum emittance A phase shift of -10deg of the RF gun with respect to the laser could lead to an
energy spread reduction by a factor 2 at the expense of an emittance degradation Whatever the goals
the linac operator must be aware of the high sensitivity of the beam parameters to the RF phase This is
illustrated in Fig 29 An error of 5deg in the acceleration section phase results in almost a doubling of the
energy spread
lsquo
25
Figure 29 Energy spread at the linac end as a function of the RF phase in the acceleration section all
other parameters keeping their nominal values
The RF electrical field in the gun also plays an important role in the beam performances The results of
simulations of the emittance variation as a function of the gun electric field are presented in Fig 210 If
this field reaches 120 MVm the emittance is reduced by 20 with respect to the 80 MVm case
However the possibility to operate the gun at such a high gradient can only be assessed experimentally
Figure 210 Injector beam emittance as a function of the electric field in the electron gun (Gaussian
laser beam 1 nC e- bunches)
The injector performances as a function of the bunch charge are summarized in Table 29 Three modes
will be available in order to reduce the effect of collective instabilities at injection
Parameters Mode 1 Mode 2 Mode 3 Unit
Q 1 05 01 nC
σxy 165 15 08 mm
εxy 47 33 11 π mmmrad
σz 44 4 33 ps
σEE 04 03 02
Table 29 Linac beam performances (rms values) as a function of the bunch electric charge
As one can see in the above Table reducing the bunch charge by a factor 10 leads to an emittance
reduction by a factor 4 Although the X-ray flux is reduced the source brightness ie the ratio of the
flux to the divergence is improved by a factor 16 As a consequence a higher number of X-rays can be
20
18
16
14
12
10
08
06
04
02
δΕ
Ε (
)
151050-5-10
φ (deg)
10
9
8
7
6
5
ε (πmmmrad)
1401201008060
E0 (MVm)
lsquo
26
transported to a sample (number per unit area) Whatever the eventual benefits to the X-ray production
the advantage for the accelerator to operate at a lower charge per bunch is clear the reliability of the
accelerator is much enhanced the laser energy requirement is reduced and it becomes also possible to
use a copper photo-cathode instead of a magnesium one easier to handle But the main advantage lies
in the ring operation Indeed a 1 nC bunch whose length is only 4 ps induce a strong wakefield
stemming from the interaction between the beam and the vacuum pipe Such a wakefield perturbs the
beam and may cause oscillations of the bunch centroid possibly instabilities and thus a reduction of the
X-ray production rate
25 Photo-injector and acceleration section vacuum
Simulations of the photo-injector vacuum have been performed assuming a pressure of 3 10-8
Pa at the
cathode location and requiring a vacuum time constant (VS) that ensures an easy gun conditioning This
constraint must be respected to maintain a 10-7
Pa pressure at the acceleration section entrance where the
system will be baked to obtain an outgassing rate τ = 5x10-10 Pa m s-1
(N2)
Pressure in the line of injection
The use of a semi-conductor cathode (Cs2Te) puts severe constraints on the vacuum and requires a preparation
and a transfer chamber To avoid these complications some tests have been made on a metallic cathode (Mg)
The first attempt ran into difficulties (it took 12 h to reach a gradient of ~ 60 MV m-1
) we observed pressure
rises and sparks In presence of only the RF power we observed hydrogen CO and CH4 outgassing (as the
main components) A slight reduction in the quantity of water and oxygen was noticed due to chemical re-
composition When the laser beam was introduced to extract electrons a pressure increase by a factor of ten
was observed The 24th
25th
and the 26th
peaks also significantly increased for the HxCy compounds but no
Mg was measured
lsquo
27
Gas analyzes during the conditioning of cathode Mg
On the basis of simulations and experimental results we identified the need for the following items
‐ 2 ion pumps 40 l s-1
(upstream of the cathode amp injector) with their baking systems and power
supplies
‐ 2 ion pumps 55 l s-1
(for the RF network) with their baking systems and power supplies
‐ 1 ion pump 150 l s-1
(for the laser introduction chamber) with its baking system and power supply
‐ 1 ion pump 300 l s-1
(for the acceleration section) with its baking systems and power supply
‐ 1 gate valve manually actuated DN 100 (injector- upstream) and a baking valve
‐ 1 gate valve electro-pneumatically actuated DN 63 (injector - downstream) and a baking valve
‐ 1 gate valve electro-pneumatically actuated DN 40 (injector- downstream) and a baking valve
‐ 3 Pirani gauges (before the injector injector laser introduction acceleration section)
‐ 3 cold cathode gauges (after the injector injector laser introduction acceleration section)
‐ 2 power supply gauges
‐ 1 residual gas analyzer (RGA)
‐ 1 turbo pump (DN 63 ~ 80 l s-1
)
‐ 1 rotary vane pump (~ 12 m3 h
-1)
‐ 4 angle valves manually actuated for pre-pumping DN 63
‐ 2 T with RF grids
lsquo
28
Chapter 3 TRANSFER LINE and RING
31 Transfer line
The transfer line (TL) has to transport the beam from the linac to the ring and to match the optics of
these two devices In addition a set of beam diagnostics needed for an efficient injection in the ring are
localized along the line as sketched in Fig 31 The transfer line main characteristics are summarized in
Table 31
Figure 31 Layout of the injection and extraction transfer lines
A particular attention is paid to ensure a versatile optics that allows the matching of different linac
beams to different ring optics Simulations have been carried out using the BETA code [31] The
transfer line optics is optimized in order to accommodate some emittance degradation which may arise
from collective effects
Screen station + dump
Linac emit + pulse length diags
Screen + Scraper
Linac energy diags
Screen station
TL emit diags
Extraction line dump
lsquo
29
Figure 32 Transfer line optics including the ring The vertical line indicates the ring entrance point
The nominal transfer line optics is shown in Fig 32 from the linac to the ring together with the ring The
nominal input bunch Twiss parameters are x = z = 34 x = z = 42 for Q = 1 nC bunch length = 4
ps (rms) and an optimized normalized emittance of about 5 πmmmrad The strong bend produced by the four
dipoles of the transfer line increases the risk of emittance degradation under collective effects but simulations
show that the emittance increase is only of about 20 including coherent synchrotron radiation (CSR [2]) and
3D space charge effects (see Fig 33)
Figure 33 Emittance increase due to chromatic (cont lines) CSR (dashed) and space charge (dot dashed)
effects for 1 nC and 4 ps (rms) bunches at 50 MeV along the TL and the first revolution in the ring
The magnetic elements are identical to those of the ring
lsquo
30
Parameter value unit
Length 14 m
Nominal energy 50 MeV
Beta max x z 70 40 m
Maximum dispersion 1 m
Dipole numberfamily 4 1
Quadrupole number 7
Biplane corrector number 5
Table 31 Transfer line parameters and components
Besides ensuring the matching between the linac and the ring the transfer line must provide the measurement
of the beam emittances the beam energy and the energy spread Care has been taken to locate the diagnostic
stations in a way that ensures accurate measurements of these important parameters
The emittance measurements will be done by means of the lsquothree-gradientrsquo scheme (The pepper-pots
alternative is also foreseen) For instance with an adequate refocusing that produces a double waist (vertical
and horizontal) at the screen it is possible to scan both planes at the same time by varying only one
quadrupole gradient (see Fig 34) Other scan variations are also possible The following figures show the
optics retuning as well as the beam spot size variation at the screens of both stations the one at the linac end
and the one downstream of the U-turn
Figure 34 Optics retuning and beam spot size scan as observed on the screen (simulation)
The electron energy and its spread are measured in between the dipoles by a dedicated optics Note that it is
also possible to retrieve the bunch length by measuring the energy spread as a function of the linac phase
(three-phase scheme)
lsquo
31
32 Extraction line
To dump the residual beam at the end of the storage time (typically from 20 to 50 ms) an extraction line
composed of one small dipole and two quadrupoles is also foreseen This line will allow the measurement of
the beam properties after it has circulated in the ring The injectionextraction devices are symmetrically
implemented in the ring injection section
33 Storage ring
331 Optics and lattice
Thom-X must produce a high X-ray flux of 1011
divide1013
phs in a stable way The flux depends on the
characteristics of the electron bunch and of the laser pulse Assuming a 1 nC electron bunch charge at
50 MeV and a 20 to 30 mJ laser pulse ( 124 microm) the optics constraints at the interaction point (IP)
are as follows
- an rms size of the electron beam less than or equal to 70 microm in the transverse plane requiring
low beta functions of 01 m with an rms geometric emittance of 50 nmrad injected from the
linac (5 mmmrad normalized)
- a vanishing dispersion at the interaction point in order to avoid a beam size increase resulting
from an e- energy spread increase as well as to avoid potential synchro-betatron coupling
resonances
- a bunch length less than 20-30 ps to ensure a good overlap with the laser pulse (the crossing
angle will be 2deg) and long enough to reduce collective effects stemming from the pipe
impedance CSR bunch internal scattering etc
Besides these specific IP constraints there are two general layout constraints namely
- a ring circumference as short as possible with enough straight sections to accommodate the
interaction region the RF the injection and possible other equipment as RF and transverse
feedback systems
- the need for some flexibility as far as the optics and the working point are concerned
The ring layout and its optics are based on a two-fold symmetric lattice with four achromatic arcs and
four straight sections two short ones (02 m) and two long ones (16 m) This layout preserves the ring
compactness and provides an adequate interaction region in one of the two straight sections between
two adjacent dipoles (cf Fig 35) The main ring characteristics are summarized in Tables 32 and 33
lsquo
32
Figure 35 Ring layout
The laser pulse frequency is the harmonic 2 of the e- bunch revolution frequency in the ring ie 357
MHz This leaves open the possibility of storing two bunches in the ring in order to increase the
Compton flux but at present this operation mode is not taken into account because of injection
constraints The optical cavity mirrors are located outside the ring vacuum chamber thus enabling a
mirror substitution without affecting the ring In addition the extraction of the X-rays can be performed
closer to their source than in the case where the optical cavity is inserted in a long straight section
The two long free straight sections accommodate the injectionextraction devices for one and the RF
and feedbacks for the other
The ring is composed of 8 dipoles 24 quadrupoles and 12 sextupoles Its circumference is 168 m long
the revolution frequency is 1785 MHz The RF frequency is set at 500 MHz ie at the 28th
harmonic
of the ring revolution frequency The nominal optical functions have been optimized with the BETA
code [31] They are plotted in Fig 36 The dispersion vanishes in the long and in the short straight
sections (RF injection and interaction regions) The beta functions decrease down to 01 m at the IP to
provide a 70 microm focus assuming that the injected beam has an emittance of 50 nmrad (5 mmmrad
normalized) in both the vertical and the horizontal planes
RF and feedback section
Injection extraction section
IP 1
Interaction Point (IP)
lsquo
33
Ring parameters value unit
Circumference 1680 m
Nominal energy 50 MeV
Betatron tunes nx nz 317 174
Beta max xz 11 11 m
Dispersion max 08 m
Beta xz dispersion IP 01 01 0 m
Momentum Compaction Factor 0014
RF frequency 500 MHz
RF harmonic 28
RF voltage 300 kV
Synchrotron frequency 360 kHz
Period Revolution frequency 56 1784 nsMHz
Natural chromaticities -32 -82
Dipole nb family 8 1
Quadrupole nb families 24 6
Sextupole nb families 12 3
Corrector nb families 12 12
Table 32 Ring parameters
Max field 70 MeV Bore length Gap Diameter
Dipole 07 T 2765 mm
(R=352 mm)
40 mm
Quadrupole 5 Tm 150 mm 41 mm
Sextupole 15 Tm2 60 mm 42 mm
Correctors 510-4
Tm
Table 33 Main magnet characteristics
Taking into account the constraints imposed by the optical functions the chromaticity and the dynamic
aperture the operating point was chosen with tunes νx=317 and νz=174 There is still enough
flexibility to vary these tunes by plusmn 02 without spoiling the main ring performances The momentum
compaction factor can also be changed over a large range in order to scan the best working point
regarding the beam dynamics and the X-ray flux
Figure 36 Nominal beta functions and dispersion over one super-period
lsquo
34
The RF acceptance for the nominal momentum compaction factor (MCF) and a larger one including the
non-linear MCF and the vacuum pipe limit are plotted in Fig 37
Figure 37 RF acceptance Left panel nominal lattice Right panel for a MCF twice as large as the
nominal one VRF= 300 kV
RF acceptance nominal r56 = - 02 m r56 = - 04 m
Linear (RF bucket) 10 10
Non linear -32 +18 - 68 +4
Pipe limit 25 25
Table 34 RF acceptance for two different values of the momentum compaction factor
The two following plots (Fig 38) show (i) the vacuum chamber occupation by the beam at injection
when the emittance is 50 nmrad and the energy dispersion is 03 and (ii) at the end of the storage
cycle when the emittance reaches 300 nmrad and the energy dispersion reaches 06 (see section
438) The orbit components (misalignment errors) are not included since they are negligible once
corrected The choice of the pipe aperture (40 mm in H by 28 mm in V full aperture) takes into account
several constraints the beam pipe occupation the magnet bore radius the vacuum and the coherent
synchrotron shielding [32]
b a
lsquo
35
Figure 38 Transverse occupation (3 rms) of the vacuum chamber at injection (left) and at the end of
the storage cycle (right)
332 Non-linear dynamics
The natural horizontal and vertical chromaticities are respectively -32 and -68 Chromaticity
corrections are achieved by means of 12 sextupoles (3 families) located in the dispersive sections
Without performing a specific optimization the dynamic aperture is large enough about 50 times the
rms transverse size of the injected beam (see Fig 39) the quadrupole fringe fields being taken into
account Furthermore this dynamic aperture remains about the same for off-momentum particles (up to
plusmn3) compared to particles having the nominal energy The fringe field of the dipole magnets
introduces high order non-linear components Since the Thom-X dipoles are short with a large gap
their magnetic design (see section 34) has been carefully worked out in order to maintain the dynamic
aperture
Pole number 6 8 10 12 18 20 28 30
Inner dipoles +10 0 +3
Outer dipoles -16 -1 -6
Quadrupoles +10 +5 -5
Sextupoles +10 -10
Table 34 List of multipole tolerances for each kind of magnets expressed in 10-4
relative field variation
at 18 mm radial position
Figures 39 and 310 show the change of the dynamic aperture with and without considering the
multipole (the fringe field of the quadrupoles is also taken into account in these plots) These
Dynamical Apertures (DA) have been calculated with two different codes BETA and TRACYIII They
lsquo
36
both give very similar results In addition the TRACYIII code dedicated to long symplectic tracking
performs Frequency Map Analysis (FMA) giving more detailed information concerning the particle
diffusion process
Figure 39 On-momentum DA and FMA for the nominal ThomX lattice Beta code left and TracyIII
code (right) sextupoles only (top) and including magnet multipole tolerances (bottom)
lsquo
37
Figure 310 Off-momentum DA and FMA for the nominal ThomX parameters Beta code left and
TracyIII code (right) sextupoles only (top) and including magnet multipole tolerances (bottom)
333 Collective effects feedbacks
At low energy and thus low beam rigidity the electron bunch is sensitive to electromagnetic self-
perturbations that act revolution after revolution in the ring Several instability sources can be
identified
‐ Interaction with surrounding metallic items such as the resistive walls (RW) the RF cavity and
various discontinuities such as bellows BPMs and flanges HOMs etc
‐ Space charge forces
‐ Coherent synchrotron radiation (CSR)
These interactions affect the bunch energy and the bunch longitudinal time-profile leading to
distortions emittance growth bunch oscillations or even above some threshold instabilities and beam
losses Similar interactions may also affect the transverse beam dynamics but to a lower extent The
two relevant parameters for the instability threshold are the bunch charge and its length The strength of
the interaction is proportional to the bunch charge and varies inversely with the bunch length
The two plots in Fig 311 show the longitudinal wakefield over one revolution at 50 MeV generated by
a 1 nC Gaussian bunch with an rms length of 12 and 6 mm (4 and 20 ps) Five different contributions
are compared in this figure
‐ the shielded CSR following the Murphy and Skrinsky [32] formulation with a beam pipe full height
of 40 mm a dipole field of 05 T and a curvature of 0352 m
‐ the longitudinal space charge (LSC)
‐ the resistive wall (RW) following the K Banes and M Sands formulation [33] with a full beam
pipe height of 40 mm and an electrical conductivity of 14 106
‐ the vacuum vessel broadband component including 12 BPM 12 flanges and 12 bellows based on
the synchrotron SOLEIL type This potential has been calculated by means of the HFSS finite
elements code [34]
‐ the 500 MHz ELETTRA type cavity broadband component This potential has been calculated by
using the HFSS finite element code
They exhibit a very strong CSR wake field (up to few per mil) with the shorter bunch case
Figu
re
311
Lon
gitu
dina
l
wak
e
field
over
one
lsquo
38
turn generated by a 1 nC Gaussian bunch at 50 MeV for a bunch length of 4 ps (left panel) and 20 ps
(right panel)
The ThomX low energy operation (50 MeV) practically suppresses the natural damping effect of the
synchrotron radiation and complicates the bunch stabilization Each kind of perturbation grows
irreversibly implying that feedback systems in the three planes are essential to cope with the induced
oscillations of the bunch center-of-mass In addition to these collective effects the unavoidable
injection jitters have also to be taken into account To calculate these effects 6D tracking simulations
that include the longitudinal impedance as well as the large mismatch between the linac and the ring
optics in the longitudinal phase space have been performed (see Fig 312)
Large longitudinal oscillations
without feedback at injection
induced by collective effects
Longitudinal oscillations damped by the
RF phase feedback system operating at
F~360 kHz
Damping lt 50 micros
Figure 312 Tracking results (50 MeV
1 nC bunch) over the first 3000
revolutions just after injection without
and with the RF phase feedback system (VRF = 300 kV)
Large oscillations start very quickly after injection when the bunch is still short (4 ps) and are of the
same order of magnitude as the ones induced by the energy-phase jitter that affect the linac injection
This effect is the one to address in the design of the longitudinal feedback As far as the transverse plane
is concerned the growth times characterizing various instability sources are listed in Table 35 The
revolution-by-revolution feedback strength indicated for each instability source is the one needed to
suppress the oscillations
Source Type Growth time Revolution-by-
revolution kicker
strength
Beam pipe
geometry
TMCI
Head-Tail
-
160 micros
gt10 nrad
Resistive Wall
600 micros gt 2 nrad
Ions lt 100 micros gt20 nrad
Table 35 Transverse instability growth time and feedback kicker strength for a 1 nC bunch at 50 MeV
lsquo
39
In contrast with what prevails in the longitudinal phase space the main transverse oscillation source is
the injection position jitter Since the non-linear sextupole components are large this jitter may produce
a large emittance increase as a consequence of the phase space painting process (see Fig 313)
Figure 313 Sketch of the transverse phase space painting process
Simulations show that a very fast increase of the transverse emittance can result from steering errors at
injection in about 500 revolutions or 25 micros To overcome this effect a fast transverse feedback is
needed it has to damp the oscillation in about 100 revolutions or ~5 micros The strength of the revolution-
by-revolution feedback has to be on the order of 2 microrad This strength is far larger than the ones
estimated to cope with the transverse instabilities For this transverse feedback a dedicated strip-line
acting in both planes is foreseen in the RF section (see Fig 314)
Figure 314 Simulated transverse emittance increase induced by a 05 mm injection jitter with and
without feedback
After N
turns
lsquo
40
Phase space Specifications Device
Horizontal amp vertical Fr Frequency = 178 MHz (F0)
Strength = 2 microrad
Dedicated strip-line for
both planes
Longitudinal Frequency = 360 kHz (Fs)
Strength = 105 rads
Main RF phase loop
Table 36 Summary of the feedback specifications
Other collective effects such as the intra-beam scattering (IBS) which is important at low energy or the
Compton recoil effect play an important role during the 20 ms storage time Their dynamics differ from
an instability dynamics but they also increase the electron energy spread and the bunch emittance mdash
and thus reduce the X-ray flux
To summarize three different contributions to the e- bunch dynamics must be dealt with
‐ at injection jitters which may affect either the injected bunch energy or its trajectory or the precise
timing of its injection in the ring generate oscillations which may be large and which must be
quickly damped by feedbacks in all three dimensions (x y z)
‐ during a transition period corresponding to the first ~1000 revolutions strong collective effects
enhanced by a possible synchronization error between the linac and the ring may lead to a complex
turbulent regime in the longitudinal plane prior to ring matching (elliptical phase space density
profil)
‐ during the 20 ms storage period IBS and CBS produce a progressive increase of the bunch size in
all three directions This evolution cannot be stopped by a feedback system In contrast RF HOMs
and other perturbations may generate instabilities that should be controlled by feedbacks
To take into account collective effects a dedicated code has been developed including a collection of
wake field as well as non-linear dynamics It is based on a 6D multi-particle symplectic tracking
restricted to perfect hard edge drift quadrupoles and dipoles of finite length Other higher order
elements as sextupoles are treated in the thin length approximation The RF cavity is simply treated as
a longitudinal cosine kick The mapping through elements is given by the exact Hamiltonian solution
[34] for the drift sections and the dipoles (including their small curvature and dipole edge effects) The
quadrupole mapping is restricted to small radial angles It has been compared to various codes
(TRACYIII ELEGANT BETA and MADX) by performing simulations of the SOLEIL and the
ThomX rings with special care given to the dipole edge effects [35]
The transverse beam size at IP and the bunch length are plotted in Fig 315 over the first 10 000
revolutions or 056 ms with a 1 nC bunch charge This study shows that a small positive chromaticity in
the horizontal plane is necessary to stabilize the horizontal emittance During the very first revolutions
the tune dependent emittance behavior has not been fully investigated nor understood It appears that
this behavior follows from the bunch length mismatch between the linac and the ring The fact that the
linac produces very short bunches of 4-5 ps (rms) leads to a complex turbulent dynamics (see Fig
316) The effect of this mismatch disappears progressively over the first 1000 revolutions then the
bunch length reaches 20-25 ps (rms)
lsquo
41
Figure 315 Red RMS beam size and bunch length at IP Magenta bunch centroid position over the
first 10 000 revolutions (056 ms) for a 1 nC bunch The beam energy is 50 MeV
Figure 316 Turbulent dynamics in the longitudinal phase space that us taking place just at injection
These plots show the phase space density after 100 and 3000 turns
lsquo
42
334 CBS and IBS effects
The electron beam dynamics that prevails in a ring where Compton back-scattering is taking place is
similar to the one resulting from synchrotron radiation (SR) in a synchrotron light source CBS gives
rise to random excitation in the transverse and the longitudinal planes In the transverse plane the
emittance turns out to remain close to the one at injection [37] In contrast the CBS excitation strongly
affects the dynamics in the longitudinal phase space It leads to a substantial energy spread increase
over 50 ms storage time (see Fig 317a) resulting in a significant bunch lengthening
Based on the lattice parameters the calculation of the Touschek lifetime leads to a value on the order of
10 s this effect can thus be neglected
Figure 317 a) Energy spread increase and b) Radial (x xrsquo) relative emittance increase during a 100 ms
storage time Blue curves CBS effect only red curves IBS effect only black curves (almost on top of
the blue ones) CBS and IBS combined effect The IBS calculation is based on the Mtingwa formalism
[38]
In contrast the multiple Touschek effect (lsquoIBSrsquo) plays a significant role in the radial phase space (x xrsquo)
as shown in Fig 317b After 20 ms this horizontal emittance is increased by a factor ~ 4 from 50 to
200 nmrad and the relative energy spread by ~20 from 03 to 036 as can be seen in Fig 317a
Figure 317 shows that the Compton back-scattering process plays a role by far more important than the
IBS does It also shows that because of the large emittance increase produced by the CBS the bunch
density decreases by a large amount making the IBS much less effective (see Fig 317b where the
curve showing the combined effects lies below the curve illustrating the IBS effect by itself)
When the electron beam energy is increased up to 70 MeV the IBS effect on the horizontal emittance
becomes less important During a 20 ms storage cycle this effect is reduced by a factor 2 On the other
hand the energy spread degradation increases by more than a factor two compared to the 50 MeV case
This change is due to the higher energy of the back-scattered photons In contrast if the electron beam
energy is decreased down to 25 MeV the energy spread is reduced by a factor two while the horizontal
emittance increases by a factor seven compared to the 50 MeV case
Assuming an optimal overlap between the e- bunch and the laser pulse in the interaction region and
neglecting the e- bunch size variation across this region (the lsquohourglassrsquo effect) as well as the variation
of the laser beam envelope near its waist (characterized by the Rayleigh length) the X-ray flux is given
by the following formula (see Eq 11)
The nominal storage time is 20 ms only
lsquo
43
where σth is the Thomson cross section Nx is the number of photons per laser pulse f is the e- bunch
revolution frequency ϕ is half the collision angle σxyze σxyzγ are respectively the three dimensional
bunch rms sizes for the electron (e subscript) and for the laser pulse (γ subscript) In fact because of the
increase of the e- bunch size during the storage time the flux is reduced by roughly a factor two at the
end of a cycle as shown in Fig 318
Figure 318 X-ray flux as a function of the storage time These results take into account the energy
spread increase and the emittance degradation due to IBS only (blue curve) and to the combined CBS
and IBS effects (black curve)
To calculate more precisely the X-ray flux produced by Thom-X the code CAIN [39] (cf sect 12 above)
has been included in the previous 6D tracking code including the non linear effects of the lattice
magnets as well as collective effects The X-rays characteristics are computed at each ring revolution
The intra-beam scattering effect is calculated also at each e- bunch revolution based on the Bjorken and
Mtingwa formalism [38] To reduce the computing time the code extrapolates the IBS effect whenever
there is only a little change from one revolution to the next (this method reduces the computing time by
a factor 4) This code runs on linux OS Using the computer farm of the IN2P3 computing center in
Lyon it takes about 15 days to simulate a full Thom-X cycle (20 ms ie 400 000 revolutions) when the
e- bunch is modeled by 500 000 macro-particles
The way the total X-ray flux decreases during the first 10 000 revolutions is shown in Fig 319 This
decrease is due to the e-
beam quality degradation mainly due to the CBS effect Also shown in this
figure is the X-ray flux evolution as a function of the storage time for three different lower cut-offs in
the X-ray energy spectrum Figure 320 shows a typical angular distribution of the X-ray emission
Figure 321 plots the bunch longitudinal phase space for the first revolutions The macro-particles in
green are those which have been hit by photons at the last crossing while those in blue have been hit
before the others are in red
( ) ( ) φσ+σ+φσ+σσ+σ
φfNNσ=N
2
γzez
2
γxex
2
γyey
γeth
x22222
sincos
2πcos
lsquo
44
Cross comparison and benchmarking of this code against other codes developed for ThomX (6D
tracking without Compton effect or Compton + IBS without tracking) are in progress and indicate a
good agreement between the codes
Figure 319 Left panel Number of X-rays produced as a function of the X-ray energy at the end of the first
10 000 first revolutions (in red) and during the last 10 000 revolutions before extraction (in blue) Right
panel Number of X-rays produced summed over 2500 revolutions as a function of the revolution number
Figure 320 X-ray intensity as a function of the emission angle in mrad (horizontal axis) and their
energy in eV (vertical axis) The color bar indicates the number of X-rays
lsquo
45
Figure 321 Upper nine panels Evolution of the e- beam longitudinal phase-space after injection The macro-
particle positions are plotted along the horizontal axis while their energy deviations are plotted along the
vertical one Green electrons which have collided with photons at the last crossing blue electrons which
have collided before red electrons which have not yet undergone any collision These plots reveal a turbulent
dynamics Lower nine panels Time evolution of the beam longitudinal phase space as a result of a turbulent
regime from the initial lsquobanana shapersquo at injection to a Gaussian distribution after a few ms (colors indicate
the electron number-density red corresponds to the most populated region)
lsquo
46
Figure 322 Evolution of the X-ray energy spectrum when the e- beam energy is increased from 475 to 505
MeV Left panel The X-ray energy spectrum is plotted vertically for seven e- beam energies Colors
characterize the X-ray intensity Right panel Each curve shows the total number of X-rays above some lower
energy cut-off Green cut-off set at 20 KeV blue cut-off at 30 KeV red cut-off at 40 KeV
335 Scattering on the residual gas
Coulomb scattering of the electrons on the residual gas molecules induces an increase of the circulating
beam emittance The amplitude of this effect is proportional to the average of the beta functions and to
the gas partial pressure Moreover it varies as the square of the atomic number of the residual gas
atoms and is inversely proportional to the electron energy With a low beam energy of 50 MeV it is
therefore essential to keep a low pressure to minimize the time-averaged emittance Fig 323 shows the
growth of the transverse emittance during the 20 ms storage time in the nitrogen case and in the
hydrogen one Assuming that one deals with nitrogen at a pressure of 8 10-7
Pa the emittance increases
by a factor of two (see Fig 323) while the degradation is negligible in the dihydrogen case We
conclude that a pressure lower than 3 10-7
Pa must be maintained to keep the emittance growth at an
acceptable level
Figure 323 Emittance increase due to Coulomb scattering assuming that the residual gas is composed
of a) 100 nitrogen b) 100 dihydrogen
lsquo
47
336 Ion effects
The beam-ion instability [310 39 312] originates in the ionization of the residual gas molecules Ions
can subsequently be trapped by the electron beam electrostatic potential We recall that the on-axis
potential is inversely proportional to the beam size
When the trapped ion number density reaches some threshold beam instability is triggered Besides this
effect trapped ions damage the beam quality by shifting the tunes increassing the emittance and
lowering the beam lifetime since the local pressure is increased
For the Thom-X ring the critical mass above which ions are trapped is on the order of one in units of
the proton mass Thus all ions whatever their mass can be trapped in our case
Once trapped ions oscillate transversally in the potential well Their oscillation frequency depends on
the electron beam size and on the ion species (see Fig 324 1c) Because of the e- beam bunching
when the ion oscillation frequency becomes larger than the beam betatron frequency a lsquofast ion
instabilityrsquo may develop the e- bunch is submitted to the electric field of the oscillating ions eventually
resulting in an exponential growth of the oscillations of all particles (electrons and ions) This instability
arises above some value of the neutralization factor which depends on the lattice
The neutralization factor and the growth time of either the vertical or the horizontal oscillations are
shown in Fig 324 as a function of the horizontal beam size for several ion species For a horizontal
beam size averaged over the ring of 05 to 1 mm (which is about what is expected) the growth time is
very short (less than one micros) but for this would happen for a neutralization factor much larger than one
obviously this is excluded To keep under control the other ion harmful effects mentioned above one
may prevent ion accumulation over many storage cycles by introducing gaps between short series of
injections or by clearing the ions by an electrostatic potential that cancels out the potential well
produced by the e- beam A simple solution consists in polarizing the beam position monitors (BPMs)
thus avoiding the insertion of electrodes along the ring This scheme should be effective since ions will
drift longitudinally as the kicks they receive at each e- bunch revolution has a longitudinal component
The amplitude of these kicks along the z direction is shown in Fig 325 where the BPMs are
represented by circles They are placed near accumulation points where the longitudinal kick vanishes
ie where ions will converge A difficulty may arise at s =125 m This point is opposite to the IP in the
lattice at this point the e- beam has small transverse dimensions and a vacuum pump is placed Indeed
with a small beam size the neutralization factor threshold is very low (10-3
) and the growth time
constant is on the order of one micros This potential difficulty can be overcome by adding an electrode in
the short section between the two dipoles to clear the ions at this accumulation point
Ion clearing with polarized BPMs or gaps introduced in the injection sequence should keep other ion
effects at an acceptable level (more work is needed to quantify this statement)
Defined as the total number of ions divided by the total number of electrons
lsquo
48
Figure 324 a) Threshold neutralization factor b) growth time and c) ion oscillation frequency for the
following fractional becirctatron tunes 1) 064 and 2) 017 for different ion species The dashed lines show
the e- bunch betatron frequency σx is the mean beam size
Figure 325 Longitudinal kick given to on-axis ions at each e- bunch revolution
337 Electron beam injection and extraction
Originally the linac beam was injected in the ring by using only a septum followed by a fast kicker
Given the high repetition rate (50 Hz) radiation safety issues and a possible noise in the X-ray
experimentation area lead to set up an e- beam extraction line The symmetric septum magnet located in
lsquo
49
the middle of one of the long straight sections is used in a double pass mode for injection and
extraction It is flanked by two identical kickers one downstream for the injection and one upstream for
the extraction (Fig 326)
Figure 326 Sketch (top panel) implementation (middle panel) and simulation (lower panel) of the
injectionextraction process
The main characteristics of the septum and the kicker magnets are listed in Table 37 Since in both
processes the beam runs through quadrupoles some margins with respect to the nominal deflection
values must be provided to cope with the fact that quadrupole fields are not uniform
Kicker Septum
Deflection 15 mrad max (11 nominal) 170 mrad (40 mTm 70 MeV)
Pulse duration lt 56 ns half sine 130 micros full sine
Effective length 200 mm
250 mm
Overall length 450 mm 500 mm
Table 37 Main characteristics of the pulsed devices
Septum center
x =14 mm
xp=9 mrad + 170 mrad
Kicker center
x =0 mm
xp=0+11 mrad
2 fast kickers
Inj Ext
1 slow septum
lsquo
50
338 Orbit corrections
A set of 12 beam position monitors (BPMs) and 12 correctors in both planes are implemented along the
lattice in order to control and correct the orbit An additional BPM located at the IP is fixed on the
table that supports the laser cavity The main error sources are listed below Uncorrected and corrected
orbits are illustrated in Fig 327
rms value dBB dL dX dZ dS tilt
Dipoles 10-4 01 mm 02 mm 01 mm 02 mrad
Quadrupoles - - 02 mm - -
Dipoles 10-4 01 mm 02 mm 01 mm 02 mrad
Quadrupoles - - 02 mm - -
Table 38 Main magnet misalignment errors
Uncorrected orbit with typical magnet alignment errors
rms value xmax (mm) zmax (mm)
Max 7 3
IP 05 03
rms value xʹmax (mrad) zʹmax (mrad)
Max 12 4
IP 6 3
lsquo
51
Corrected orbit
rms value xmax (microm) zmax (microm)
Max 200 140
IP 200 30
rms value xʹmax (microrad) zʹmax (microrad)
Max 600 300
IP 350 60
Figure 327 Uncorrected and corrected closed orbits Red lines horizontal projection blue lines
vertical projection
Quadrupoles and sextupoles are fixed on girders with a maximum error of 50 microm The magnetic field
center error relative to the magnet yoke being of the same order the expected total alignment errors are
~100 microm in each plane The girders supporting all the elements will be aligned as solid blocks Since
lsquo
52
dipole misalignments have a large effect on the orbit dedicated additional support tools will allow the
readjusting of the dipole positions relative to the girder
One needs a maximum corrector strength of 05 mrad when this strength is computed by assuming just
one sigma alignment errors to cope with alignment errors which would be twice as large corrector
strengths of 1 mrad are thus necessary
Figure 328 Orbit bumps (red horizontal blue vertical) that can be produced at the IP
In addition to this standard orbit correction procedure a dedicated orbit scan at the IP will be very
useful to ensure an optimal overlap between the e- bunch and the laser pulse in the interaction region
(see Fig 328) To provide a scan range of up to 1 mm or 1 mrad the corrector strength must be further
increased by 1 mrad Thus the corrector maximum strength will be 2 mrad
The orbit correction will be performed by a slow feedback operating around 01 The correctors are in
the sextupoles magnets (combined magets) The orbit feedback correction will also control the RF
frequency in order to keep the orbit length at the ring design value A specific synchronization scheme
that ensures the proper injection timing and that maximizes the electron-photon collision rate is
described in chapter 5
Position amplitude scan at IP location up to 1 mm
Angle amplitude scan at the IP
location up to 1 mrad
lsquo
53
34 Magnets
Table 39 describes the magnets that will equip the transfer line the ring and the extraction line
Magnet type Dipoles Dipoles for inj Quad Sext Corr
Number 4+8+2 2 7+24+2 12 4+12+1
PS number 3 2 33 3 34
Strength 0180 T m 5 Tm 40 Tmsup2 05 mTm
Length 2765 mm 150 mm 60 mm 30 mm
Table 39 ThomX magnet parameters
341 Dipole magnetic design
The dipole bending angle is 45deg A magnetic field of 05 T is needed at 50 MeV Since their radius of
curvature is very small (0352 m) special attention has been paid to the field profile between the magnet poles
and to the pole edge shape where large multipole components are present Lateral shims and pole edge
specific profiles have been optimized with the TOSCA3D code to minimize the impact of the multipoles on
the beam dynamics
Characteristics unit value
Gap mm 40
Field at centre T 063
Overall dimension mm3 350 x 400 x 400
Current A 263
Voltage V 177
Pole width mm 100
Pole height mm 100
Yoke length mm 2765
NI Aturns 13000
Coil area mmsup2 67x40
Current density Ammsup2 116
Number of turns turns 48
Transverse size of the conductor mmsup2 6x6
Cooling channel mm 4
Water pressure drop bar 7
Temperature increase degC 47
Flow rate (per dipole) lmn 86
Electric power (per dipole) W 3100
Table 310 ThomX dipole parameters
lsquo
54
Figure 329 Drawing of a ThomX dipole
342 Quadrupole magnetic design
The quadrupole design (see Fig 330) was also performed with the TOSCA software The magnetic field
is produced by four coils carrying a DC current The conductor is wrapped around four poles which
concentrate and define the quadrupole magnetic field map
Figure 330 Quadrupole magnetic design
A quadrupole can be manufactured in four identical parts each one comprising one pole and one quarter
of the yoke or the yoke and the four poles can be manufactured separately The quadrupole will be
equipped with special shims which will be adjusted during the magnetic measurement to ensure a good
alignment of the magnetic axis and low harmonic components The ThomX quadrupoles are air-cooled
to reduce the infrastructure cost This is possible since the maximum current density is 15 Ammsup2
Figure 331 shows the magnetic field intensity versus the radial distance from the quadrupole axis The
maximum gradient is 5 Tm Figure 332 shows the magnetic field intensity at a distance of -20 mm
from the magnet axis versus the longitudinal coordinate z The maximum magnetic field is 01 T
lsquo
55
Figure 331 Magnetic field (in T) versus the distance (in mm to the magnet axis
Figure 332 Magnetic field intensity (in T) versus z (the longitudinal coordinate in mm)
lsquo
56
The main parameters of the ThomX quadrupoles are given in Table 311 Each quadrupole is powered
by a 10 A 32 V DC power supply thus the released heat power remains below 32 W A dedicated
magnetic bench equipped with a rotating coil will be used for the quadrupole magnetic measurements
This bench has been developed for the magnetic measurements of the SOLEIL storage ring
quadrupoles
Parameters unit value
Bore mm 41
Gradient Tm 5
Current density Ammsup2 15
Overall dimensions cm3 19x19x18
Current A 10
Voltage V 35
NI Aturns 810
Pole width mm 20
Pole height mm 50
Coil dimensions mmsup2 15x36
Number of turns turns 81
Wire dimensions mmsup2 5x125
Yoke length mm 150
Yoke thickness mm 20
Chamfer mm 5 (not optimised yet)
Table 311 Main parameters of the ThomX quadrupoles
343 Sextupole magnetic design
The ThomX sextupole design (see Fig 333) was also performed with the TOSCA software The magnetic
field is produced by six coils that carry a DC current The coils are wrapped around six iron poles which
concentrate the field and define the sextupole field map These magnets are combined function and are also
equipped with additional coils to produce the corrector dipole fields
Figure 333 Sextupole magnetic design
lsquo
57
Each sextupole yoke is manufactured as a single piece and the six poles are screwed on this yoke Then a coil
is wound around each pole During the design phase a chamfer was calculated with TOSCA to guaranty a low
harmonic content The sextupoles will be air-cooled to reduce the infrastructure cost The maximum current
density is 15 Ammsup2
The magnetic field intensity versus the distance to the magnet axis is shown in Fig 334 The maximum
sextupolar strength is 40 Tmsup2
Figure 334 The ThomX sextupole magnetic field versus the distance to the magnet axis
Fig 335 shows the variation of the magnetic field at a distance of -20 mm from the sextupole axis versus the
longitudinal coordinate z The maximum magnetic field is 0016 T
Figure 335 ThomX sextupole magnetic field versus z (the longitudinal coordinate)
Table 312 gives the main parameters of the ThomX sextupoles They will be powered by a 10 A 05 V DC
power supply
lsquo
58
Parameters unit value
Bore diameter mm 42
Sextupolar Strenght Tm2 40
Field 20 mm T 0016
Current density Amm2 25
Overall dimension cm3 19x19x8
Current A 10
Tension V 05
AmpereTurn At 100
Pole width mm 20
Pole height mm 40
Coil dimension mm2 6x40
Number of turns Tours 10
Conductor dimension mm2 2x2
Yoke length mm 60
Yoke thickness mm 20
Chamfer mm 15
Table 312 Main characteristics of the ThomX sextupoles
A dedicated bench equipped with a rotating coil will be used for the magnetic measurements of the sextupoles
This bench has been developed for the magnetic measurements of the SOLEIL storage ring sextupoles
344 Corrector magnetic design
Simple window frame correctors based on an already existing SOLEIL design will be used for the orbit
corrections of the two transfer line
Figure 336 Magnetic design of the window frame correctors
lsquo
59
345 Injection dipole
The two injectionextraction dipoles compensate the septum deflection with a 170 mrad deflection (cf sect 337
see Fig 326) They are also air-cooled Their design and their technical characteristics are illustrated in Fig
336 and Table 313
Figure 336 Magnetic design of the injection dipole
Parameters unit value
Gap mm 40
Field at the magnet centre T 0224
Current A
10
Voltage V 6
Pole width mm 60
Pole heigth mm 70
Yoke length mm 150
AI (per coil) At 3600
Coil cross section area mmsup2 60x60
Current density Ammsup2 09
Number of turns tours 360
Conductor size mmsup2 10 x 1
Pressure drop of the cooling water bar NC
Temperature increase degC NC
Power (per dipole) W 120
Table 313 Parameters of the injection and extraction dipoles
lsquo
60
35 Power supplies
In order to provide a high operation flexibility ThomX will be equipped with the following power supplies
ndash One power supply for the ring dipoles (the latter will be connected in series) one for the TL dipoles
and one for the DL
ndash Individual power supplies for all the other magnets (quadrupoles sextupoles and correctors)
All the power supplies will have a 16 bit-resolution and a plusmn20 ppm reproducibilitystability over 24 hours The
technology is a switched-mode one in order to achieve good efficiencies (gt 85)
Magnet Nb PS Nb mag current
(A)
voltage
(V)
type load
R
(mOhm)
L
(mH)
Dip ring 1 8 300 170 Unipol 565 48
Dip TL 1 4 300 85 Unipol 280 24
Dip DL 1 2 300 45 Unipol 150 12
Dip inj 2 1 12 18 Unipol 1330 180
Quad 33 1 12 6 Unipol 500 10
Sext 12 1 12 3 Unipol 250 08
Corr ring 24 1 plusmn12 plusmn3 Bipol 250 08
Corr TL DL 10 1 plusmn12 plusmn3 Bipol 250 08
Table 314 Overview of the ThomX power supply equipment
351 Pulsed magnets
3511 Layout
As discussed above the e- beam injection in the storage ring and its extraction will take place in the
same straight section (see Fig 337)
Figure 337 Injectionextraction section layout
Eddy current septum magnet
(under vacuum) Fast Kicker Fast Kicker
lsquo
61
The first injection step involves a thin septum magnet It bends the incoming bunch trajectory to bring it
inside the ring vacuum chamber close to the stored beam orbit Then a fast kicker compensates for the
residual trajectory angle and brings the injected bunch on-axis
Extraction is realized by another fast kicker followed by the same septum magnet as the one used for
injection This kicker deflects the bunch trajectory so that it enters into the septum channel in turn the
thin septum bends the bunch trajectory which enters then into the extraction channel that brings the
bunch into a beam dump
In this configuration the same thin septum magnet is used for both the bunch injection and the bunch
extraction This requires a symmetrical design with two flanges one dedicated to the injection and the
other to the extraction The magnetic gap is wide enough to accommodate the two bent trajectories
When operating at a 50 Hz injection rate the septum current pulse consists of a slow full sine whose
same positive half sine will be used successively for the lsquoold bunchrsquo extraction and the for the lsquonew
bunchrsquo injection with a suitable delay between the two steps
The two fast kicker magnets are identical their length has been reduced as much as possible because of
the very limited space available The current pulse in the kickers will have a half-sine shape One must
achieve a rise time (from zero current to peak current) and a decay time (from peak current to zero)
each less than a ring revolution period Thus the current pulse can have a maximum duration of two ring
revolution periods Consequently their charging voltage reaches high values even though most of the
deflection is performed by the septum magnet
The design of these three devices ensures that they can operate with a beam energy of 50 MeV
(nominal) or 70 MeV (maximum) and at a 50 Hz repetition rate
3512 Slow septum magnet and its pulsed power supply
The main constraints on the septum design are as follows This septum must produce a large trajectory
deflection (160 mrad nominal peak value 170 mrad maximum peak value) to bring the injected beam
very close to the ring straight section axis with no significant disturbance of the stored beam The
magnet mechanics should preserve the ring vacuum chamber geometry to keep its impedance as low as
possible Furthermore its stray field must remain very low within the vacuum chamber where the stored
beam circulates as the septum excitation is done by a full sine pulse whose width (130 micros) is very long
compared to the ring revolution time (56 ns)
An eddy current septum magnet installed in vacuum will fulfil these constraints The effective septum
blade thickness can be as small as 3 mm thus ensuring the closeness of the stored beam orbit and the
desired bunch trajectory at the output of the septum yoke An internal chamber connected to the magnet
yoke extends the ring vacuum chamber and preserves the impedance budget The injected bunch
deflected by the septum magnet field penetrates through a small hole in this internal chamber Such a
septum magnet is very asymmetric in the transverse plane it is driven by a copper loop that wraps the
steel yoke
In order to contain the magnetic flux the whole yoke is enclosed in a copper box which carries the eddy
current On the side facing the stored beam orbit the copper box thickness is reduced so that it
constitutes the thin septum blade As a result of the electric shielding provided by the eddy current
circulating in the copper septum such a structure is very effective to avoid a large magnetic stray field
outside the magnet gap The fact that the excitation pulse is a full sine reduces the duration and the
lsquo
62
amplitude of the stray field (the slow delayed stray field tends to be the sum of the two opposite half
current arcs this feature brings about a reduction of the stray field)
An additional magnetic shielding is required to strongly reduce the residual strayfield This shield is
provided by a 05 mm thick sheet of high permeability mumetal that will completely surround and
enclose the internal chamber (thus shielding the stored beam orbit from the strayfield)
Such a design that combines an eddy current shield a full sine pulse excitation and the use of a high
permeability magnetic material reduces the stray field to a very low level (a few 10 ppm of the main
pulsed field) [313]
Assuming a septum opening of H x V = 30 x 12 mm2 and an active length of 250 mm (the length was
chosen as short as possible because of constraints coming from the septum implementation in the
straight section) the septum magnet inductance is estimated to be 16 microH Connexions and cables
increase this inductance up to 23 microH
This design leads to the following parameters for the septum magnet and its power supply (values for a
70 MeV ring operation)
Septum deflection field integral field peak current charging voltage
170 mrad 40 mTm 160 mT 1500 A 226 V
Table 315 Septum main parameters
The pulsed power supply will be based on the lsquoclassicalrsquo resonant discharge circuit scheme composed
of a storage energy capacitor (190 microF) a fast high-current thyristor switch and a transmission line to
the magnet comprising a few low impedance coaxial cables
A 300V-20A power supply is foreseen It will be connected to the storage energy capacitor through a
current-limiting power resistor
3513 Fast kicker magnet and its pulsed power supply
The two fast kicker magnets must be identical Because of the very limited space available their length
has been reduced as much as possible The final numbers are 200 mm and 450 mm for the active length
and for the overall length respectively They will be located outside the vacuum vessel with their
lsquowindow framersquo rectangular ferrite yokes surrounding the ceramic vacuum chamber (see Fig 338)
Figure 338 Front view of the kickers
The two ceramic sections of the vacuum chamber will have a flange-to-flange length of 450 mm Their
aperture will be the same as the one in the ring standard sections but with a racetrack transverse shape
Both extremities of each ceramic chamber include a smooth shape transition and a short bellow that
lsquo
63
allows for RF finger contacts These bellows can accommodate thermal dilatations especially during
bake-out The ceramic chambers will be coated with a metallic deposit that will carry the bunch mirror
current and thus avoid an increase of the chamber impedance This coating needs to be very thin It will
consists of a titanium layer with a thickness le0050 microm The thickness has been calculated so as to
avoid the deterioration of the fast pulse shape and to avoid the temperature rise due to the mirror current
[314]
Figure 339 Time profile of the kicker pulse versus the titanium layer thickness
The kicker dimensions follow from
- the aperture needed by the stored beam H x V = 40 x 28 mm2
- the required deflection kick nominal 11 mrad maximum 15 mrad
- the beam energy nominal 50MeV maximum 70MeV
- the short revolution period of the stored e- bunch Trev = 56 ns
- the very short length of the injected bunch from 4 to 25 ps (rms)
With a ceramic thickness of 6 mm the effective magnet aperture must be H x V = 55 x 55 mm2 These
parameters determine the current and voltage pulse characteristics
The kicker magnets will be powered by a circuit delivering half-sine pulses [315] In fact because of
the e- bunch short revolution period (56 ns) the field pulse duration has to be less than twice the ring
revolution period (less than 112 ns) and thus must a have very fast rise time and decay time (both less
than a ring revolution period) On the other hand with such a very short bunch length (4 or 25 ps (rms)
whether one deals with the incoming bunch or the extracted bunch one may assume that the pulses do
not have to present a marked flat top
These considerations lead to the following kicker parameters
00000E
-2
17000E
-10
34000E
-10
51000E
-10
68000E
-10
85000E
-10
10200E
-9
11900E
-9
0000E+00
2000E-01
4000E-01
6000E-01
8000E-01
1000E+00
1200E+00
Thom-X injection kicker pulse field attenuation relating to Titanium thickness
Bext(t)
Bint(t) 002microm
Bint(t) 004microm
Bint(t) 005microm
Bint(t) 006microm
Bint(t) 008microm
Bint(t) 01microm
Bint(t) 02microm
Time
Fie
ld a
mp
litu
de
lsquo
64
Kicker beam energy deflection field integral
Operation at Enominal 50 MeV 15 mrad 25 mTm
Operation at Emax 70 MeV 15 mrad 35 mTm
field peak current estimated voltage
92 mT 406A 10 kV
175 mT 771 A 18 kV
A specific RampD program is necessary to guarantee that these constraints will be fulfilled by the power
supply and by the magnet Preliminary studies were based on the standard circuit used for such a pulsed
power supply (see Fig 340) involving a high voltage (HV) capacitor to store the energy in series with
a HV switch and a load whose inductance is close to the one of the kicker magnet In this circuit the
HV switch is a very fast MOSFET switch followed by a HV fast recovery diode module
Figure 340 Power supply circuit for the ThomX kickers
These first studies lead to unsatisfactory preliminary results
- The BEHLKE Ultra-Fast 17 kV 480 A MOSFET module can switch ON in a very short time less
than 8 ns Then the pulse shape is normally determined by the passive component parameters in the
circuit provided that the negative current is quickly interrupted either by HV diodes in series or by
a turn Off control of the HV switch
- But the different types of HV diodes used are not able to switch OFF the pulsed current quickly
enough Even the lsquofast recovery diodesrsquo have a reverse recovery time too much long to stop cleanly
the pulse at its zero crossing Thus it was necessary to work on other topologies and to find better
suited components
Further simulations were performed to see how the circuit could be improved It turned out that
controlling the switch both at the closing and at the opening can improves the shape of the current
pulse A prototype using the same Behlke MOSFET modules and including a coaxial transmission line
was built and tested A plot of the current pulse in the kicker coil that was obtained is shown below
lsquo
65
This plot was obtained with the following prototype circuit
As one can see more work is still needed to improve the shape of the current in the coil (namely one
should reduce the level of reflections improve the current pulse shape and work on the peak current to
meet the 70 MeV requirements) We are also looking for an other supplier who could provide controlled
HV switches ensuringr both ON and OFF fast operation
Mechanical and magnetic studies of the kicker magnets are ongoing
36 Storage ring RF system
The choice of 500 MHz for the RF frequency is a compromise between several constraints the cavity
fundamental and high order mode (HOM) impedances space requirements the availability of RF power
sources and other components The required accelerating voltage of 500 kV can be provided by only
one 500 MHz single-cell cavity with about 40 kW of RF power As shown in the following the choice
of the cavity design is mainly dictated by the need for a strong damping of the HOM
lsquo
66
361 HOM impedance thresholds as regard to beam instabilities
When a cavity mode is excited in resonance it generates beam oscillations with a rise time
approximately given by one of the two following expressions
(1τ)l = Ib α fm Rl (2 Qs Ee) in the longitudinal case and
(1τ)t = Ib βt fo Rt (2 Ee) in the transverse case
where fm is the HOM resonant frequency Rl [Ω] and Rt [Ωm] are respectively the longitudinal and the
transverse HOM impedances Qs = fs fo is the synchrotron tune (fs and fo are the synchrotron and the
revolution frequencies) βt is the beta function amplitude at the cavity location α is the momentum
compaction factor Ib is the average beam current and Ee is the beam energy in eV
Assuming typical values for the cavity HOM impedances (Rl fm = 01 to 1 MΩ GHz Rt = 1 to
10 MΩm) and the machine parameters listed in Table 315 the instability growth time is around 10 micros
in both the longitudinal and the transverse cases
Orbit length C 168 m
Electron energy E 50 MeV
RF frequency fRF 500 MHz
Revolution frequency fo 1786 MHz
Harmonic number h 28
Momentum compaction factor α 0015
β-function amplitude at the cavity location βxz 3 m 4 m
Relative energy spread σE E 40 10-3
Peak acceleration voltage VRF 300 (500) kV
RMS bunch length σs 27 (21) ps
Synchrotron frequency fs 358 (462) kHz
Bunch charge Qb 1 nC
Number of stored bunches Nb 1
Maximum current Ib 1786 mA
Table 315 Ring parameters used to estimate the rise time of the HOM induced instabilities
In a low energy ring like ThomX radiation damping is so weak (the damping time is ~ 1 s) that a
stationary state is not reached during the beam storage time (20 ms) On the other hand to ensure a
good ring operation it is sufficient to make all instability rise times much longer than this storage
period This can be achieved by attenuating the HOM impedances very strongly typically by a factor
104
362 Cures to the HOM induced instabilities
There are essentially two ways of coping with such HOM induced instabilities either by performing a
strong de-Qing of the HOM resonances or by tuning their frequencies away from the beam spectral
lines in order to prevent resonant excitation
In practice HOM de-Qing is achieved by extracting the cavity HOM power and transferring it towards
external dissipative loads But in this approach it is difficult to reach attenuation factors higher than
102 - 10
3 over a large frequency range and this technique requires heavy and cumbersome equipment
close to the cavity (coaxial lines with coupling antennas or ferrite loaded waveguides) The DAMPY
lsquo
67
cavity designed at BESSY [316] and which is used in ALBA [317] and the PEP II cavity developed
at LBNL [318] are two variants of this technique
The other method which consists in controlling the HOM frequencies is better suited to a small
circumference machine like ThomX where the beam spectral linesrsquo spacing (~ 18 MHz) is very large
compared to the HOM bandwidth (this bandwidth is of a few tens of kHz for Qo equal to a few 104) As
long as the HOM density is not too high and the modes can easily be tuned far enough from the beam
spectral lines (δf gtgt fm Qo) it should be possible to reduce their effective impedances (the ones lsquoseenrsquo
by the beam) down to tolerable levels ie Reff asymp Rs (2 Qo δf fm) 2 ltlt Rs
We are investigating the potential performance of this technique by applying it to an ELETTRA-type
cavity (see Fig 344) [319] which combines three frequency tuning means The HOM frequencies can
be precisely controlled by an appropriate choice of the temperature of the water cooling the cavity while
the fundamental frequency is maintained constant by a mechanism which deforms the cavity
longitudinally In addition a movable plunger tuner also called HOM frequency shifter (HFS) located
in the cavity equatorial plane provides an additional degree of freedom [320]
Tables 316 a and b list the characteristics of such a cavity fundamental and HOM resonances They
have been calculated with the 3D GdfidL code [321] One finds 18 significant HOM (9 monopoles and
9 dipoles) with resonant frequencies lower than the cavity cut-off frequency Figures 345 a and b
represent the corresponding HOM spectra fm Rl(f) and Rt(f) folded over a single base band of
18 MHz The horizontal dashed line in the graph represents the impedance threshold corresponding to
an instability rise time of 20 ms the beam storage time Although they remain somewhat critical these
results tend to confirm that it should be possible to find acceptable operating conditions as long as the
HOM frequencies can be shifted by a few MHz by combining temperature and HFS tuning
lsquo
68
Figure 344 Drawing and photo of the ELETTRA cavity
lsquo
69
Figure 345 (a) Longitudinal and (b) transverse HOM spectra folded over a single base band of
18 MHz for the ELETTRA-type cavity
τL = 20 ms
lb
os
I
eEQ
τα
2
ttob
o
fI
eE
τβ
2
τt = 20 ms
lsquo
70
a) Longitudinal modes fm [MHz] Q0 Rs Q0
[Ω]
Δfm (T)
[MHz]
Δfm (HFS)
[MHz]
L0 (fundamental) 499132 44164 765 000 000
L1 958618 48158 336 037 -100
L2 102781 57861 036 080 369
L3 14087 52562 220 174 021
L4 151528 69088 783 112 086
L5 155129 66375 850 240 721
L6 186912 55785 097 142 -092
L7 19495 81752 008 183 143
L8 203985 62729 086 400
L9 206896 87300 530 445 667
Table 316 Main characteristics of the monopole (a) and dipole (b) modes computed with GdfidL for
an ELETTRA-type cavity Δfm (T) and Δfm (HFS) are the effective temperature and HFS tuning ranges
assuming a constant fundamental frequency
It is worthwhile mentioning that in any event fast longitudinal and transverse feedback systems are
required to damp the beam oscillations resulting from injection transients (see section 333) They will
contribute to the HOM damping as well
363 RF power source
An RF power of about 40 kW is required to generate the desired voltage of 500 kV in a single 500 MHz
ELETTRA-type cavity For this frequency there exists a wide choice of power sources based either on
vacuum tubes (klystron IOT diacrode) or on solid-state technology The low operating voltage and
high modularity of the solid-state version offer significant advantages compared to vacuum tubes
Experience acquired at SOLEIL after few years of operation has definitely demonstrated all the benefits
of this technology [322] Figure 346 shows the 35 kW - 352 MHz solid-state amplifier used for the
SOLEIL booster synchrotron It consists of a combination of 147 modules Each one has a power of 330
W a gain of 11 dB and an efficiency of 62 This amplifier was commissioned about 7 years ago
During a running period of more than 35 000 hours it tripped only once and this trip was caused by a
human mistake
b) Transverse modes fm
MHz]
Qo Rsperp Qo
[Ω]
Rsperp[MΩm] Δfm(T)[MHz] Δfm(HFS)
[MHz]
T1 73701 50267 335 2599 04 075
T2 74606 52566 92 7557 155 285
T3 115202 47196 264 3006 049 -312
T4 125565 55386 51 7428 068 725
T5 130249 71674 36 7039 097
T6 169981 69325 103 2542 188
T7 173650 91039 165 5463 gt 2
T8 177337 59409 32 7061 gt 2
T9 183617 77697 55 1643 gt 2
T10 206142 88484 59 2254 gt 2
T11 222475 99092 11 5079 gt 2
T12 225933 56641 13 3484 gt 2
lsquo
71
This technology has already been extended to 476 MHz in the framework of a SOLEIL-LNLS
collaboration aimed at realizing two 50 kW amplifiers for the LNLS storage ring [323] More recently
the SOLEIL team has developed modules which can deliver up to 650 W at 500 MHz with a gain of
17 dB and an efficiency of 62 while still reducing the thermal stress [322] About a hundred of such
modules will be combined to obtain the 50 kW required by ThomX
Figure 346 35 kW - 352 MHz solid-state amplifier of the SOLEIL Booster (147 amplifier modules
and DCDC converters bolded on 8 water-cooled dissipaters)
364 Low level RF system and longitudinal feedback
The proposed low level RF (LLRF) system is conventional As shown by the synoptic diagram of Fig
347 it consists of three relatively slow loops (frequency amplitude and phase) aimed at controlling the
cavity voltage with an accuracy of about plusmn 02 in amplitude plusmn 02deg in phase and a response time
around 1 ms This scheme is suitable for compensating the pressure and temperature drifts as well as
the power supply ripples
Such a system is not fast enough to deal with the beam phase oscillations at the synchrotron frequency
fs on the order of 400 kHz that originate in injection transients These oscillations can produce an
undue emittance blow up and a phase jitter at the interaction point Beam dynamics simulations show
that preserving the beam quality requires a damping of these oscillations within a few tens of a micros (see
section 333) As the radiation damping is negligible during the storage time a longitudinal feedback
600 W 280 Vdc 28 Vdc converter
Power combination
330 W amplifier module
lsquo
72
(LFB) must be implemented It can be performed by a dedicated broad-band longitudinal kicker that
acts on the beam Since ThomX is operating with a single bunch we propose to use the main cavity as a
longitudinal kicker despite its narrow bandwidth of 25 kHz ltlt fs This difficulty can be overcome by
making the cavity effective bandwidth much larger than this value in fact larger than fs We will use a
high gain direct RF feedback (RFFB) that allows the operation of a fast phase feedback (ϕFB)
Assuming a cavity-to-amplifier distance of about 10 m the loop delay will be less than 150 ns Then
one expects an RFFB gain Go larger than 50 which leads to an effective bandwidth larger than 1 MHz
Simulations show that a LFB that combines such a RFFB with a ϕFB (see Fig 348) can damp injection
phase errors of up to 10 degrees within a few micros ie faster than the above specification
Figure 347 Basic LLRF system scheme with lsquoslowrsquo frequency amplitude and phase loops
lsquo
73
Figure 348 Schematics of the longitudinal feedback system combining a phase feedback with a direct
RF feedback
365 RF equipment status
For the ThomX commissioning we will use an ELETTRA cavity The design of all the components of
the 50 kW solid-state amplifier (600 W modules combiners power supplieshellip) was validated in 2011
The next two milestones are first the validation of a 9 kW unit (combining 16 modules) and secondly
the commissioning of the complete amplifier on a dummy load
Several versions of the LLRF system including LFB are under investigation either fully analogue (see
Fig 349) or mixed analogue for the fast feedback loops and digital for the slower ones (see Fig 350)
or even fully digital based on the new generation of fast ADC DAC and FPGA
366 RF system conclusion
For the RF system of the ThomX storage ring we plan to use a 500 MHz single-cell cavity of the
ELETTRA-type Maintaining at a tolerable level the impact of the RF cavity HOMs on the beam is a
challenge However this goal should be achievable with a proper control of the HOM frequencies It
can be done by combining three different means of tuning the cavity temperature control longitudinal
elastic deformation of the cavity and motion of a plunger The fast longitudinal and transverse feedback
systems - which in any event are required to damp the beam oscillations resulting from injection
transients - will contribute to the damping of the HOM excitations as well
In order to provide the required accelerating voltage of 500 kV the cavity will be powered by a solid-
state amplifier of the lsquoSOLEILrsquo type able to deliver a power of up to 50 kW The cavity voltage will be
controlled with an accuracy of plusmn 02 and plusmn 02 degree by means of conventional amplitude and phase
loops (either analogue or digital) with a few kHz bandwidths In addition a fast phase feedback system
combined with a high gain direct RF feedback will provide the longitudinal damping which is required
to preserve the beam quality
lsquo
74
Figure 349 Complete LLRF system longitudinal feedback included (analogue version with phase and
amplitude modulation ndash the option with IQ modulation is also considered)
Figure 350 Complete LLRF system longitudinal feedback included (mixed analogdigital version)
37 Transverse feedback
371 Specifications
Table 317 summarizes the instability growth time constants and the corresponding kicker strength
requirements for the main types of instability It shows that for Thom-X the betatron oscillations due to
the injection jitter in position and angle are the most critical ones They require a feedback damping
time constant of 5 micros or less (cf chapter 3)
lsquo
75
Source type growth time constant kicker strength
Beam pipe
geometry
TMCI
Head-Tail
-
160 micros
gt10 nrad
Resistive Wall 600 micros gt2 nrad
Ions lt 100 micros gt20 nrad
Injection Jitter 5 micros 2 microrad
Table 317 Overview of expected instabilities (1 nC 25 ps rms at 50 MeV)
372 Electronics
To cope with these instabilities we will use a SPring-8 based high performance digital transverse
feedback system which consists of a wideband detector (a BPM) an RF front-end an FPGA based
processor a power amplifier and a stripline kicker (see Fig 351) This system is capable of detecting a
coherent transverse motion and applying a counter kick to damp it bunch-by-bunch and turn-by-turn
with one or two bunches stored in the ring
Figure 351 Spring-8 based digital feedback system
Two BPMs (one for each plane) will provide the beam position information The RF front-end will
extract the betatron oscillation information which will be sampled by ADCs From the sampled data the
FPGA will compute a suitable kick to damp the oscillation The signal processor output will be
amplified by a 10 W wideband amplifiers before being fed to the kicker
In the RF front-end (see Fig 352) a baseband down-converter will be implemented in order to amplify
the beam signal and to minimize the jitter effect for the same reason the digital kick will also be
extended over four RF periods
lsquo
76
Figure 352 Front-end of the Thom-X RF electronics
For the commissioning phase a digital system (rather than an analog one) will be used to benefit from
the tuning flexibility provided by the digital parameters of the FIR filter Once the machine tune will be
fixed a more suitable low cost analog system will be installed
373 Kicker
The kicker is under development It consists of a stripline composed of four electrodes connected to
vacuum feedthroughs on each side (see Fig 353) The amplifiers will be connected to the downstream
ports whereas upstream ports will be used for signal detection and they will be equipped with 50 Ω
loads The same kicker will be used to act on the beam in both planes
Electrode line impedances must be adapted (50 Ω) Electromagnetic 3-D simulations are being
performed to optimize the bandwidth and shunt impedance (ie to maximize the stripline efficiency)
and also to minimize the kicker impedance to wakefields
Figure 353 Schematics of the Thom-X stripline kicker
lsquo
77
38 Vacuum
381 Transfer line vacuum
Concerning the design of the transfer line vacuum system three main constraints have been taken into
account
- Pressure of 10-7
Pa at the acceleration section output
- Ion pump operating at 10-6
Pa
- Ring pressure of 10-8
Pa
Simulations have been performed assuming that two 03 m3 s
-1 triode pumps will be installed one at the end of
the acceleration section and the other at the septum The vacuum chamber is a DN40 The septum output
conductance is 018 m3 s
-1 Without baking the outgassing rate would be σ τ = 2x10
-8 Pa m s
-1 and the
pressure at the ring entrance would be about 10-7
Pa Thus the septum and its environment must be baked to
bring τ down to 5x10-10
Pa m s-1
and obtain a pressure of 10-8
Pa at the ring entrance
Pressure in the line of transfer
The vacuum equipment envisaged for the transfer line is the following one
‐ 3 ion pumps 150 l s-1
with their baking systems and power supplies
‐ 1 ion pump 500 l s-1
with their baking system and power supply
‐ 4 Pirani gauges
‐ 4 cold cathode gauges
‐ 2 power supplies gauges
‐ 2 gate valves electro pneumatically actuated DN 40 (acceleration section output ring entrance beam
dump)
‐ 1 turbo pump (DN 63 ~ 80 l s-1
)
‐ 1 rotary vane pump (~ 12 m3 h
-1)
‐ 4 angle valves manually actuated for pre-pumping DN 63
lsquo
78
382 Ring vacuum
Concerning the ultra-vacuum to be maintained in the ThomX ring one must take into account first the
pipe relatively low specific conductance c = 452x10-3
m3 m s
-1 and secondly the pipe specific
outgassing rate θ τ = 108x10-10
Pa m2 s
-1 Also the pumpsrsquo effective pumping speed is reduced by the
T port with the grid Seff = 19x10-2
m3 s
-1 In a first stage the ionic pumps chosen for the simulations
are 0040 m3 s
-1 diode pumps This corresponds to S = 0030 m
3 s
-1 at 10
-8 Pa and with a grid pumping
port whose conductance is 00362 m3 s
-1 to an effective pumping speed Seff = 0019 m
3s
-1 The pressure
has been estimated by modeling a quarter of the ring including half of the non-baked optical cavity ie
in the most unfavorable case
Figure 354 Pressure distribution in the vacuum chamber
The calculated average pressure is 306x10-8
Pa It can be improved by increasing the grid pumping port
conductance by replacing the DN35 with a DN63 Monte-Carlo simulations show an improvement of ~
16 in efficiency
The entire ring must be baked Geometrical and temperature constraints (respectively a distance of only
07 mm between the magnet coils and the vacuum chamber a baking temperature of 150 degC for the
chamber while keeping the coilsrsquo temperature below 80deg C) are strict An ultra-thin baking system has
been constructed tested and validated
Figure 355 The vacuum chamber ultrathin baking set-up
lsquo
79
The following is the vacuum equipment needed for one quarter of the ring
- 3 ion pumps (40 l s-1
) with their baking equipment and their power supplies
- 1 residual gas analyzer (RGA)
- 2 turbo-pumps (DN 63 ~ 80 l s-1
)
- 2 rotary vane pumps (~ 12 m3 h
-1)
- 2 Pirani gauges
- 2 cold cathode gauges
- 1 gauge power supply
- 1 DN 63 gate valve electro-pneumatically actuated with an HF feed-through (one on each side of
the RF cavity and similarly for the septum) and the baking equipment
- the vacuum chamber baking equipment
- 3 angle valves manually actuated for pre-pumping DN63
- 3 T with the grid
lsquo
80
Chapter 4 INJECTOR TRANSFER LINE AND RING DIAGNOSTICS
Several diagnostic devices will be implemented in the ring to measure the electron beam parameters
listed in Table 41
Parameter unit value
Energy MeV 50
Average current mA 1784
Bunch charge nC 1
Energy dispersion 03 at injection to 06 at storage end
Emittance x mrad 5 e-8 at injection to 30 e-8 at storage end
Normalized emittance x mrad 5 e-6 at injection to 30 e-6 at storage end
Emittance y mrad 5 e-8
Normalized emittance y mrad 5 e-6
Bunch length ps-rms revolutions 1 ~ 4
revolutions 100-200 ~ 25
revolutions 200-300000 ~ 50
RF frequency MHz 500
Revolution frequency MHz 1784
Period ns 56
Injection repetition rate Hz 50
Ring circumference m 1680
(a) Ring parameters
Magnetic field T 05
Radius of curvature m 0352
R51 m 0 to -025
R52 m 0 to -045
R56 m 07 to -07
Beta x m 06
Beta y m 6
Eta x m 02 - 03
Transverse dimension x mm (rms) 06
Transverse divergence x mrad 2
Transverse dimension y mm (rms) 054
Transverse divergence y mrad 2
(b) Parameter values at the location of the bending magnets
Table 41 ThomX beam parameters
41 Charge measurement
411 Current transformers
The beam charge will be measured non-destructively with commercial in-flange high resolution
current transformers (ICT) Three devices will be installed
- one in the linac (LIDGICT010) located before the accelerating sections
- one at the transfer line entrance (TLDGICT010)
lsquo
81
- one at the transfer line exit (TLDGICT020)
In-flange ICTs are directly mounted in the beam line They are UHV compatible can be baked up to
150deg C (185deg C on request) and they are available for many pipe diameters (also with an elliptical
aperture or any other aperture shape) We will use the CF6rdquo model its internal diameter is 604 mm and
it is 60 mm long This device includes a ceramic gap a shield and a wall current bypass Bellows are
not required
The signal from each integrating current transformer will be processed by a BCM (beam charge
monitor) A built-in calibration generator provides an on-line check of the measurement reliability An
Integrate-Hold-Reset (BCM-IHR) device is suited for pulse repetition rates ranging from 10 kHz down
to single pulses and for bunch charges up to 2 nC The acquisition will be performed by using a 100
kHz 12 bit ADC
412 Faraday cups
Interceptive absolute beam charge measurements will be performed by Faraday cups There will be two
of them placed inside the two beam dumps (see Fig 2 of the Introduction) They should provide a
better resolution for small electric charges
42 Transverse profile measurements
421 Diagnostics stations
Five diagnostics stations will be installed to measure the transverse beam profile as shown in Fig 41
Diagnostics station 1 (DS1 ndash LIDGSST010) will be located after the RF gun at the beginning of the
linac Diagnostics station 2 (DS2 ndash TLDGSST010) will be located between the end of the linac and
the linac beam dump Diagnostics station 3 (DS3 ndash TLDGSST020) will be placed in the middle of the
transfer line Diagnostics station 4 (DS4 ndash TLDGSST030) will be located at the end of the transfer
line and Diagnostics station 5 (DS5 ndash ELDGSST010) will be located before the ring dump
lsquo
82
Figure 41 Location of the diagnostics stations (see text for the nomenclature)
The ThomX station design draws from the station built by LAL for PITZ (itself drawn from a SOLEIL
design) It is illustrated in Fig 42
Figure 42 3D view of a ThomX diagnostic station
Each station will have a motorized arm on which several screens will be mounted A stepper motor will
move this arm and insert the screens laterally The following screens will be available
- 1 YAG(Ce) screen in all stations
- 1 optical transition radiation (OTR) screen in all stations
lsquo
83
- 1 target to allow an online calibration of the camera in all stations
- one Cerenkov screen for bunch length measurements in station DS2 (described in more detail
below)
The YAG(Ce) and OTR screens and the calibration target will be tilted at 45deg with respect to the beam
axis so that the emitted or reflected light will travel downward and exit the vacuum through a DN60 CF
fused silica window A CCD camera will be located below the station to image the screens The
Cerenkov screen will be titled so that its light exits at the top of the diagnostic station Shutters will be
used to close either the Cerenkov window or the YAG(Ce)OTR window when not in use to prevent
measurement noise due to background photons Several LEDs located just after the bottom window will
be used to illuminate the inside of the chamber when needed
At the beginning of the Linac at station DS1 the beam size will be about 05-07mm (rms) in both
planes at the end of the Linac at DS2 the beam size will be 15 mm (rms) in both planes In the middle
of the transfer line at DS3 the beam size is expected to reach 25 mm (rms) in x and slightly less than
1mm in y At the end of the transfer line at DS4 it will be about 1 mm in both planes After the
extraction at DS5 the beam size will be about 05-07mm (rms) in both planes Given these beam sizes
we chose a screen diameter of 25 mm for all stations
Screen photon yield
The YAG(Ce) screen will be 100 microm thick Such screens have a light yield of 8 photons per deposited
keV [41] The YAG radiation length is 35 mm [42] assuming the usual model for energy deposition in
matter [43] each electron will deposit about 143 keV in the screen this corresponds to the emission of
~1100 photons Thus the total number of photons produced by an e- bunch crossing the screen will be
71 1012
We recall that YAG(Ce) emission occurs in all directions
The second screen will be of the OTR type The screen support design allows either the use of a 100 microm
aluminized silicon wafer or of an ultra-thin aluminized mylar film The silicon wafer has the advantage
of giving a better image quality while the mylar film allows a beam injection through the screen without
significantly affecting its quality The OTR yield within a cone of half-aperture θ for photons whose
wavelengths range between λ1 and λ2 is given by [44]
Thus for a standard CCD camera whose sensitivity ranges between 400 nm and 800 nm and assuming
an acceptance cone of 003 radian this expression gives ~0002 photon per electron at 50 MeV (station
DS2-5) A similar yield is reached at 5 MeV with an acceptance of 03 radians (station DS1)
To image the screens a lens will be located 500 mm away from them A small telescope made of two
lenses will be used to focus the light on the CCD camera At least one of the telescope lenses will be
movable to provide the needed dynamic range A Gigabit Ethernet camera with at least 600 x 400 pixels
will record the images The camerarsquos square pixels are about 74 x 74 microm2
With such a configuration one expects that a 50 MeV bunch passing through the YAG(Ce) screen will
emit 71x1012
photons and assuming that the CCD chip has an average efficiency of 50 1010
counts
will be recorded by the camera taking into account the solid angle covered by the lens
lsquo
84
The same bunch passing through the OTR screen will emit 82 107 photons within the lens acceptance
Thus 3 106 counts will be recorded by the camera
For a 5 MeV bunch (station DS1) the YAG(Ce) screen will emit 2 1012
photons and 109 counts will be
recorded by the camera The OTR screen will emit 82 107 photons within the lens acceptance and 3 10
6
counts will be recorded by the camera
Assuming a transverse beam size of 15 mm one expects 8 106 counts per pixels with a YAG(Ce)
screen and 25 105 counts per pixel with an OTR
If the 50 MeV beam is defocused (for example during a quadrupole scan) to produce a 5 mm spot size
these numbers become 8 105 for the YAG(Ce) screen and 2 10
4 for the OTR
Number of OTR
photons emitted
Number of
OTR
photons
recorded by
the camera
Number of
YAG
photon
emitted
Number of
YAG
photons
recorded
DS1 82 107 3 10
6 2 10
12 10
9
DS2-5 82 107 3 10
6 71x10
12 10
10
Assuming a YAG Cerium dopant density of 018 we get a Cerium density of 25 1019
cm-3
[45] and
a 15 scintillation efficiency It follows that the scintillating site density is 37 1018
cm-3
Therefore the
100 microm YAG(Ce) screen will saturate when the photon density reaches 36 1014
mm-2
This last figure
translates into an electron density of 32 1011
mm-2
for an e- beam energy of 50 MeV In these
conditions the YAG(Ce) screen will saturate if the beam is focussed on a spot smaller than 20 microm This
is five times less than the smallest beam foreseen at ThomX
Additional targets will be used to calibrate the cameras when necessary They will be made of stainless
steel plates with etched grids Each square of the grid will be 2 by 2 mm2 In some of these squares
smaller lines of known thickness will be etched Images of these targets will allow one to check the
whole optics
To minimize a possible radiation deterioration of the cameras lead-glass windows will be inserted
between the vacuum windows and the focusing lenses
43 Emittance measurement
The beam emittance will be measured with the quadrupole scan technique In this technique a triplet
(set of three quadrupoles) is used to find the minimum transverse beam size in each plane A fit of the
beam size variation versus the quadrupole strength enables one to retrieve the projected transverse
emittance
The sets of triplets and the diagnostic stations implemented along the ring allow an emittance
measurement at three different locations at the exit of the linac with DS2 in the transfer line with DS4
and in the ring with DS5 by using the extraction kicker The beam emittance measurement with DS5
lsquo
85
will provide a check of the turn-by-turn emittance growth thus bringing precious information on the
transverse beam dynamics
Pepper-pot measurements are also foreseen The advantage of this technique with respect to a
quadrupole scan is that it consists in a single shot measurement independent of machine operation
fluctuations
A pepper-pot set-up comprises a plate with evenly distributed holes Such a plate is placed in the beam
path together with a scintillating screen located downstream and an imaging system A pepper-pot
mounted on a movable arm could be located at the end of the linac between the last dipole and the
diagnostic station DS2 and another one could be located just before station DS5
At station DS2 the beam size will be about 15 mm (rms) and the geometric emittance about 5 10-8
m
rad Therefore the maximum beam divergence will be about 50 microrad By adjusting the quadrupoles one
may focus the beam down to a 20 microm spot and increase its divergence up to 250 microrad To measure the
transverse emittance one would have to position this focal point two meters before DS2 (or DS5) and
to position a pepper-pot mask with 100 microm holes or slits one meter upstream of DS2 (or DS5)
44 Beam position measurement
The beam position will be measured by a set of BPMs placed all along the ring Their locations are
shown in Fig 43
lsquo
86
Figure 43 Location of the BPMs along the ring (note the BPM located at the IP has been suppressed
since this drawing was made)
441 Beam Position Monitors (BPM)
Two types of BPMs will be used short circuited striplines in the linac and the transfer line and buttons
in the storage ring
Compared to button BPMs striplines generate a higher signal but need more space and contribute more
to the wake-field generating impedance For these reasons striplines are preferred for the linac and the
transfer line where we need single pass measurements and where wakefield is not an issue In the
storage ring button BPMs will be chosen for their small size and small contribution to the ring
impedance The lower signal produced by the small electrodes will be compensated by the high
repetition rate of the stored beam (averaging is possible)
BPM electronics detection frequency on the storage ring is usually equal to the RF frequency as it is the
only common frequency component present in the BPM data whatever the filling pattern is Even
though only one bunch will be stored in ThomX the same approach is chosen Therefore the storage
ring BPM electronics detection frequency is 500 MHz
Since it is more convenient to adopt the same electronics for all the BPMs (linac transfer line and
storage ring) stripline BPMs on the Linac and transfer line are optimized to give a signal with the
highest possible component at 500 MHz Their electrode length is thus adapted to this frequency which
means that it has to be equal to λ4 (150 mm 500 MHz) where λ is the RF wavelength
4411 BPM for the linac
The linac BPM equipment will be as follows
- Number of BPM 1
- Type short circuited stripline
- Electrode length 100 mm
- Required resolution 100 microm for a single pass measurement and a 1 nC bunch charge
Figure 44 Linac BPM
lsquo
87
Due to lack of space in the linac section where a stripline must be installed the electrode length cannot
be 150 mm but must be reduced to 100 mm Transfer impedance calculations show that at 500 MHz
the signal amplitude will be reduced by 13 compared to the optimal length of 150 mm (see Fig 45)
This amplitude reduction is acceptable
Feedthroughs will be soldered on CF16 flanges
Figure 45 Stripline transfer impedance versus frequency for various electrode lengths With an
electrode length of 100 mm the signal is reduced by 13 compared to its maximum value for a
frequency of 500 MHz
4412 BPMs for the transfer line
The transfer line will be equipped as follows
- Number of BPM 4
- Type short circuited stripline
- Electrode length 150 mm
- Required resolution 100 microm for a single pass measurement and a 1 nC bunch charge
lsquo
88
Figure 46 Drawing of the BPMs that will equip the electron transfer line
Feedthroughs will be soldered to CF16 flanges
4413 BPMs for the storage ring
The specifications relevant to the ring BPMs are as follows
- Number of BPM 12
- Type button
- Button diameter 107mm
- Required resolution ~1 microm 50 Hz and for a bunch charge of 1 nC
- Absolute precision plusmn 250 microm
In order to provide additional BPM signals for other applications (see below) 4 BPM blocks will be
equipped with a double set of electrodes In that case only 4 electrodes (among 8) will be used for
position monitoring The button size and the beam pipe size remain unchanged
Figure 47 Simple and double BPM blocks for the ThomX storage ring
The button positions and dimensions have been determined by simulations with Poisson Superfish (an
electrostatic simulation software) An evaluation of the impedance due to the button discontinuities has
shown that their effect will be negligible
Feedthroughs will be soldered directly to the BPM block They are bakable Connectors will be SMA
males to prevent any deformation during baking
lsquo
89
Alignment
BPM blocks will have an external mechanical reference This reference will be used for BPM alignment
during the installation phase and to position precisely the block in a calibration bench
Calibration
Storage ring BPMs will be calibrated by using a dedicated bench An antenna inside the block will be
fed with a 500 MHz wave of constant amplitude Calibration is done by measuring the signal output by
the 4 buttons A precise position of the antenna is not needed as position errors can be eliminated by
repeating the measurement with the BPM block turned by 180deg and averaging the results
The second halves of the blocks equipped with two sets of buttons will be dedicated to
‐ longitudinal (beam phase) feedback (one set of BPMs)
‐ transverse feedback (one set of BPMs)
‐ ion clearing To suppress ions which may generate beam instabilities BPM buttons will be
polarized with a high voltage (max plusmn200V) This electric field will deflect the ion trajectories
toward the vacuum chamber
Electronics
Electronics will be the same for all BPMs (whether equipping the linac the transfer line or the storage
ring)
Data rate
Revolution-by-revolution beam position measurements would be best But because of the high
revolution frequency (178 MHz) it might be difficult to get electronic devices that handle the
corresponding data rate Then one may work with averages over two turns This data will be provided
on demand and the acquisition will be triggered The buffer size should accommodate data collected
during at least 20 ms to cover full storage cycles Revolution-by-revolution data will be used for
machine physics studies and tune measurement The position measurement resolution must be better
than 100 microm
Slow acquisition
Slow data acquisition will be used for slow orbit correction and slow orbit feedback It will be obtained
by averaging the fast data flow with a sampling rate of ~10 Hz A data rate of 50 Hz would provide just
one measurement per storage cycle The resolution aimed at for this measurement is one micron or
better
Acquisition must be synchronous with the machine revolution frequency Moreover modules must be
synchronized together
Connections and cables
The four cables that carry the signals of each BPM should be put together in a strand Their electrical
lengths will be matched so that phase differences are kept below 13deg 500 MHz The cable attenuation
should not differ much from one BPM to another If the cable lengths cannot be the same for all BPMs
cable sections will be chosen in order to ensure the same attenuation for all cables To minimize both
lsquo
90
attenuation and cost cable lengths below will be kept below 30 m The LMR240 cable is an example
of what could be used its attenuation is ~6 dBm 500 MHz over a distance of 33 m
As radiation close to the machine can damage the cable insulator short (~1 m) pigtails (that can be
easily replaced if needed) will be inserted between the BPMs and the long cables
442 Orbit feedback system
A feedback system will keep the beam orbit stable within plusmn1 microm It will use the position measurements
provided by the 12 ring BPMs The correction rate will be comprised between 01 Hz and 10 Hz
Correction signals will be applied to the 12 correctors located along the ring The orbit feedback system
will be a matlab based application
443 Tune measurement and feedback
The tune will be obtained from the revolution-by-revolution data provided by the BPMs There is no
need for any kicker to excite betatron oscillations as the beam transverse position will naturally oscillate
after each injection (ie every 20 ms) If necessary a tune feedback will be implemented to keep the
tunes constant
444 Energy measurement
In the middle of the transfer line ie downstream of the dipole located at the end of the linac the
dispersive function is non-zero Therefore the beam energy and its spread can be deduced from the
electron horizontal distribution shown by the YAG screen in DS3 The same measurement will be
performed with DS4 using the dispersion induced by the extraction magnet
45 Bunch length measurement
The bunch length will be measured at two different places at the end of the linac and in the ring
Depending on the observation point two different physical processes are used to produce visible light
pulses which are then transported by dedicated optical lines through the ThomX shielding wall to a
double-sweep streak camera located in the experimental hall
451 Cerenkov radiation
Cerenkov radiation (CR) produced in a target placed at the end of the linac will be used to measure the
e- bunch length Cerenkov radiation is emitted when a charged particle passes through a medium at
velocity greater than the speed of light in that medium It is emitted at an angle θcr ndash with respect to the
particle momentum ndash determined by n(λ) the medium index of refraction and β the particle velocity as
follows
Above 10 MeV the emission angle is almost independent of the particle velocity Ncr the number of
radiated photons whose wavelengths range between λ1 and λ2 summed over a path of length d along the
charged particle trajectory is
( )
minus
minus
λnβλλπαd=N
cr 22
21
11
112
( )
minus
minus
λnβλλπαd=N
cr 22
21
11
112
( )( )λβn
=θcr
1cos
lsquo
91
where α=73 10-3
is the fine structure constant
Two types of radiator are usually used to produce Cerenkov radiation an aerogel or a solid target
Aerogels dramatically spoil the vacuum and therefore have to be mounted in a separate vacuum vessel
to maintain ultra-high vacuum conditions Solid targets are UHV compatible This is why they are
preferred
At the end of the linac when the bending magnet is off the beam goes straight into a beam dump
Before the dump several types of targets can be inserted in the beam trajectory for diagnostic purposes
For the production of Cerenkov radiation a sapphire target is adequate This materialrsquos index of
refraction is 177 With a 50 MeV beam the Cerenkov angle is θcr=5563deg Assuming that the window
thickness is 025 mm 16 photonse- are created with a wavelength ranging between λ1 = 300 nm and λ2
= 800 nm Their spectrum is given in Fig 48 For a 1 nC electron bunch 1011
photons are created The
energy deposited in such a target is negligible
Figure 48 Spectrum of the Cerenkov radiation emitted by a 50 MeV e- beam passing through of
sapphire window 025 mm thick
For simplicity the sapphire target is oriented so that the Cerenkov radiation exits in the vertical
direction For a Cerenkov angle θcr = 5563deg the window angle with respect to the beam axis must be θP
= 11475deg (see Fig 49)
8x10-8
6
4
2
0
In
ten
sity
[au
]
800700600500400300200
Wavelength [nm]
lsquo
92
Figure 49 Orientation of the sapphire target used to observe the Cerenkov light emitted by the e- beam
at the linac end
At the target location the beam dimensions will be σxy= 14 mm (rms) It will produce a light spot
whose dimensions will be σx = 14 mm (rms) and σy = 154 mm (rms) in the sapphire To intercept the
whole light beam and to leave some margin for the alignment we will use a target whose diameter will
be equal to 2 x 6 x σy ~ 20 mm
The Cerenkov radiation will exit the vacuum chamber via a UHV viewport made of fused silica (FuSi)
with a 90 transmission over the 300-800 nm wavelength range The viewport is mounted on a CF63
flange that provides a 68 mm clear aperture At the exit of the viewport a FuSi lens is used to focus the
light This collimated beam is then transported via a set of planar mirrors across the shielded area and
then through the ThomX shield to a dedicated hutch in the experimental hall (see sect 453)
452 Synchrotron radiation
Most of the synchrotron radiation (SR) emitted when the electrons circulate in a bending magnet (BM)
will be in the far infrared but visible light will also be present The cut-off wavelength of this radiation
corresponds to ~077 eV Thus the X-ray content is negligible This last feature makes the extraction of
the visible part of the spectrum easy The spectral flux per unit bandwidth per unit surface ΦBM
[Photons01bwmmsup2] is shown in Figure 410
Figure 410 On-axis spectral flux 1 m downstream of a bending magnet Photon energy in eV The
visible spectral range corresponds to [400 nm - 800 nm] or [155 eV - 31 eV]
The radiation is extracted at the exit of the BM ndeg5 through a porthole which provides a 63 mrad x 32
mrad aperture at 15deg with respect to the 0deg incident angle in the dipole The maximum power density is
lsquo
93
4 microWmmsup2 in the horizontal plane of this extraction porthole thus no dedicated absorber is required
(But for the porthole region the radiated power will be dissipated in the vacuum chamber wall) The
radiation will exit the vacuum chamber through a UHV viewport made of MgF2 (this material offers
the highest transmission coefficient over the 200-800 nm range) The synchrotron radiation distribution
at 5 eV one meter downstream of the bending magnet is around 6 mm (FWHM) So the spectral flux
integrated over a collecting area of 6 mm x 63 mm is about (around 5 eV)
Φe [Phs] = ΦBM [Phs01bwmmsup2] x 6 mm x 63 mm x 1000
Since photons of lower energy thus with a higher divergence will also contribute to the total spectral
flux this value gives a low limit estimate of the real total spectral flux Figure 411 shows the Φe
spectral flux
As was the case for the Cerenkov radiation a FuSi lens will be used at the exit of the viewport to focus
the beam The collimated radiation will then be transported via a set of planar mirrors across the
shielded area and then through the ThomX shield to a dedicated hutch in the experimental hall (see next
section)
Figure 411 Spectral flux Φe in a 63 x 32 mrad2 aperture 1 m downstream of a bending magnet
453 Transport of the Cerenkov and synchrotron radiations to the diagnostics hutch
Both the Cerenkov light and the synchrotron radiation have to be transported to the diagnostics hutch
located in the experimental hall Starting from different locations both radiations will be transported via
a set of planar mirrors to a lsquomeeting pointrsquo inside the shielded area A flip mirror (ie a double-
orientation mirror) will enable the selection of one of the radiations The one selected will be further
transported again with a set of planar mirrors through the shielding wall to the diagnostics hutch
Inside the diagnostics hutch the last lens will focus the radiation on the streak camera entrance The
lens aperture and its focal length will be adapted to the selected incoming radiation All the line
components and their parameters are chosen to minimize spatial aberrations and to maximize the
number of photons entering the streak camera
454 Streak camera for the bunch length measurement
We plan to use a double-sweep streak camera to measure the bunch length and to visualize its evolution
over short time scales Since the bunch length may vary from 4 to 20 ps (rms) a 2 ps (FWHM)
resolution streak camera is required
lsquo
94
The time resolution of the bunch length measurement depends on the radiation type (CR or SR) which
is used and involves several contributions the electron transverse beam size at the light emission point
chromatic effects etc All these contributions will be minimized as much as possible to maintain the
effective resolution as close as possible to the one of the streak camera
46 Bunch imaging using synchrotron radiation
The synchrotron radiation transported to perform the bunch length measurement (see section 45) can
also be used to image the bunch while it circulates For this purpose a beam splitter could be installed
at the radiation exit port the main part of the radiation would be sent to the streak camera while a small
fraction would be directed to a CCD camera mounted nearby A focusing lens placed between the
viewport and the CCD would enable the imaging of the stored beam
In the orbit section where the bunch emits the synchrotron radiation used for the above measurements
the e- beam transverse size is about 05 mm (rms) Since the size of a CCD chip is typically 5 mm the
lens gain should be about 10 Such a set-up would enable one to follow the evolution of the beam size
in real time (we will use a CCD whose acquisition rate is about 10 Hz) It would be a non-destructive
and cheap diagnostic Similar setups are currently used on storage rings to measure the electron beam
emittance assuming that the beta function is known
47 Beam loss measurement
Particle losses will be measured by using optical fibers [46] When a charged particle crosses a glass
fiber Cerenkov radiation is emitted within the fiber and propagates along the fiber It can be detected
by a photomultiplier that collects the light at the fiber end The measurement eg with an oscilloscope
of the pulse production (in the fiber) time enables one to recover the particle loss location
Two fibers will be installed One will be used to monitor e- losses along the linac and the transfer line
The second fiber will be used to monitor particle losses along the ring Each one of these fibers will be
connected to a fast photomultiplier (PMT) The PMT time resolution will determine the accuracy with
which the losses can be localized
A fused silica fiber has a yield of 227 photons per cm of a relativistic electron track [47 48] Therefore
a 5 mm thick fiber will have a yield of ~100 photons per relativistic electron which crosses it Such a
signal should be easily detected by a PMT
As it may be difficult to place the fiber close to the beam pipe in some parts of the ring the fiber may
have to be located at a distance d away from the beam axis Then the number of lost particles which will
cross the fiber will be reduced by a factor dsup2
48 Beam dumps
The ThomX set-up will include two particle dumps to stop the e- beam in such conditions that the
radioactivity then generated is minimal The first dump will be located at the linac exit The second
dump will be located downstream of the injectionextraction section The latter dump can be used to
stop the beam either at the exit of the transfer line or after any number of bunch revolutions in the ring
Both dumps are based on the SOLEIL design and can handle the 35 W power deposition (1 nC 70
MeV 50 Hz repetition rate)
lsquo
95
Chapter 5 SYNCHRONIZATION
51 Overall scheme
An ultra-low noise 500 MHz synthesizer will be used as the main clock for ThomX The 10 MHz
output of this synthesizer will be used to drive a second synthesizer (the so-called lsquolinac synthesizerrsquo)
operating at 3 GHz The RF output of the latter synthesizer will be used for the linac operation It will
be split in two lines The first one will be used to synchronize the gun laser oscillator (after appropriate
down conversion of the frequency) The second line will be used to drive the gun and the linac RF
The RF output of the main synthesizer (500 MHz) will be used for the ring RF it will be split in three
lines The first line will drive the ring RF (after appropriate phase shifting) The frequency of the second
line will be divided by 14 this line will synchronize the Compton cavity As in the MightyLaser
experiment [51] the laser used to inject power in the Compton cavity will be synchronized with that
cavity by using the PDH method The frequency of the third line will be divided by 28 and will
therefore be synchronous with the electron revolution frequency This line will be used to synchronize
programmable delays delivering turn-by-turn triggers and injectionextraction triggers To ensure that
injection is always taking place at a zero crossing of the mains the programmable delays will also be
synchronized with the mains A counter will reduce the mains frequency to the operations frequency
(when lower than 50Hz) The gun trigger will be generated when the mains will be at a zero crossing
and the ring RF at a preset phase that takes into account the time it will take for the electrons to travel
from the linac gun to the ring accelerating cavity
To ensure that the e- bunches will always be injected at a triple zero (50 Hz 500 MHz and 3 GHz) we
need to specify the tolerance on the 50 Hz zero crossing We may take 1 (ie 02 ms) for this
tolerance this means that the coincidence between the ring RF (~500 MHz) and the linac RF (~3 GHz)
must occur at a frequency greater than 5 kHz Therefore by choosing a heterodyne frequency (ie the
frequency difference between the linac RF divided by 6 and the ring RF) greater than 5 kHz we ensure
that there will always be a common zero crossing between the ring RF and the linac RF within 1 of
the mains zero crossing This sets a constraint on the ring radius of 10-5
ie 168 micrometres on a
168 m long orbit (see Table 2)
The RF bucket must be positioned with an accuracy of 3 ps in the ring RF Thus the phase noise
between the linac RF and the ring RF must be lower than 3 ps when integrated between 50 Hz and
500 MHz Most good-quality synthesizers achieve such a performance
The information collected by the ring diagnostics will be used to adjust the exact value of the frequency
(around 500 MHz) generated by the main synthesizer so that it matches the exact ring length This
information will also be used to adjust the phase between the injection trigger and the ring
To reach the correct signal level each RF line may have to be amplified An isolator will be added to
avoid reflections that may degrade the overall performance of the system
To ensure long-term low frequency synchronization a Network Time Protocol (NTP) server will be
available to allow all computers working at ThomX to synchronize their internal clock
lsquo
96
52 Specifications
Synthesizer
The main synthesizer will be used to generate a 500 MHz adjustable signal Its phase noise jitter
integrated between 50 Hz and 20 MHz must be less than a few picoseconds It must support a frequency
modulation of at least 100 kHz Typical mid-range synthesizers meet such specifications
Measurements done at LAL in collaboration with SOLEIL have shown that the addition of an external
frequency reference does not necessarily improve the phase noise of these synthesizers in the frequency
range of interest
The linac synthesizer specifications are the same ones except that it must be able to generate a
frequency of 3 GHz
Programmable delay
Two low jitters units with 8 channels each will be used
Phase shifter
The phase shifters must be voltage controlled and cover 360deg (two 180deg units)
Figure 51 Synchronization layout
lsquo
97
Chapter 6 OPTICAL SYSTEM DESIGN
Since the writing of the ThomX CDR three directions have been pursued improvement of the feedback
system improvement of the laser amplification and mechanical design of the optical resonator The
results are described in the following three sections
In the first section the ThomX feedback system is described During the past year a strong RampD
program has been developed on the locking of a TiSapph picosecond laser beam on a two-mirror high
finesse cavity at OrsayLAL Very good results in operating for the first time a 30 000 finesse cavity
in the picosecond regime were obtained The feedback techniques used for this task are directly usable
for ThomX and are reported in this section A 3000 finesse four-mirror cavity at KEK was also installed
and operated and we succeeded in producing γ rays off the ATF electron bunches (Mightylaser project
[61]) Hardware damages were however caused by the Japanese 2011 earthquake At the time of
writing this document we are recovering all our equipment to reach our final goal locking a 30000
finesse four-mirror cavity to produce a high γ ray flux at ATF Thanks to a contact with the SYRTE
laboratory we have acquired a new laser oscillator with a 1030 nm wavelength that we are testing and
that we plan to use for ThomX With this oscillator by embedding an intra-cavity EOM we are able to
increase our feedback bandwidth by more than one order of magnitude
The second section illustrates the laser amplification developments done at CELIA First a non-all-
connected fiber amplifier for the Mightylaser project was set up that allows us to extract an average
power of 50 W This amplifier was used to produce γ rays at ATF A new setup has been designed and
built to reach both the reliability and the power level required by ThomX An important activity is
devoted to the construction of a fully all-connected amplification chain As mentioned above the new
laser oscillator and the new laser amplification chain are being integrated in order to fill a 30000 finesse
high finesse cavity to conclude the ANR Mightylaser project
In the third section the details of the implementation of the cavity in the electron ring and the cavity
vacuum system are described with the opto-mechanical system that will be used for a precise alignment
of the cavity mirrors
61 Feedback system and results on a Fabry-Perot cavity locking
To produce a high X-ray flux the ThomX facility produces high intensity laser pulses and collides them
with electrons stored in a ring Hence the locking system (see figure below) has a double goal
- Stabilize the synchronization of the Fabry-Perot cavity (FPC) photon pulse with the electron bunch
- Stabilize the synchronization of the laser pulses with the FPC to obtain amplification
For this last point as will be seen in section 62 firstly the laser beam will be amplified with a CPA
technique using optically pumped fibers and then it will be amplified again by accumulation during
many round trips into a high finesse FPC (the target is a gain of 10000 which leads to a finesse of
30000)
This section describes the following topics related to the stabilization of the laser pulses relative to the
FPC
- Parameters to be stabilized repetition rate and carrier-envelop phase (CEP)
- Error signals the Pound-Drever-Hall (PDH) measurement technique
lsquo
98
- Actuators dynamic range and bandwidth
- Behavior of the digital feedback system (DFS)
General scheme for the locking system Detection and actuation elements are shown in red
The results on the locking of a TiSapph laser oscillator on a 30000 finesse cavity will also be
illustrated
Figure 61 Locking scheme for the synchronization of the electron bunch and the FPC pulse for the
ATF experiment at KEK
611 Electron and FPC pulse synchronization
This kind of stabilization has already been achieved for the MightyLaser experiment at KEK (ATF) a
1785MHz repetition rate A digital feedback system was used (i) to acquire two clocks one from the
laser and one from the master oscillator of the electron ring (ii) to perform a digital phase detection
(iii) to build a digital integrator filter to remove phase drifts and (iv) to react on a PZT inside the FPC
to change its length according to the measured phase drift (see Fig 61)
lsquo
99
612 Laser parameters to be stabilized
We will choose a laser with a repetition rate corresponding to twice the e- bunch revolution frequency
in the ring (two laser pulses per electron bunch revolution) around 500MHz14 = 357 MHz A phase
noise measurement on a similar product operating 1785 MHz was already carried out to characterize
the repetition rate phase noise and the drift (see Fig 62 and Fig 63)
Figure 62 Repetition rate phase noise spectrum Figure 63 Repetition rate drift measured
X-axis offset frequency from the carrier in Hz during about one hour X-axis repetition
Y-axis single side band phase noise power rate drift in Hz Y-axis time duration in
density in dBcHz seconds
The global shape of the phase noise in 1f will permit to use three feedback actuarurs a high bandwidth
low dynamic one (EOM for repetition rate and AOM or pump current for CEP) a middle
bandwidthmiddle dynamic one (PZT for repetition rate) and a high dynamiclow bandwidth one
(mirror motor and isochronic wedge motor)
For this kind of laser the harmonic content (index m) is well known and can be represented either in the
time domain or in the spectral domain (see Fig 63 and Fig 64)
( )2
CEL REP CE REPf m m f f m f
φ
π
Δ = + = +
lsquo
100
Figure 64 Time domain and spectral representation of the laser pulses
Since one is dealing with two parameters (the repetition rate and the CEP) one needs two different
sources of information to measure them The standard method to measure these two parameters consists
in using the Pound-Drever-Hall (PDH) technique [62] which gives the position of the laser harmonics
relative to the FPC resonances
( )FPC CAVf m m f=
One of the parameters is deduced from the whole beam signal using a central photodiode the second
one is derived from a part of the spectrum selected by a grating and a slit and measured by the grating
photodiode
If the repetition rate and the FPC round trip frequency are close enough the error signal coming from
the central photodiode (index m0) is proportional to the difference between the laser harmonics and the
corresponding resonances of the FPC
1 0 0
2
CEREP CAVm f m f
φε
π
Δ prop + minus
Frequency
domain
lsquo
101
As the locking system cancels the error the central laser harmonic m0 is perfectly aligned on the FPC
resonance but all other laser harmonics (m=m0+Δm) are slightly detuned from the corresponding FPC
resonances This introduces a correlation between the CEP and the repetition rate
( )( )
1 0 0
2 0
0 0
02
1 12
CEREP CAV
CEREP CAV REP CE
m f m f
m mm m f f f f
m m
φε
π
φε
π
Δ = rArr + =
Δprop minus minus = minus = minus
As a result of these equations the signal received by the PDH system located after the grating (ε2) is
directly proportional to the offset frequency (fCE) and then to the CEP Thus if we change the CEP of
the laser we should clearly see on the grating photodiode PDH signal ndash which measures the laser-FPC
detuning on the border of the spectrum (for an m index far from m0) ndash a linear relation between the error
signal and the CEP
This method well known for femtosecond lasers has been tested in our laboratory in the picosecond
regime Figure 65 shows our results (submitted to Applied Optics) on the locking of a 76MHz
TiSapph picosecond laser oscillator on a 30000 finesse cavity
Figure 65 Top experimental measurement of the CEP value of a TiSapph laser oscillator Bottom
measurement of the corresponding grating photodiode signal
In Fig 65 one sees that when the CEP is measured close enough to zero the grating photodiode
exhibits a linear relation with the CEP otherwise the photodiode is saturated (due to the PDH signal
saturation) In our experiment the ratio between the cavity reflected power and the incident beam
power was ~20 which leads to a value of 80 for the coupling of the incident beam power to the
cavity
These results demonstrate for the first time that a picosecond laser oscillator can be locked to a 30000
finesse cavity (implying a power enhancement factor of 10000) with a power coupling of 80
613 PDH Measurement technique
Having shown that we are able to measure independently both parameters of interest (the repetition rate
and the CEP) we will now describe the apparatus to transform these measurements into error signals by
lsquo
102
the PDH technique (see Fig 66) Side-bands to the laser optical harmonics are produced by an electro-
optic modulator (EOM) modulated by a generator coming from the digital system Side-bands and laser
harmonics are reflected by the FPC the first ones without acquiring any phase the second ones shifted
in relation with the discrepancy between the laser and the FPC frequencies All the harmonics and the
side-bands of the reflected laser are combined into the photodiodes and the front-end electronics giving
an error signal at the modulation frequency
Figure 66 Pound-Drever-Hall scheme with digital phase detection
The photodiode front-end (see Fig 67) amplifies these signals at a level compatible with the digital
feedback system (DFS)
Figure 67 Electronic scheme and picture of the PDH front-end
Inside the DFS the PDH signals are filtered by a band-pass filter to remove unwanted frequencies and
multiplied by the same modulating signal with a phase shift to allow for the phase detection A low-pass
filter at the end of the chain removes the image signal at twice the modulation frequency Figure 68
shows an example of the input and the output signals
lsquo
103
Figure 68 Modulated input (blue) error signal (yellow) FPC transmitted signal (pink)
614 Actuators
In the previous sections we described how the two parameters (the repetition rate and the CEP) of a
pulsed laser have to be controlled in order that all the laser harmonics match the FPC resonances and
we also detailed how to measure the variations of these two parameters We will now describe the
devices inside or around the laser on which we need to react to correct the variations of the laser
harmonics relative to the FPC resonant frequencies
Figure 69 Typical scheme of a laser with all its actuators
The actuators will produce an action on the two parameters to be controlled with different dynamic
ranges and different bandwidths (see chart in Fig 610)
Figure 610 Dynamic ranges and bandwidths of the laser actuators
lsquo
104
All of the actuators are located inside the laser except the AOM (see Fig 610) They are the key items
to achieve a good stabilization
All the actuators measurements have been made on the 1785MHz laser version used at KEK
- Mirror motor dynamic range precision and bandwidth
The motor is a step motor with a precision of 140 nm on the round trip variation (equivalent to 15 Hz on
Frep) Figure 611 shows the linearity and the maximum dynamic range available with the motor stage
while Fig 612 shows a step response which indicates the maximum bandwidth we can obtain
Figure 611 Mirror motor range Figure 612 Time response of Frep for
1k motor steps X-axis repetition rate
offset (Hz) Y-axis repetition rate
The repetition rate range is 178033 - 179244 MHz (gt 1MHz)
- Piezoelectric sensitivity dynamic range and bandwidth
The PZT sensitivity is approximately 3Hz V 28 nmV (length variation to be compared to the
length of a round trip of the FPC pulse)
The PZT can be operated up to 150V providing a total dynamic range of 42 microm (round trip length
change) Its first peak resonance has been measured and found above 40 kHz
lsquo
105
Figure 613 Repetition rate change produced by a Figure 614 PZT frequency response X-
PZT voltage variation from 0 to 28 V frequency range 01-5 kHz Y-axis
Attenuation (dB) ndash arbitrary offset
Figures 613 and 614 show the PZT sensitivity and its transfer function
- EOM sensitivity measurement
Measuring the sensitivity of the EOM is not direct thus we chose to apply a 10 V amplitude ndash 10 Hz
frequency modulation ndash to the EOM and to measure the power of the harmonics (see Fig 616)
Assuming a pure frequency modulation the power of the harmonics fis a Bessel series that depends on
the sensitivity Thus taking the ratio of two consecutive harmonics gives directly the sensitivity
Figure 615 EOM modulation spectrum used for sensitivity measurement X-axis frequency centered
on the repetition rate Y-axis relative optical power around the repetition rate due to the modulation
The table on the left gives the frequency and the power of each harmonic
0 5 10 15 20 25 300
10
20
30
40
50
60
70
80
90
100
PZT voltage (V)
Fre
p v
ariation (
Hz)
Frep variation vs PZT voltage
AJ0(z)
AJ1(z)
AJ2(z)
lsquo
106
We now check that the sensitivity calculated from the ratio of the first harmonic to the fundamental and
the one obtained from the ratio of the second harmonic to the fundamental are the same
∆
∆
~1
2
∆
10~ minus 2909 rarr ∆~07
∆
∆
~1
8
∆
10~ minus 6398 rarr ∆~07
Since the ratio is 07 Hz for an applied sinusoidal signal of 10 V the sensitivity is around 007 HzV
- Isochronic wedge
It is made of two glass prisms with different refraction indexes chosen to maintain a constant group
delay in order to avoid repetition rate variation
We measured that the maximum variation of the repetition rate for a full CEP range (several times 2π)
is about 300 Hz for a repetition rate of 1785 MHz (see Fig 616 which shows the measured repetition
rate shift plotted along the X-axis versus the date of the measurement plotted along the Y-axis) Thus
the influence of this actuator on the repetition rate is about 210-6
Figure 616 Measurement of the influence of wedge displacement on Frep
- Acousto-Optic Modulator (AOM)
Such an actuator has already been used with a laser which is not the same as the one used for the other
actuators namely a 76 MHz laser locked on a two-mirror FPC
In Fig 617 we present the closed loop measurement The lowest frequency part (lt10 kHz) is
attenuated by the loop gain If the loop were open this part of the transfer function would be quite flat
To estimate the AOM bandwidth limits we can use the frequencies where the gain starts to drop
significantly this bandwidth is about 300 kHz
300 Hz
lsquo
107
Figure 67 Frequency response of the AOM in closed loop
615 The digital feedback system
The feedback system will deal with several coupled inputs and outputs To ensure flexibility we have
chosen a digital system Since we also want to work with a large bandwidth in closed loop (gt 1MHz) a
low latency system (lt 100 ns) is needed A digital system featuring such performance does not exist on
the shelf Thus we decided to design and build such a system at LAL (see Fig 618)
Figure 618 Scheme of the DFS and picture of the DAC daughterboard
This system will perform several functions (see Fig 619)
103
104
105
106
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
Frequency (Hz)
Magnitude (
dB
)
Bode plot
lsquo
108
Figure 619 Organization of the digital feedback functions
- The first important function is to produce the error signal from the modulated signal coming from the
photodiodes It is a phase detection block (Digital PDH DPDH) the main problem being to reduce the
latency as much as possible This will be achieved by optimizing the low-pass filter and the modulation
frequency (see Fig 620 for a digital implementation)
Figure 620 Digital phase detection (DPDH) to produce the PDH error signals
- The second important function is to provide Infinite Impulse Response (IIR) filters to build
Proportional-Integrator (PI) filters in order to cancel the system drifts
lsquo
109
Figure 621 Digital IIR filter implementation with adders and multipliers
These filters can be programmed by the user to change the filter coefficients and it can be cascaded to
realize complex filters They have been cut in Second Order Sections (SOS) to minimize the
propagation of fixed precision errors due to quantized data and coefficients (see Fig 621)
- The last important function integrated in the digital system is performed by an identification block
which works like a network analyzer by (i) sending a small harmonic signal (ii) looking at the system
response in closed loop at this frequency and (iii) sweeping this small harmonic signal frequency to
cover all the range to be measured (see Fog 622)
Figure 622 Identification system
lsquo
110
This kind of identification system can provide information on (i) the feedback system itself (ii) the
system to be stabilized or (iii) the open and closed loop transfer functions by producing Bode or
BlackNichols diagrams (see Fig 623) These informations are very useful for system analysis and for
tuning or designing the feedback filters
Figure 623 Typical Bode (top-left magnitude bottom-left phase) and Black-Nichols (right) diagrams
produced by the identification system
62 High average power laser system and its coupling to a high gain FP cavity
The all-fibered laser system will draw from the system developed in the MightyLaser project It will
consist in an oscillator associated with three amplifying stages involving single and double clad Yb-
doped fibers (see Fig 625) At present the amplifiers are based on commercial components to facilitate
a potential replacement of damaged parts The architecture is designed to provide the mandatory
diffraction-limited beam This relatively simple and compact architecture will implement the lsquoChirp
Pulse Amplificationrsquo (CPA) scheme In order to limit non-linearity effects in the fiber the signal is
initially stretched in time thus reducing its intensity in the waveguide then amplified in the fiber and
finally recompressed to its minimal duration The difference between the MightyLaser and the ThomX
repetition frequencies (respectively 1785 MHz and ~357 MHz imposed by the different repetition rates
of the two accelerators) requires an adjustment of the laser design
Figure 625 Sketch of the all-fiber laser system
lsquo
111
621 The MightyLaser oscillator
This oscillator is an all-normal dispersion mode-locked fiber laser based on the nonlinear polarization
evolution technique equipped with a fibered output It uses an ytterbium doped fiber which allows an
emission centered around 1030 nm with a full width at half maximum of 10 nm (see Fig 626) It
delivers short pulses at a repetition rate of 1785 MHz with a low average output power (~10 mW) in
order to minimize the noise level (see Fig 628) The main advantage of the all-normal dispersion
regime resides in the significant peak power reduction provided by the pulse stretching during the
propagation along the cavity Non-linear effects are thus reduced allowing a high-energy extraction
The output pulses are naturally chirped with an estimated duration of 2 ps
Figure 626 Emission spectrum of the oscillator
The oscillator is passively mode-locked by taking advantage of the non-linear evolution of the
polarization This technique is based on the optical Kerr effect The polarization of a pulse propagating
in a fiber evolves non-linearly as a function of intensity Thus by combining polarization controllers
and a polarizer one may generate losses that depend on the intensity (see Fig 627) Beyond the
polarizer all the pulse components are linearly polarized The wave plates transform the linear
polarization into an elliptic one The latter polarization state then evolves in a non-linear way while it
propagates along the fiber This evolution results from the combined effect of the self-phase and the
cross-phase modulations induced on the two polarization components The polarization rotation of the
more intense central part of the pulse is larger than the polarization rotation of the pulse wings Since
the pulse center crosses the analyzer with a reduced loss compared to the wingsrsquo loss the pulse duration
is shortened
Figure 627 Principle of mode locking based on the non-linear polarization evolution
lsquo
112
Figure 628 Spectrum of the oscillator phase noise noise level (dBcHz) versus offset frequency (Hz)
622 Amplification stages
The amplification chain is divided in three stages in order to distribute the gain and to saturate the
second and the third amplifying stages The first and the second stages are based on the same
architecture It is an all-fibered amplifier whose central component is a polarization maintaining single
mode using an Ytterbium doped fiber with a core diameter of 6 microm and a clad diameter of 125 microm (see
Fig 629) A wavelength division multiplexer (WDM) is spliced on the doped fiber to transfer the pump
power into the gain medium and to couple the signal of the picosecond oscillator to the amplifier The
first stage is pumped by a single mode diode centered at 976 nm and delivering 200 mW The second
one is pumped at a higher average power namely600 mW The two amplifying stages are separated by
an optical isolator to avoid any parasitic feedback from one amplifier to the other
Figure 629 Schematic of the first and the second amplification stages
The average power level at the input of the first amplifying stage is about 1 mW A limitation is
imposed on the output of this stage which should remain below the damage threshold of the isolator
fixed at 100 mW Consequently we adjust the pump level in order to obtain 80 mW at the output of the
ytterbium-doped fiber The 3 dB losses of the isolator allow one to inject 40 mW into the second
amplifying stage The evolution of the average output power versus the pump level is shown in Fig
630 For the maximum pump level of 600 mW the second amplifying stage delivers 330 mW
lsquo
113
Figure 630 Second stage output power plotted versus the pump power
Beyond the second isolator the third amplifying stage (see Fig 632) is built by using as its central
component a polarization maintaining double-clad Ytterbium-doped fiber with a core diameter of 25
microm and a clad diameter of 250 microm The amplifier is clad-pumped through a special combiner 6+11
(see Fig 631) In practice this component has an input signal fiber 6125 microm directly spliced to the
previous amplification stages an output signal fiber 25250 microm spliced to the double-clad Ytterbium
doped fiber and six multimode fibers of 105 microm to couple the pump diodes Here only three pump
ports are spliced to multimode diodes emitting up to 55 W at 976 nm corresponding to a pump power of
at most 165 W The development of a mode field adapter was necessary to splice the pump combiner to
the double-clad Ytterbium-doped fiber because their numerical apertures are different Losses entailed
by this component are compensated by the last amplification stage
Figure 631 Pump combiner Figure 632 Schematic of the third amplification stage
Figure 633 is a plot of the average output power versus the pump level For a pump power level of
76W the last amplification stage delivers 52W This was the first objective The fiber laser power
stability was measured during 1 12 hour at this power level (see Fig 634) The residual pump power
the pump and signal feedbacks into the pump diodes are also plotted in this figure
lsquo
114
Figure 633 Third stage output power Figure 634 Laser stability at 52 W
versus the pump power
New measurements were made after optimizing the central wavelength of the multi-mode pump diodes
The efficiency was increased to reach 787 for an output power of 24 W (see Fig 635) The fiber
laser power has been tested at this level during two hours (see Fig 636) The observed power variation
is less than 01
Figure 635 Output power versus pump power Figure 636 Laser stability at 24 W
after wavelength optimization
The all-fibered amplifier setup has been integrated in a 19rsquorsquo rack as shown in Fig 637 An FCPC
connector placed in the front panel allows the connection of the fibered oscillator to the master
amplifier module Several 955 optical fiber couplers have been inserted in our setup at strategic
locations to monitor the power level in real time by using photodiodes they may also signal potential
problems The output fiber comes out of the module through its front panel The back-side is dedicated
to all the electronic controls (diode pump photodiodes and modulators for noise management described
elsewhere)
lsquo
115
Figure 637 Pictures of the 52 W all-fibered setup integrated in a 19rsquorsquo rack
623 Stretcher and compressor
A chirp volume Bragg grating (CVBG) is used in our setup to stretch and compress the pulse with a
very good efficiency of 94 The CVBG transmission bandwidth is around 2 nm (see Fig 638) leading
to a pulse duration ranging from 200 to 250 ps depending on the grating dispersion It is possible to
recompress the pulse after amplification by using a second CVBG Then the pulse length ranges
between 20 and 87 ps depending on the second CVBG dispersion (see Fig 639)
Figure 638 Transmission bandwidth of the CVBG
1‐2 couple 3‐2 couple 3‐1 couple
Figure 639 Time profile of the recompressed pulses for different grating pairs
624 ThomX development
Some changes must be made to the all-fibered lsquoMightylaserrsquo design in order to use it in the ThomX
project The oscillator repetition rate needs to be scaled down to match the frequency imposed by the
accelerator As far as the laser design is concerned the stretchercompressor couple must be modified to
limit non-linear effects The increase of the output power from 50 to 150 W imposes an upgrade of the
second amplification stage and eventually of the third one
lsquo
116
6241 Oscillator
The oscillator for ThomX will be based on the same principle as the one used for Mightylaser The only
difference is the repetition frequency imposed by the accelerator (357 MHz for ThomX) The low
average output power around 5 mW permits to limit the phase noise A particular attention will be paid
to the noise level of such a long cavity as no commercial products have been tested at this repetition
rate
6242 Amplification stages
For the ThomX project the lower repetition rate and the higher average output power require a change
of the second amplification stage architecture in order to limit non-linear effects and to saturate the last
amplification stage Consequently the second pre-amplifier will be based on a polarization-maintaining
double-clad Ytterbium doped fiber with a core diameter of 15 microm and a clad diameter of 130 microm (see
Fig 640) The amplifier will be clad-pumped through a special combiner 2+11 with an input signal
fiber 6125 microm directly spliced to the previous amplification stages an output signal fiber of 15130
microm spliced to the double-clad Ytterbium doped fiber and two multimode fibers of 105 microm to couple
the 9 W pump diodes
Figure 640 Schematic of the second amplification stage of the ThomX laser
Concerning the final amplification stage various fibers are available (see below) Micro-structured
fibers can be engineered and tailored but suffer from the lack of splicing techniques Hence their use in
an all-fibered setup will require additional developments dedicated to splicing (see Fig 641) and
integration If splicing is not possible we still have the possibility of proceeding with air propagation
and injection However this approach requires precise mechanics control and advanced engineering
The lsquocoherent beam combining techniquersquo will also be considered at this point In such a configuration
amplification would be supported by two parallel amplification stages dividing by two the energy level
in the gain medium Yet a first version will be developed with commercially spliceable flexible fibers
and preliminary tests of cavity locking with high power from these fibers will be performed to validate
or invalidate operating with a high level of non-linear effects
lsquo
117
Fig641 A micro-structured fiber
6243 Stretcher and compressor
Because of the high average output power required by ThomX we must modify the stretcher size Two
techniques can be considered First we may continue to use a CVBG which can stretch the pulse up to
450 ps with a maximum size of 25 cm (see Fig 642) It would be possible to stretch the pulse up to
900 ps with an efficiency of 88 through a double pass in a CVBG or a single pass in cascaded CVBG
The second way involves a chirp fiber brag grating (CFBG) as depicted in Fig 643 When associated
with an optical circulator it allows a pulse stretching with up to 800 ps in an all-fibered setup The
efficiency is around 70 (92 for the CFBG and 75 for the circulator) in this configuration This
solution is less efficient than the CVBG but the all-fibered configuration ensures the set-up robustness
and stability and the losses can be easily recovered by the amplifier chain However dispersion issues
have been reported for these devices and a detailed investigation is needed prior to a definitive
integration
Fig 642 Examples of CVBG Fig643 Example of CFBG
The high output average power forces us to use a CVBG to compress the pulse one or two units
depending on the stretcher configuration The final pulse duration can be varied between 1 ps and 50 ps
thus leaving the possibility to optimize the X-ray production by adjusting the pulse duration
We also performed simulations to evaluate the influence of the couple stretchingfiber core diameter on
the non-linearity with 3 different fiber cores 25 microm 30 microm and 40 microm Our results are summarized in
Table 61
lsquo
118
Fiber
dimensions
mode field
diameter stretching B-Integral
25250 20 microm 250 ps 47 rad
25250 20 microm 500 ps 263 rad
25250 20 microm 1 ns 118 rad
30150 22 microm 1 ns 09 rad
40200 30 microm 1 ns 0525 rad
Table 61 Pulse stretching as a function of the fiber core diameter (simulations)
A first configuration will be developed with a stretching of 500 ps If the non-linearity effects remain
too high to allow for an efficient locking on the Fabry-Perot cavity we will modify the setup to induce
a larger stretching factor or insert a fiber with a larger core diameter in the last amplification stage or
both
63 Mechanical design
Collisions between the laser pulse and the stored electrons will take place between two dipoles (see Fig
643) The constraints on the optical cavity mechanical design are as follows
- High stability level (vibrations)
- Good accessibility to the cavity mirrors
- Weak impact on the electron beam dynamics
- Ultra high vacuum (UHV) system with baking capability
- Motorized granite table for micrometric scan oof the electron beam
lsquo
119
Figure 643 Implementation of the cavity in the electron ring
The cavity mirror system is mounted on a massive granite table which can be driven to perform a
micrometric electron beam scan (see Fig 643)
lsquo
120
Figure 644 Scheme of the Fabry-Perot cavity mounted on the optical table
The Fabry-Perot cavity is composed of two cylindrical UHV vessels linked by a dipole chamber
Bellows are placed between each element to absorb the dilatation due to the backing (see Fig 644)
e‐
e‐
X‐Ray
1rsquorsquoMirrors
IP
Bellows
1rsquorsquoMirrors
Vessel
Vessel
Dipole
chamber
lsquo
121
Figure 645 On the left detail of the standard mirror assembly and on the right detail of the piezo
electric mirror assembly
The Fabry-Perot cavity is composed of two spherical and one flat mirror mounted in a standard
assembly and one flat mirror mounted on a special assembly with a piezo electric actuator for very
small longitudinal displacements (see Fig 645)
Figure 646 Detail of a truck system
Mirrors
Piezo
electric
actuator
Mirror
Gimbal
Actuators
lsquo
122
Figure 647 Cut of an UHV actuator
All the mirrors are mounted on a truck system that porvides three degrees of freedom (two rotations and
one translation) driven by three micrometric actuators These actuators are specially designed to be
compatible with UHV and backing constraints (see Fig 646 and Fig 647)
Figure 648 Cut of a vacuum vessel
Trucks (two trucks per vacuum vessel) and plugs with special cables composed of ceramic connectors
oxygen- free copper cables and ceramic pearls and spacers for electrical isolation are placed in a UHV
vacuum vessel One of the vacuum vessels has a beryllium window to extract the X-ray beam (see Fig 648)
Micrometric Actuator
Capsule
Trucks
Beryllium
window
lsquo
123
631 Optical cavity vacuum
A simulation was carried out in the most unfavorable case when the whole optical vacuum chamber is not
baked The area of one half of the optical cavity is about 2 m2 the conductance of the rectangular chamber
connecting the two cavity halves is C =70810-1
m3s
-1 (value for one half of that rectangular chamber) The
conductances of the slits connecting the optical cavity to the ring are respectively C = 45210-2
m3s
-1 (9 x 48)
and C = 16010-2
m3s
-1 (6 x 30) The outgassing rate of one optical cavity half without any baking (σ = 210
-8
Pams-1
) is Q = 410-8
Pam3s
-1 The outgassing rate of one rectangular chamber half (σ=10
-9 Pams
-1) is Q =
7610-10
Pam3s
-1 The outgassing rates of the two slits (σ=10
-9 Pams
-1) are respectively Q = 22810
-13
Pam3s
-1 (9x48) and Q=14410
-13 Pam
3s
-1 (6x30) The diode ion pump used for the optical cavity has a
pumping speed of 05 m3s
-1 DN 150 S = 0320 m
3s
-1 at ~10
-10 mbar The conductance of the pumping port
being C = 136 m3s
-1 the effective pumping speed is Seff= 0278 m
3s
-1 The diode ion pump used for one half
of the rectangular chamber has a pumping speed of 015 m3s
-1 DN 100 S = 0090 m
3s
-1 at ~10
-10 mbar The
conductance of the pumping port being C = 0046 m3s
-1 the effective pumping speed is Seff= 0075 m
3s
-1
The outgassing rate of the optical cavity without baking is very high 2 x 410-8
Pam3s
-1 (which is
approximately 40 times the outgassing rate of the whole ring) With such a rate one would need a pumping
speed of 8 m3s
-1 to reach the required pressure level of 10
-8 Pa (the present pumping speed is 07 m
3s
-1) The
conductance of the rectangular chamber that connects the two halves of the optical cavity (710-1
m3s
-1) and
the conductance of the two slits between the optical cavity and the ring (610-2
m3s
-1) are high These figures
do not allow a differential pumping between the cavity and the ring Simulations of the ring vacuum level
show that it is mandatory to bake the whole optical cavity
Figure 649 Location of the cavity pumps
The vacuum equipment needed for the optical cavity is as follows
‐ 2 ion pumps 150 l s-1
with their baking equipment and their power supplies
‐ 2 ion pumps 500 l s-1
with their baking equipment and their power supplies
‐ 2 gate valves manually actuated (pipe diameter DN100) and their baking equipment
‐ 4 Pirani gauges
‐ 4 cold cathode gauges
‐ 2 power supplies for the gauges
‐ 4 residual gas analyzers (RGA)
‐ 1 baking equipment for the vacuum chamber
‐ 4 angle valves manually actuated for pre-pumping (pipe diameter DN63)
lsquo
124
Chapter 7 X-RAY BEAM LINE AND EXPERIMENTS
Introduction
The equipment of the X-ray beam line should firstly ensure a continuous detection and monitoring of
the beam (mainly in the first optical hutch) and secondly allow a complete characterization of the
beam (in the experimental hutch) This characterization consists in measuring the beam absolute
intensity its total energy range its spectral and angular distributions and its degree of transverse
coherence Time analyses of the bunched X-ray beam will also be performed This line also aims at
confirming the ThomX assets in such fields as cultural heritage biomedical and materials sciences by
performing demonstrative experiments
71 Available extracted X-ray beam
The X-ray beam is extracted through a 50 mm diameter beryllium window as schematized in Fig 71
Because of limitations due to the dimensions and the location of the Fabry-Peacuterot cavity mirrors the X-
ray beam available for characterization and experimentation is a cone with a half-aperture of 16 mrad
Figure 71 The X-ray beam extracted from ThomX through a beryllium window The beam fills a cone
whose total aperture is 2 x16 mrad
In the case of a lsquoperfectrsquo beam (ie without taking into account the electron beam emittance) the X-ray
spectral and energy distributions are shown in Fig 72 as a function of the scattering angle for two
electron beam energies 50 and 70 MeV and a laser wavelength of 106 microm In Fig 73 the energy
bandwidth and the flux per unit solid angle are shown for on-axis Compton photons produced by either
a 50 MeV or a 70 MeV electron beam
72 Global view of the X-ray lne
As can be seen in Fig 74 the positioning of the machine in the Igloo imposes the splitting of the X-ray
line into two sections a first optical table (3x2 m2) located just after the beryllium exit window and a
second table (4x2 m2) located outside the Igloo in hall D1 Along its path and until the optical table 2
the beam is transported in the primary vacuum inside stainless steel pipes with the exception of a short
upstream section of a few tens of cm between the beryllium window and the optical table this
insulating section will prevent the transmission of harmful vibrations to the Fabry-Peacuterot cavity This
primary zone is kept oxygen free by a nitrogen flow that prevents ozone production
lsquo
125
The beam diameter at the entry of the first optical table is 45 cm After this first optical table the beam
reaches the second table via a 13 cm diameter beam pipe inserted in a hole which was drilled long ago
in the Igloo wall About 8 mrad beam angular divergence are preserved at the entry of the second
optical table where the beam diameter is 13 cm
Figure 72 X-ray spectral distributions (left panel) and X-ray energy (right panel) versus the scattering
angle for an e- beam of 50 MeV (black) and 70 MeV (red) The laser wavelength is 106 microm Grey
areas correspond to emission angles larger than 16 mrad
Figure 73 X-ray flux (left panel) and X-ray energy bandwidth (right panel) of on-axis photons per unit
solid angle versus the emission angle for an e- beam of 50 MeV (black) and 70 MeV (red) The laser
wavelength is 106 microm
lsquo
126
Figure 74 Global view of the X-ray line in ThomX
73 Chraracterization of the X-ray beam
731 Optical table 1
A global view of the optical table 1 design is presented in Fig 75 Just upstream of this optical table a
60 mm diameter beam shutter activated when one needs to take background data will allow stopping the
beam A few millimeter thick tungsten is sufficient to stop the full beam (tungsten attenuation mass
factor microen ~ 4043 cm-1
for 100 KeV photons) This shutter does not have to be cooled since the ThomX
radiated power will not exceed 100 mW A drawing of the shutter mechanical design is shown in Figure
76 (a)
Figure 75 Global view of the optical table 1 showing the location of the main devices placed on it
lsquo
127
Beam monitoring
The optical table 1 equipment (see Fig 75) is mainly devoted to the detection and the continuous
monitoring of the X-ray beam The first device monitoring the beam presence is a fluorescent screen
mounted on a two-position motor (up or down) as shown in Fig 76 (b) It is inserted in the beam path
and viewed by a CCD camera Its screen absorbs almost the totality of the beam and thus has to be put
in the up position when not in use Downstream of the fluorescent screen a device composed of two Si
PIN diodes measures the beam scattered by a thin Kapton foil inserted in the beam path This setup
shown in Fig 76 (c) will monitor the beam flux variations and beam position displacements The
Kapton foil is fixed at an inclination angle of about 60o with respect to the beam path The sum and the
difference of the two diode currents will provide a relative measurement of the beam flux and of the
beam position respectively This set-up which is nearly transparent to 20-90 keV radiations is a non-
intrusive monitor and can be used continuously Then a high precision double slit [71] intercepts the
X-ray beam and allows one to let through only the desired part of the Compton cone (see Fig 76 (d))
The minimum dimensions of the slit aperture are 10 microm in the horizontal and the vertical directions
Beyond the double slit a wire monitor [72] shown in Fig 76 (e) is inserted in the beam This device
consists in two tungsten wires stretched horizontally and spaced by a few tens of a millimeter This
monitor will be driven vertically through the beam The difference of the two wire signals allows the
measurement of the absolute beam position
Figure 76 Drawings of the X-ray line devices located on the optical table 1 (a) Beam shutter
(b) Motorized two-position fluorescent screen (c) Kapton foil and Si diodes (d) Double slit (e) Wire
profiler
lsquo
128
Focusing device
Downstream of the beam monitoring instruments a focalizing device will be installed (see Fig 75) to
make the X-ray beam parallel andor to focus it on a sample It can be either a device based on
compound refractive lenses [73] (a so-called lsquotransfocatorrsquo) or two bendable mirrors in the Kirkpatrick-
Baez configuration (KB Optics) or a capillary optical device These three techniques will be
investigated to evaluate the advantages and disadvantages (aperture efficiency focusing strength etc)
of their eventual implementation in the ThomX facility Such a focusing apparatus has to be located as
close as possible to the X-ray source because of the rapid growth of the beam transverse dimension
Finally at the end of the optical table 1 some space is kept free for any specific study which might
concern optics detectors or samples
732 Beam transport
As shown schematically in Fig 77 beyond the optical table 1 the X-ray beam crosses the Igloo wall
inside a beam pipe that runs through a 15 cm diameter hole and reaches the D1 hall where the second
optical table is installed in the lsquoexperimental hutchrsquo At the entrance of the wall hole a second beam
shutter in lead around 15 cm in diameter and 25cm thickness is installed This shutter is connected to
an interlock system that ensures radioprotection It absorbs radiations coming from the electron
machine which propagate through the Igloo cylindrical hole and allows X-ray users to work in the
experimental hutch without stopping the electron machine
Figure 77 Schematic of the X-ray beam transport across the Igloo wall from the optical table 1 located
in the Igloo until the optical table 2 located in the D1 hall
733 Optical table 2
Several kinds of instruments will be placed on the optical table 2 The goal is to study in detail the X-
ray beam produced by ThomX firstly to determine how the beam characteristics depend upon the
machine operation parameters and secondly to prepare the demonstration experiments A global view
of the optical and detection instruments placed on that table is provided by Fig 78
lsquo
129
Figure 78 Global view of the optical table 2 indicating the main instruments that will be used to
characterize the beam
Absolute and spectral flux measurement
The measurement of the X-ray beam absolute flux and of its energy spectrum will be performed by
using two detectors a calibrated Si diode [74] and a cooled high-purity Ge solid-state detector (SSD)
[75] The Si diode will be bought already calibrated [76] A lsquogoodrsquo Ge SSD has an energy resolution
(ΔEE) around 1 at 20 keV 06 at 45 keV and 045 at 90 keV [77] The energy calibration of the
Ge SSD will be carried out in the X-ray beam line by using the Kα fluorescence lines of reference
materials (see Table 71)
Element Ge Mo Sn Cs Ce
EKα1 (keV) 9886 17479 25271 30973 34719
EKα2 (keV) 9855 17374 25044 30625 34279
Element Eu Er W Pt Pb
EKα1 (keV) 41542 49128 59318 66832 74969
EKα2 (keV) 40902 48221 75981 65112 72804
Table 71 Kα fluorescence lines of chemical elements used to calibrate the Ge SSD in the 10-90 keV
energy range
The two detectors will be mounted alternately on a (X Y) translation stand to measure a large number
of points of the emission spectrum
- the mean emission energy E (Ge SSD)
- the energy spread ΔE (Ge SSD)
- the absolute flux (photon numbers) (Si diode)
These beam characteristics will be measured as a function of the size of the selected X-ray sample the
selection being carried out with a double slit device (see Fig 76 (d)) Our goal is to check
lsquo
130
experimentally the theoretical angularspectral distributions shown in Fig 12 of the Introduction by
measuring the number of Compton photons per second per mrad2 and per 01 of energy bandwidth
Time analysis of the bunch structure
The X-ray source has a time structure which involves two main scales a 50 ns period which is the
interval between two successive collisions between the laser pulse and the electron bunch and the 20
ms e- storage cycle To perform a pulse by pulse analysis we will use a high resolution time detector
(~1-5 ns) with a high repetition frequency (~10-100 MHz) eg a micro-channel plate (MPC) detector
[78] or an avalanche photo-detector (APD) Such time measurements have already been performed at a
similar facility by Sakaue et al [79] MCP Hamamatsu devices can reach a 07 ns (FWHM) time
resolution which is good enough not only to resolve each one of the collisions between the laser pulse
and the electron bunch but also to measure their relative intensity accurately from the beginning of the
e- storage cycle to the end These measurements will be performed on the X-ray beam axis They will
have to be synchronized with the ThomX clock
Transverse coherence of the beam
Some simple diffraction and interference measurements will be performed in order to analyze the
source spatial coherence The diffraction of the X-ray beam by a slit of a known aperture or the
interference effect produced by the insertion of a phase-shifting object in the beam allows one to
quantify the transverse coherence of the beam [710] In these experiments the on-axis photon intensity
distribution will be recorded with a 2D high resolution detector (such as for instance an XPAD CdTe
camera with 130 microm pixels) This measurement will be repeated at several positions along the beam
The pattern analysis will inform us about the degree of coherence of the source A monochromator will
be inserted upstream of the diffractive slit or the phase-shifting object to select a well defined energy
Later on Young double-slit experiments could also be carried out [711] to measure the beam
coherence length beam
74 Demonstration experiments
In parallel with the complete characterization of the ThomX beam described in the previous
sections some simple experiments will be performed in order to demonstrate the potentialities
of the ThomX source In this phase we will focus on medical and materials science
applications An important aim of the ThomX project is to perform medical and materials
imaging exploiting the high and tunable energy of its beam A second aim of the ThomX
project is to show its capability in the structural investigation of materials using X-ray
diffractionscatteringspectroscopy techniques
741 Experiments using the full X-ry beam
Three experiments issued from the expertise of the ESRF-INSERM teams in medical applications of
synchrotron radiation have been chosen to be carried out as demonstrators on the ThomX prototype
The goal of setting up such experiments is to demonstrate (i) the suitability of ThomX of performing
both imaging and therapy experiments (ii) the tenability of the source adjustable at energies
corresponding to the K edge of high Z elements routinely used in medicine (Gd I) or of those of new
experimental radiotherapy adjuvants (Au Gd nanoparticles CisPt) (iii) suitability of the source for
phase contrast imaging (iv) exploitation of the advantage of using a 2D beam with respect to the typical
laminar synchrotron X-ray beam Some imaging and therapy experiments in the biomedical field
require a wide 2D beam Using the natural divergence of the ThomX beam a sample of a few
lsquo
131
centimeters in diameter located at 10-20 meters from the ThomX interaction region can be fully
illuminated by the a conic beam with no need of optical elements Calculations have been performed in
order to show the potential of using the ThomX source in comparison with a hospital X-ray generator
(fluoroscopy unit type) as well as in comparison with the beam from the biomedical line ID17 at ESRF
Results of these comparisons are given below for each medical experiment we intend to perform with
ThomX
An imaging experiment
For imaging experiments the energy range of the beam has to around 60-90 keV According to Fig 73
the corresponding half-aperture will be 5 mrad leading to a photon flux of 61012
phs When the beam
reaches the sample stand placed on Table 2 at 20 m from the ring interaction region it will have a
diameter of 20 cm This conical beam will then have a cross section of ~300 cm2 and permit
tomographic acquisitions of a full human head using a 2D detector This wide 2D beam is a major
advantage of the ThomX source for biomedical imaging with respect to the laminar synchrotron
radiation beam The dose rate (DR) in Gys can be expressed as
(71) DR = 16 10-13
Ф[phscm2] E[keV] (microenρ)[cm
2g]
where Ф is the photon flux per cm2 E is the mean energy of the beam and (microenρ) is the mass
attenuation coefficient For an X-ray beam of 75 keV (ie the average of our 60-90 keV energy range)
the mass attenuation coefficient microenρ for soft tissues is 0028 cm2g leading to a dose rate of 67 mGys
delivered to a 300 cm2 area The corresponding expected lsquodose area productrsquo (DAP) of the ThomX
source for such imaging experiments is DAP = 20 Gyscm2 For comparison at the ESRF ID17 line a
dose rate of 800 mGys is delivered to an area of 150x2 mm2 corresponding to a DAP of 24 Gyscm
2
As far as conventional sources are concerned typical laboratory devices used in 2D fluoroscopy deliver
a DAP of 06 Gyscm2 [712]
A feasibility experiment will be performed by using a standard human head phantom (see Fig 79)
which will be mounted on a rotating stand that allows a 360deg rotation It will be possible to translate this
stand in three directions (along the X-ray beam axis and in the two other perpendicular directions) to
optimize the sample position in the beam Acquisition will be performed using the 60-90 keV pink
beam before and after the insertion of a high-Z element solution (typically iodine or gadolinium) in a
slot available in the phantom head Several images will be acquired using different concentrations of
high-Z element chosen in the range of concentrations that can be typically reached into a tumor (in
humans the high Z element is injected intravenously) The study will also aim at finding for each high-
Z element the lowest concentration that can be measured with the ThomX imaging system
Figure 79 Typical phantom featuring a human skull from X-ray absorption point of view
lsquo
132
The detector will be placed downstream of the sample A typical detector that may be used is a large
field of view 2D detector equipped with Si or CdTe sensors with ~100-200 microm pixels [713] Finally a
standard computed tomography (CT) algorithm will be used to reconstruct the tomography images and
to calculate the concentrations (in mgml) of the given high Z element in the skull The results will be
compared to previous work done at the ESRF [714]
A radiotherapy experiment
For this experiment the targets will be rats implanted in their right brain hemisphere with rodent brain
tumors (F98 9L) at an appropriate tumor stage (typically at D10 or D14 after implantation) For the
irradiation we need a pink beam with a 10 keV energy bandwidth and a median energy above the K-
edge of the high-Z element to be injected as a radiosensitizer (ie in the 45-80 keV range for both
iodine and gadolinium) The beam size will be a few centimeters in diameter Figure 710 shows the X-
ray beam energy and beam flux generated by a 69 MeV electron beam versus the photon back-
scattering angle X-rays of energy 80 keV within a 10 keV bandwidth are contained in a cone of half-
aperture 27 mrad and their flux is ~31012
phs At the sample location on optical Table 2 at 9 m from
the ring interaction region the beam has a diameter of 5 cm ie a cross section of ~20 cm2
The center of the ratrsquos right hemisphere will be placed on the rotation axis of the irradiation system and
slits will limit the beam transverse extension in order to define a precise field of irradiation Assuming
an X-ray flux (Ф) of 31012
20 phscm2 with an average energy (E) of 80 keV a mass attenuation
coefficient of 0025 cm2g (soft tissue 80 keV X-rays) Eq (71) leads to a dose rate DRe of 48 mGys
close to the tissue region where the X-ray beam penetrates Before reaching the tumor the radiation is
attenuated by the bone (~ 02 cm thickness (microρ)att = 022 cm2g ρ=192 gcm
3) and by the skull healthy
tissues (~ 15 cm thickness (microρ)att = 018 cm2g ρ = 10 gcm
3) Thus the dose rate DRt delivered at
the tumor is DRt = TDRe (where T = exp (-02219202) exp (-0181015) = 070) and is
expected to be 337 mGys The dose will be delivered while the rat undergoes a 360deg rotation The total
dose to be delivered to a tumor ranges between 10 to 20 Gy Therefore according to the expected rate
of 337 mGys the irradiation will take typically from 5 to 10 minutes with the big advantage of not
having to move the rat A dosimetry protocol (such as the one used at ID17 [715]) will be set to check
the homogeneity of the irradiation and to measure the absolute dose with a 2 to 5 uncertainty
Figure 710 X-ray energy (left panel) and X-ray flux (right panel) versus the photon back-scattering
angle for stored electrons of 69 MeV The laser wavelength is 106 microm
For humans and a tumor placed at 8 cm into the skull the transmission coefficient becomes T= 019
Therefore the expected dose rate DRt delivered by the ThomX source at the tumor is 91 mGys One
may compare with the ESRF ID17 beam-line where 800 mGys can be delivered for irradiation on a
lsquo
133
surface of 2x50 mm2 ie with T=019 a dose rate DRt of 015 Gys on 2 mm in height at the tumor
which is equivalent to a rate of 6 mGys to cover 5 cm of height with a scanning rate of 50 mms
ThomX could therefore deliver doses for human skull irradiation larger than those available at
synchrotron radiation facilities and also perform the irradiation without having to move the irradiated
sample Concerning the conventional hospital devices they deliver low doses deposited by X-rays
whose energy spectrum is broad and moreover they are not made to operate continuously for such long
time irradiation
Also the tunability of the ThomX beam will allow testing several high Z drugs containing iodine
gadolinium platinum or gold proven to be promising radiosensitizers [716]
Conformal radiotherapy
A final step in this sequence is performing conformal radiotherapy experiments ie doing both imaging
and therapy at the same time In fact one major issue of radiotherapy is to spare as much as possible
healthy tissues located around the tumor and at its border In this regard it is a key issue to adapt the
field of irradiation to tumor geometry size and positioning by performing first a tomography
Phase contrast imaging
The absorption medical imaging technique is based on the fact that different parts of a sample absorb X-
rays differently But the fact that X-ray absorption by biological material is weak constitutes also the
limitation of this technique To enhance or replace absorption-based imaging refraction diffraction and
interference of X-rays can also be used A better visibility at the edge of different regions of a specimen
can therefore be obtained by using the information contained in the phase of an X-ray wave For this a
spatially coherent X-ray source is required and was available only since the development of modern
synchrotron sources Experimentally the simplest way to visualize phase-contrast imaging is provided
by the propagation-based imaging (PBI) method In this method a spatially quasi-coherent radiation
illuminates an object which gives rise to a spatially varying phase shift in areas with spatial variations
of the refractive index As the radiation propagates beyond the sample parts of the wavefront which
have experienced different deflections on both sides of an edge of the sample produce a serie of fringes
due to the classical Fresnel-edge diffraction phenomena [717]) This gives rise to a characteristic phase
contrast pattern which is then recorded by a 2D detector placed at a proper distance downstream of the
object (see Fig 711 left panel) The actual phase-contrast signal is convolved with the detector point
spread function which has to be sufficiently narrow to avoid the smearing of the interference pattern
Additionally the phase contrast signal can be visualized if and only if the transverse coherence is
sufficient ie if Lcoh = λRs is larger than the details to be imaged (here R is the source-sample distance
and s is the source size) In contrast the requirements in terms of longitudinal coherence
(monochromaticity) are rather weak even a beam with a bandwidth of several keV does not lead to a
significant smearing of the phase contrast signal [717]
Assuming s = 70 microm R = 15 m a sample-detector distance of 5 m and a detector point spread function
of 50 microm the phase contrast signal expected when the ThomX beam illuminates a nylon wire of 300
microm diameter ranges from 10 to 3 when the X-ray energy increases from 20 to 80 keV (see Fig
711 right panel)
lsquo
134
Figure 711 Left panel The set-up for propagation-based phase contrast imaging Right panel The
simulated signal produced by a nylon wire of 300 microm diameter illuminated by the ThomX source
(simulation)
The experiment with ThomX will comprise two parts a planar radiography and a computed
tomography For the radiography three kinds of samples will be used (i) a custom-made phantom
composed of nylon wires of different diameters (ii) the ACR (American College of Radiology)
accredited phantom used in mammography quality (iii) a 4 cm thick formalin-fixed human breast tissue
obtained from a hospital bank The X-ray energy will be chosen in the 20-80 keV range For each
sample and for several energy values images will be acquired at two different sample-detector
distances in the absorption regime for comparison purposes (the detector will be in contact with the
sample) and at 5 m A silicium detector with 50 microm pixels will be used Images will be acquired for
different doses (measured with a standard PTW dosimeter) in order to obtain the signal-to-noise ratio as
a function of the dose These measurements will allow one to qualify the phase contrast imaging set-up
at ThomX
Computed tomography will be applied to (i) a meacutethyl polymeacutethacrylate (PMMA) custom-made
cylindrical phantom containing insets of different compositions and densities (water ethanol paraffin
etc) (ii) a formalin-fixed human breast tissue (5 cm in diameter) Images will be acquired using the
same parameter values as the ones chosen for the radiography Samples will be installed on a rotating
stand positioned on a double cradle that will permit an alignment with the beam and with the detector
Images will be taken during the sample rotation over 360 degrees
The final step will consist in applying standard algorithms to the images in order to calculate the phase
and to extract quantitative information about the samples This data will be compared with equivalent
images that will be acquired at the ESRF
742 Experiments using the central part of the beam
Most specimens from cultural heritage and functional materials are heterogeneous these lsquorealrsquo
materials often exhibit complex hierarchical structures involving various levels of order and
heterogeneity To understand the properties of such materials better one needs at least a description of
their structure at the atomic scale and at the macroscopicmesoscopic scale which takes into account
weak structural signals (relating to disorder minor constituents microstructure hierarchical orderhellip)
lsquo
135
This requires much probing as already demonstrated with synchrotron radiation sources A major goal
of materials science is to link global properties to local atomic features (orientation strain structural
distortion grain size) of some components in relation with neighboring domains and to deduce from
that the macroscopic properties As the functionalities of these materials often stem from the association
of different phases or constituents or of specific inter-relation between them (microstructure) highly
selective structural probes need to be used to characterize them Also in the particular domain of
cultural heritage many specimens require non-destructive methods and probes In order to understand
how they were made and to preserve them from degradation over time their physical and chemical
properties must be determined There is an increasing demand for material probing with complementary
techniques Fluorescence emission techniques excel in revealing the elemental composition of the top-
most layers of samples Diffraction is suited for studying quantitatively the phase composition the
structural details of the constituent phases and the characteristic distribution of sizes and orientations of
crystallites Tomography techniques excel in revealing the 3D mapping of the chemical andor the
density heterogeneities within a sample This is why the cultural heritage community is today strongly
involved in the development of new instruments that may characterize patrimonial artifacts by mapping
not only the chemical composition of a variety of samples but also their physical phases
Experiments described in section 741 require a large 2D beam and exploit the natural divergence of
the ThomX beam In this section diffraction scattering and spectroscopy experiments require a
focalized or a parallel beam and it should also be monochromatic or lsquopinkrsquo The optics will therefore
select the central part of the beam ie its higher energy part Let us give an order of magnitude
assuming a perfect 50 MeV electron beam a collimator of 2 mm diameter located at 2 meter from the
IP will cut the X-ray conic beam at 05 mrad leading to a flux of ~8 1010
phs with an energy of 45 keV
and an energy bandwidth ΔEE on the order of 10-3
(see Fig 73) In practice this bandwidth will be
broadened and will reach a few percent because of the actual e- beam divergence and energy spread For
experiments requiring a narrower energy bandwidth an additional selection will be performed with a
monochromator For instance selecting an energy bandwidth of 10-4
would lead to a flux on the order
of 109 phs
Since the goal of the first series of demonstration experiments is to qualify ThomX as an X-ray source
these experiments will be performed with samples previously characterized with other sources such as
synchrotron radiation sources
Diffraction and scattering
Diffraction and scattering experiments are major ways of performing materials structural and micro-
structural analyses This concerns highly ordered samples as well as powders or ill-ordered samples
Complete scattering analyses are based not only on Bragg diffraction but also on diffuse scattering in
order to look beyond the average structure and to investigate the local structure or short-range order
The crystalline quality of powders depends on the preparation methods Grain sizes vary from about 10
nm to several hundreds of microm In the case of nano-materials Bragg peaks are strongly broadened and
X-ray diffraction performed with standard laboratory sources does not provide suitable data for
structural investigations As illustrated by Fig 712 laboratory X-ray sources do not allow the
identification of minor phases in a poorly crystallized powder Figure 713 shows an example of a
diffraction pattern obtained with an excellent resolution The sample used for this analysis is an
Egyptian cosmetic powder dated to 2000-1200 BC The patterns were obtained with the ESRF
synchrotron source (bottom panel) and with a classical laboratory source (top panel)
lsquo
136
Figure 712 Powder X-ray diffraction patterns obtained by using a laboratory X-ray source Top panel
Poorly crystallized Prussian blue sample Bottom panel Prussian blue crystals From [718]
Figure 713 Powder X-ray diffraction patterns obtained by using a laboratory X-ray source (top panel)
and the ESRF synchrotron source (bottom panel) Arrays indicate minor phases From [719]
We intend to use the ThomX source to record diffraction patterns and scattering plots in order to
characterize short-range ordering and identify minor phases in such heterogeneous samples
For the first demonstration diffractionscattering experiments we will study ill-ordered Prussian blue
and white lead samples Pigments used for paintings or frescos have various compositions Since their
usage followed a definite chronology such pigments can sign an era Pigment characterization can also
help the undertaking of a restoration while a thorough knowledge of pigment synthesis through
centuries can help to apprehend the degradation mechanisms better
lsquo
137
For ordered samples diffraction will be performed with a single crystal by using a monochromatic
beam but also by using a lsquopink beamrsquo as done in the Laue method For ill-ordered samples the whole
scattering pattern will be recorded and analyzed with the Pair Distribution Function (PDF) method This
analysis technique (often called the ldquototal scattering methodrdquo) can be used alone or associated with
diffraction For experiments using the lsquopink beamrsquo Laue method the beam will be focused directly on
the sample (by using convergent refractive lenses (CRLs) or a capillary optic system as mentioned in
Section 731) To determine powder crystalline parameters diffraction analyses require the
monochromatization of the beam in order to reach ΔEE ~ 10-4
In the optic hutch (optical Table 1) the
diverging beam will first be made parallel by the CRLs or the capillary optic method and then
monochromatized in the experimental hutch (optical Table 2) by a monochromator The set-up for these
measurements needs a goniometer to align the sample and a low noise 2D-detector sensitive to high
energy photons
Fluorescent spectroscopy
X-ray fluorescence spectroscopy experiments allow one to determine the chemical composition of
materials and the mass concentrations of elements Indeed depending on the atomic number of a given
chemical element energies of the fluorescence lines emitted after an X-ray excitation are a signature of
the element An illustration of this approach is presented in Figure 714 where the fluorescence
tomographic images of a Globorotalia inflata specimen are shown and compared with a classical
radiography image
Figure 714 Fluorescence tomographic imaging (a) Set-up (b) Radiography (c) Fluorescence
spectrum measured at a given angleposition of the sample (d) 3D tomography reconstruction From
[720]
We plan to carry out fluorescence experiments since the energy of the ThomX beam can reach the
excitation energy of most chemical elements usually involved in material science for instance the
critical excitation energy of lead regarded as a heavy element is 88 keV) For this experiment the
excellent energy resolution of cooled Ge detectors will be an asset to detect the fluorescence light (the
detectors will be oriented at 90o with respect to the incident beam)
lsquo
138
K-edge and XANES spectroscopy
In K-edge experiments two images of a given element present in some sample are performed one at an X-ray
energy just below the K edge of the chosen element and the other at an energy just above Then they are
subtracted (see Fig 715 left panel) The K-edge imaging technique is used in particular in the medical and
the cultural heritage domains In XANES (X-ray Absorption Near Edge Structure) the absorption by a
material is measured over an energy range of about 100 eV on both sides of the absorption edge This
technique provides information about the local structure prevailing close to a particular chemical element (see
Fig 715 right panel)
Figure 715 Left panel K-edge subtraction imaging Right panel Principle of the XANES method
The first XANES spectroscopy experiments with ThomX could exploit the K-edge of heavy elements
such as W Pt or Pb By first selecting the central part of the ThomX beam with an energy spread of a
few 10-2
a monochromator could then provide a beam with an energy spread of 10-4
Samples will be
aligned by using a goniometer and the transmitted intensity will be collected by a low noise 2D-
detector
Combined diffractionabsorptionfluorescence tomography
A major limitation of diffraction methods comes from the fact that only average properties can be
obtained while most lsquorealrsquo materials are heterogeneous and complex Thus structural studies of such
materials are difficult to carry out A new tool for structure mapping of materials by means of X-ray
diffractionabsorptionfluorescence combined with tomography allows one to perform non-destructive
3D structure selective mappings of heterogeneous samples This way one may identify minor phases
and determine crystallite orientation and grain size Furthermore the selectivity of the method allows
one to separate and locate the origin of the diffuse or diffracted patterns to identify amorphous phases
In archaeology such complementary micro-structural analyses could shed light on the mechanical
history of artifacts
A recent experiment carried out with a micrometer beam at synchrotron radiation sources have
established the validity of such a local probe by analyzing a mixture of chalcedony and iron pigments
containing haematite [721] This experiment illustrated in Fig 716 consists in (i) performing a
structure tomography mapping directly from the diffraction data (top panel) (ii) obtaining selective
structure patterns of unknown minor phases from these maps (bottom panel) This study was realized by
the Neacuteel institute and the C2RMF and was one of the first diffraction tomography experiments
performed with a micro-beam
With the ThomX source we will use poorly crystallized samples and combine in the same experiment
and simultaneously tomography diffraction with fluorescence and absorption measurements Diffraction
lsquo
139
tomography is beam-time consuming it will be performed to demonstrate its feasibility at ThomX The
experimental set-up will be similar to the one used for multimodal analyses (cf the top panel of Fig
716) The sample will rotate and this rotation will be combined with a translation to provide a 2Θ scan
of 2D diffraction patterns obtained with a 2D detector sensitive to high energy photons The diffracted
2D fluorescence patterns will be recorded with our Ge detector while the sample absorption will be
recorded with our calibrated Si diode
Figure 716 Top panel Experimental set-up for multimodal spectroscopydiffractiontomography
analysis Insets Images of the chalcedonyndashFe mixed-pigment sample are reconstructed by combining
the various signals X-ray absorption (bottom) Fe X-ray fluorescence (top left) and chalcedony
diffraction (top right) Bottom left panel Reconstructed diffraction image based on the whole
diffraction diagram (black) compared with the calculated pattern for pure chalcedony (red) Bottom
right panel Reconstructed image that only uses the iron pigment scattering contribution (black)
compared with the calculated pattern for hematite and siderite grains (red) From [721]
75 Summary
As shown in this chapter two types of beams will be available at the ThomX compact X-ray source
(i) wide beams well adapted for imaging (conventional or phase contrast imaging) and therapy in the
biomedical field and (ii) almost monochromatic beams that can be either focused or made parallel for
diffraction and spectroscopy experiments in the field of materials science
The X-ray line is designed
- to ensure a continuous monitoring of the X-ray beam
- to measure the beam brightness and its transverse coherence
- to demonstrate the potential of this new type of compact source and identify the benefits it can
bring to the currently available X-ray sources
lsquo
140
Chapter 8 CONTROL SYSTEM
Introduction
The ThomX control system combines those hardware and software elements involved in the
management of all the facility components But more than a system dedicated to carrying out operating
processes (processes launched from the X-ray beam control room in order to undertake some specific
task such as a performing an imaging experiment or a material chemical analysis) it must also be an
advanced tool for various studies (data processing new equipment integration operating mode
evolution) and scientific experiments which will be performed with the X-ray beam line Hence the
control system has to be a versatile tool easily scalable by various users (scientists operators IT
technicians)
In the following a lsquoSub-systemrsquo is a physically connected set of hardware items sharing a global logical
function The ThomX sub-systems are as follows the linac (LINAC) the transfer line (TL) the ring
cell 1 (RI-C1) the ring celle 2 (RI-C2) the optical cavity (OC) the extraction line (EL) the section
of the X-ray line located in the Igloo (XLI) and the section of the X-ray line located in the X-ray hall
(XLH) A lsquoDomainrsquo is a set of hardware items related by a common purpose The ThomX domains are
as follows alignmentmetrology (AM) control-command (CC) diagnostics (DG) infrastruture (IF)
magnetic elements (ME)n optics (OP) pulsed elements (PE) power supplies (PS) radio-frequency
(RF) safety (SA) synchronization (SY) and vacuum (VA)
81 General principles
Some general principles drive the design of such a control system In broad terms the following criteria
will serve as guidelines
- Since monitoring is a major role of a control system comprehensive monitoring interfaces must be
provided to all users Moreover the monitoring system must take into account sub-systems specificities
dedicated GUIs must be designed for every need for instance in synoptic form to give a global system
status overview or as charts to show the time evolution of some specific parameters of a particular
device (eg pressure readings on TPG300)
- Operating processes management a control system also aims at the prioritization and sequencing
of processes It has to provide high-level functions in order to remotely launch complex operations (eg
machine startstop interlockshellip) Computing and data analysis tools which could launch such particular
procedures will be integrated or developed to maximize the automation of the machine
- Default management the facility control system must be able to manage operation defaults It will
be necessary to design graphical tools to quickly identify defaults when they occur Color display of
warnings and errors according to a criticality-based convention will be implemented in addition to the
recording of running process logs
- Safety for obvious reasons the ThomX radioprotection and safety system must standalone It should
not use any data generated by the control system Nevertheless each end-user must be able to make
sure that the ThomX facility does not present any danger whenever the accelerator is running This
information must be available in every control room through a continuous display of all the safety
equipment status (radioprotection sensors door openingshellip) A full historical record of safety-related
lsquo
141
events will be made available for analysis and a procedure of detailed acknowledgement (operatorrsquos
name action to close an incident) will be defined in order to guarantee safe operations
- Ease of use The ThomX control system is intended for various usages therefore it will concern
various users including operators who are not familiar with software development Hence it is crucial
to make it modular through relevant to sub-systems GUIs with various visualization interfaces control
buttons graphs sub-system overviews in synopsis or detailed display of subset status The definition of
new interfaces will be possible through visual development frameworks to fit new needs
82 Technical considerations
In order to guarantee a proper accelerator operation to make its use as easy as possible and to limit
costs some technical points have to be taken into account
- Distributed architecture an accelerator is a complex structure which results from various designs in
different fields The hardware part of the control system relies on dedicated IT devices (industrial PCs
programmable logic controllers) whose role is to drive sub-systems Each of these nodes will be fully
and remotely controlled by the system software from the control rooms (or from any authorized user
terminal in the ThomX network) independently of possible other issues All of these components must
communicate with each other when required by the operation needs Moreover the supervision part of
the control system must be available in different places (main control room X-ray line control room
perhaps others) The chosen system must meet these requirements and must run on several terminal
types
- Standardization of the hardware items and of the software codes is necessary in order to simplify
their integration in the control system (eg same brandmodel for all the stepper motors) Then it will be
possible to mutualize high-level controlling tools whether pre-installed or homemade Additional
benefits of this approach concern the overall maintenance the deployment of new releases and the
replacement of hardware parts
- Reliability The service continuity of the control system must be guaranteed A spare strategy must be
set up to make sure that the replacement of defective parts will be done on short notice In particular
storage memory redundancy will guarantee that none of the machine parameters or of the collected data
will be lost Periodic backup of main software systems (device databaseshellip) will be performed right
from the start Uninterruptible power supplies are foreseen to protect major elements of the control
system from electrical failures
- Upgradability Special attention will be paid to communication features when choosing hardware
devices Ethernet is now one of the major protocols and must be preferred when possible For the PLC
driven devices the fieldbus Profibus [81] is recommended since one single master interface can
manage more than 100 slave devices without data losses This product is also valuable as far as cabling
cost is concerned
- Safety Since several users may wish to perform various machine-related tasks (integration
maintenance measurementshellip) a policy must be established to control the access to the ThomX
network and its control system The management of these access permissions will be provided It will
take into account the role and the field of expertise of each project member
lsquo
142
83 The TANGO control system software
Tango [82] is the software chosen to be the backbone of the ThomX control system It is an open-
source software initially developed at the ESRF It is now used in several Institutes to control huge
accelerator facilities like the SOLEIL synchrotron In fact the SOLEIL staff is playing a major role in
maintaining and developing new Tango features
831 Software bus
An accelerator equipment is usually highly heterogeneous Thus its control system must enable various
devices to communicate with each other even if they use different protocols or communication
standards like Ethernet RS232 USB PCI DAQ Cardshellip
The first asset of Tango is the communication standardization that it provides between any type of
devices Tango implements a CORBA-based communication protocol so that a common network is
virtually created for the whole hardware If needed the parameters of a given device are thus available
to every other element of the control system This is achieved by adding an extra Tango-compliant
software layer above the hardware low-level libraries This layer is specific to a device brand and model
and is a simple executable file which can be instantiated as many times as required by the number of
devices This key element of the Tango system is called a lsquoDevice serverrsquo
832 A control system for accelerators
For basic reasons the Tango core and many of the tools developed around this software bus are open
source tools It is therefore an accessible system fully customizable for specific ThomX needs and a
system which can be maintained Since Tango is Linux and Windows compliant issues originating in
driver availability for the Linux OS can be avoided while Tango enables the use of scientific
applications which are often developed for Linux
Furthermore Tango is developed and maintained by a community which involves several accelerator
Institutes Even if Tango is not an accelerator-dedicated tool a number of developments have been
carried out or are being carried out that deal with devices currently used at such facilities Thus a large
Tango library of device servers is presently available to any new user
Finally more than a simple tool which provides a network access to a set of devices Tango is also a
framework that provides high level operational services such as configuration tool logging archiving
services and alarms permission management For developers specific tools are provided for the
generation of codes that fit in the common structure required by all device servers Other tools allow
easy GUI development As a result the integration of new devices or the upgrade of control and
supervision windows do not need in-depth coding As far as the compatibility with commercial software
is concerned dedicated bindings can be freely used eg applications developped for Matlab or
Labview can be used to perform a variety of tasks involving the ThomX devices
lsquo
143
84 Control system architecture
841 Equipment to control
On a complex machine like ThomX a wide range of devices and control signals must be taken into
account motors cameras scopes diodes or valves will deal with analog and digital inputsoutputs
complex signals transmitted through GPIB USB Ethernet or profibus interfaces
The inventory of devices to be controlled is presented in the following table Safety equipment is not
mentioned as it is a fully standalone sub-system
LINAC TL Ring OC XL
Power Supplies
Coils 2
Steerers 2 4 24 2
Dipoles 5 4 8 3
Dipole chicane 1
Quadrupoles 7 24 3
Sextupoles 12
Pulsed elements
Septum 1
Kickers 2
Synchronization
3GHz synthesizer 1
500 MHz synthesizer 1
Phase shifter 5
Programmable delays 2
Counter 1
Vacuum
Ion pump 8 4 17 4
Gauge 8 8 22 8 1
Valve 4 4 4 2
Primary pump 1
Diagnostics
BPM 1 4 12
Low frequency Signal
generator 2
ICT 1 2
Stepper motors 1 3
Camera CCD 2 3 1
RF
Klystron coil 1
Modulator 1
Amplifier 1
Canon 1
Feedback 2
Source laser
Photodiode 1
Camera CCD 1
lsquo
144
Motor 1
Mirror 2
Laser ndash FP cavity
Camera 4
CCTV Camera 10
Motor 16
Thermal control unit 2
Laser equipment 3
Scopes 3
FPGA 1
X‐ray Line characterisation
Xpad camera 1
Si PIN diode 3
Ge diode 1
Motor 13
Stepper motor 25
Others
Translation table 1 1
Slot 1
Table 81 Devices to control (TL=Transfer line OC = Optical cavity and XL = X-ray line)
842 Inputsoutputs
All devices listed above communicate with each other by digital inputsoutputs (DI DO) ie one bit
signals analog inputsoutputs (AI AO) or more complex signals as n-bit digital signals (n = 8 12 or
16) Several communication protocols mentioned in sect832 above will be used
They are detailed in Table 82
PLC InputsOutputs Bus Ethernet
connections
DAQ (ADCampDAC)
Vacuum 26 DI 13 DO 26 profibus
Diagnostics 2 gpib 30 3 x ADC 12 bits 100 kHz
Linac 2 DI 2 DO 5 AI 3 ADC 8 bits 1 GHz
Magnets 35 profibus
Feedbacks RF (x2) 10 DI 10 DO 10 AI
10 AO
USB
Ampli RF 64 DI 8 DO 32 AI 16
AO
RS232 2 4 channels ADC 12 bits
1MHz
Modulator RF 23AI 4 AO 69 DI 5 DI 5 DO
8 channels ADC 8 bits
Pulsed Elements (x3) 12 DI 3DO profibus ADC 12 bits
Laser ndash FP cavity RS232 22
Timing 2 gpib 14 15 channels DAC 16 bits
X Line 26 DI 26 DO 7
Table 82 Equipment IO to control (not complete at the time of writing)
lsquo
145
843 Front-end machine architecture PLCs and industrial PCs
The front-end architecture will comprise equipments individually dedicated to sub-systems These
machines are controlling nodes which will be located in sub-system server racks as close as possible to
the equipment to be controlled (when required)
Two kinds of front-end machines will be used
- PLCs programmable logic controllers are automation OS-based machines able to manage digital and
analog IO and to control processes carried out by devices connected to a field bus (eg Profibus) or
specialized devices They are reliable devices designed to operate in an industrial environment A
proprietary operating system runs on these machines which are powerful tools (eg automats) to
implement logic appropriate to experimentation Interconnection with the other Tango network
equipment is made through an Ethernet adapter interface
- Compact PCIs (industrial PCs) these crates can embed complex operating systems like a Linux
distribution or some Microsoft Windows Operating Systems hard drives Ethernet connections and
USB adapters Such crates are based on a PCI bus backplane to interconnect specific cards (DAC
ADC counters optoelectronics convertershellip) cPCI is a standard widely used in an industrial
environment it is compatible with numerous card suppliers These front-end machines will also allow
the running of Tango processes or specific computing tasks locally installed in a sub-system
The ThomX distributed control system will be build on these controlling nodes This structure will
allow the bandwidth optimization of the network main links and will provide cabling simplifications
The control equipment may be placed in the accelerator hall as well as in the technical rooms where
most of the electronics that must be controlled will be clustered in 19rsquorsquo standard server racks
‐ Some devices need not be controlled through these front-end machines They will be directly
connected to the control network (eg ethernet cameras or electronics for motor control) Three
ways to interconnect such devices are shown in Fig 81
‐
Figure 81 Control architecture for one sub-system
lsquo
146
These acquisition and controlling units can be Profibus PLC or a cPCI crate Each driven element will
be controlled through one of these acquisition and controlling unit
The overall control system will be composed of more than 25 front-end machines as described in Table
83
PLC cPCI Crate
Linac (wout vacuum diags modulator
magnets) 2 1
Vacuum 2
Diagnostics 2
Feedbacks 2 2
RF amplifiercavity 1 1
RF modulator 1 1
Pulsed elements 3 3
Fabry-Peacuterot cavity 1 1
X-ray line 1
Synchronization 1
Magnets 2
Total 13 14
Table 83 List of front-end machines (not complete at the time of writing)
85 IT infrastructure
- Network The ThomX local network will be organized around an IP backbone It will be based on the
Ethernet TCPIP standard in order to link user terminals front-end machines specialized servers etc
Because of the large number of devices and the requirements bearing on the bandwidth (eg for image
storage) 1 Gbitss compliant equipment will be chosen (LAN adapters switcheshellip) Depending on the
actual number of cameras an image-dedicated network could be added On the cabling side shielded
copper cables will be used as interconnection wires to limit electromagnetic interferences as much as
possible
- Data storage During the ThomX commissioning or for the operation monitoring or for the X-ray
experiments it will be crucial to store a large amount of data It is foreseen to implement the three
storage processes provided by Tango historical (long term storage at a slow rate) temporary (short
term storage at a fast rate) and snapshot (storage of parameters on operator demand or event triggered)
It will also be possible to meet the X-ray line needs for the storage of large size images during short
periods (at most for several days) This specific storage will not be necessarily on local servers it could
be convenient to move these data and store them on servers close to the final users possibly outside the
ThomX network
Two storage-dedicated servers will be installed for analyses replay of accelerator operation scenarios
and for some specific needs of the X-ray line These storage units will be specific machines which
prevent data outages or losses by redundant power supplies stored data redundancy (RAID) hard disk
hot swap etc The first of these two storage units will be a 2 To server equipped with a disk array
dedicated to the accelerator data management The second one will have a 10 To capacity it will be
available for the large size images of the X-ray line
lsquo
147
- Equipment of the control rooms Users will work in control rooms on system administration
accelerator control development and integration (device servers GUIs high level computing software
safety analysis tasks) Administration and end-user terminals should be provided in sufficient number
(see below)
The main control room will be equipped with
- 5 PCs 1 for the Tango core system 2 for operations 1 for development 1 for safety (dedicated to
visualization storage and analysis of safety related events)
- 1 server to run high-level applications and device servers
- 2 screens one displaying the ThomX status with synopses and the other one for the safety
supervision
- camera monitors for some direct controls
- 1 UPS (uninterruptible power supply)
The X-ray line control room equipment will consist of
- 2 PCs for the operation equipment and for macro development
- 1 server to run high-level applications and device servers
- 2 screens dedicated to the monitoring of the machine status to safety synopses and to the display of
safety related events
- Software needs
- Both the Linux and Windows OS must be installed in the control room PCs in order to run Tango and
many other operating processes (see the Introduction above) for development purposes and to use
configuration software Together with the Tango core and specific tools high-level software will be
installed to meet usersrsquo needs Labview and Matlab for science applications SPEC to control the
motors and for the analysis of the X-ray line data
- Data bases will run on the MySQL open-source database managing system
- An open-source http server Apache will allow the implementation of WEB interfaces for the control
system authorized users this way these users will be able to remotely visualize experiment data or
equipment status
- Finally all end-user stations will provide standard office tools
The following Figure gives an overview of the control room equipment
lsquo
148
Figure 82 Schematics of the equipment foreseen for the main control room and the X-ray line control
room
Overall the ThomX control system ndash designed to ensure harmonization and standardization and based
on open source tools ndash should be cost effective The investment cost is estimated around 200 keuro
including IT small equipment not described in this document cables a gateway to provide internet
access extra screens to display the ThomX status in the laboratory etc
lsquo
149
Chapter 9 SAFETY AND RADIOPROTECTION
91 Introduction
The ThomX facility will be sited in an existing building called lsquoIgloorsquo where a source of hard X-rays was
operated until 2003 by the Laboratoire pour lrsquoUtilisation du Rayonnement Synchrotron The ThomX linear
accelerator and storage ring will be built inside this building while the facility control room and the one of the
X-ray line will be located respectively in the D1 and D2 experimental halls The D4 hall will be devoted to
experimental tests
The ThomX operation will produce high-energy electrons and other particles such as X-rays and neutrons
which generate ionizing radiations through their interactions with matter An adequate protection against these
radiations must be ensured for people working at the facility The safety logic adopted is the so-called
lsquopositive safetyrsquo which means that any system dysfunction will immediately cause a complete stop of the
accelerator Furthermore redundant safety devices will be implemented allowing various actions that will
guarantee the staff safety such as
- Shining the beam into an area shall be submitted to prior people evacuation and to the interlocking of this
arearsquos doorways
- Access to a sensitive area shall be submitted to the prior stop of the upstream beam by using at least two
different means independent of each other
Complementary to these two procedures safety measures will be applied involving hard-wired logic safety
PLCs shielding beam dumps shutters personnel exclusion interlocks radiation monitoring of the
experimentation areas staff dosimeters and radiation hazard training of the personnel In case of dysfunction
radiological monitoring hard-wired logic or safety PLCs will act through interlocks that will break
connections to the mains voltage (Pre-Amplifier modulator or electron gun depending on the situation) in
order to stop the accelerator
92 PLC safety
The safety hard-wired logic works in association with an automaton PLC system via two completely separate
circuits A Safety Programmable Logic Controller (PLC) is a special purpose machine which is used to
provide a critical control and safety applications to automaton users This controller is an integral part of
lsquosafety instrumented systemsrsquo (SIS) which are used to detect situations involving potentially dangerous
processes When such a situation occurs the SIS is programmed to bring the process to a safe state
automatically
A safety PLC is specifically designed to accomplish two important tasks (ensured by a diversitary redundant
architrecture)
- lsquodo not fail but if this cannot be avoidedrsquo
- lsquofail only in a predictable safe wayrsquo
Hard-wired logic is a safety circuit fully independent of automatons It takes in charge in particular the
access door interlocking feedback contacts which provide a large number of checks bearing on the magnetic
field the high voltage the pre-amplifier status etc and the running control panel
The PLC safety level must be as high as the one of a positive safety hard-wired logic It manages access doors
emergency stop buttons safety rounds and reports issue authorizations to operate from a command-control
system
lsquo
150
93 Radiation monitoring
Concerning the radiation protection the ThomX facility design satisfies the following principles
- No one will receive a dose of radiation in excess of the regulatory dose limits as a result of normal
operation
- Professionnal occupation exposures will be kept As Low As Reasonably Achievable (ALARA)
- All practical steps will be taken to prevent accidents with radiological consequences and to minimize the
radiological consequences of any accident
Arrangements will be made for the monitoring of working areas such as the Igloo the bunker and the Optical
hutch For this purpose several monitoring modes will be available such as thermo-luminescent dosimeters
measurements made by the qualified person in charge of radiation protection (Competent Radiological
Protection Person) and a modular system for continuous monitoring A continuous monitoring of gamma rays
and neutrons will provide radiation dose rates and will manage alarms its measured values will be transmitted
to a central data acquisition unit Placed in the Compton room a special treatment unit will also be
implemented that will provide a local display of the dose rate signage and will manage alarms
94 Radiation protection zoning
The D1 D2 and D4 halls are not classified lsquosupervised arearsquo therefore their access is free to people who
satisfy access requirements In contrast the Igloo hall is classified lsquosupervised arearsquo
The D1 hall contains the X-ray line and technical equipment
- The X-ray line is not a classified area with the exception of the Optical hutch
- The Optical hutch is alternately classified lsquoprohibited arearsquo and lsquonot classifiedrsquo area depending on the
permission conditions issued by functional elements important to safety (shutter position permanent
radiological level)
The D2 hall contains the ThomX control room and a room which contains the laser equipment
The D4 hall is devoted to tests and mechanical assembly
The Igloo hall contains the electron accelerator located inside a bunker The Igloo is classified lsquosupervised
arearsquo therefore only workers subject to individual dosimetry are admitted However in exceptional
circumstances public access might be authorized as long as regulations and procedures are respected
The Igloo bunker on the one hand and on the other hand the basement area devoted to the facility are
alternately lsquoprohibited arearsquo during the accelerator operation and respectively lsquosupervised arearsquo and lsquonot
classified arearsquo when the facility operation has stopped
The accelerator energy is high enough to expect nuclear reactions of photons and neutrons with matter
leading to the activation of structural materials This is why it is particularly important to use appropriate low
activation structural materials or structural materials that are activated for short periods of time only
Furthermore one must ensure a nuclear waste management which will require a specific room to store and
monitor activated materials This room should be a lsquosupervised arearsquo located in the Igloo
95 Access control
951 General statement
It is a matter of absolute necessity to have appropriate procedures and devices in place to identify and screen
all persons entering the ThomX facility Consequently the LAL access control should be extended to the D1
D2 and D4 halls and to the Igloo
lsquo
151
952 Particular requirements concerning the lsquorestricted areasrsquo
The validation of door contacts as well as the validation of the position of protective shields and real-time
radiation measurement are prerequisites to access the restricted areas (ie Optical hutch and the accelerator
area) As it applies to all functional elements important to safety signage and door statements (openedclosed
lockedunlocked) from classified areas must be transmitted to the control room Local indications are
displayed alternately with the status messages The measured values of the dose rate and alarm symbols are
shown permanently
953 Access to the accelerator area and to the Igloo basement
The accelerator and the storage ring will be located in a concrete enclosure that will provide an efficient
shielding to protect people against ionizing radiations inside the igloo in line with regulations Prior to the
validation the floor load-bearing capacity must be checked by experts
The concrete containment structure will have a removable roof made of staggered concrete slabs Civil
engineering will be needed for the anchorage and supports to be used to adapt the bunker concrete blockds to
the igloo circular wall which will be part of the enclosure and to make sure that the roof of the whole
lsquoprohibited arearsquo provides a complete shielding Since doors will be made of standard steel the bunker access
will be indirect ie through (short) mazes to ensure protection against ionizing radiations
Access to the bunker and to the basement area which is part of the ThomX facility will be determined by the
equipment that guarantees peoplersquos safety in accordance with the facility operation As mentioned above
these devices ensure redundancy by using wired logic safety PLCs and also special safety door locks which
are simple but effective devices A mechanical system called lsquotrapped key lockrsquo will be implemented the key
is trapped in the lock when the door is open and it can be recovered only once the door is properly locked
Turning on the beam in lsquoprohibited areasrsquo is allowed only if the safety systems have been unlocked which
implies that the collection of access keys is complete as will be checked by the control board To meet the
safety requirements new access doors must be installed and some existing doors must be modified
954 Access to the X-ray line and to the Compton room
The D1 hall will not be a lsquoclassified arearsquo and will be open to authorized people The X-ray line sections will
be in open access during normal operation with the exception of the Optical hutch To meet the radiation
protection requirements the Optical hutch partition and its roof will be made of thick metal sheets A thick
lead shield will be installed at the end of the X-ray beam axis
The Optical hutch will be alternately classified lsquoprohibited arearsquo and lsquono classifiedrsquo area (under the worst-case
conditions it might become a lsquoregulated arearsquo) depending on permission conditions issued by safety functional
items External and internal light-signaling warning will indicate the zoning status Thus access to the Optical
hutch is firstly subordinated to the removable shutter position and secondly to the local permanent
radiological measurement values The shutter is dimensioned to protect people from high radiation rates
effectively A fail-safe closer device triggers the shutter closing automatically in case of electrical power
failure too (compressed air power supply) When a door is open the shutter command is prohibited
A local radiation detector located in the Compton room will check he radiation exposure and the dose levels
down to as low as reasonably achievable values Measured high dose rates will automatically shut down the
accelerator and activate the evacuation alarm The validation of these safety devices is mandatory to grant
access permission
lsquo
152
96 Beam dumps
The ThomX facility will comprise tow beam dumps made of concrete that will contain high energy particles
They are very important to ensure an adequate protection of people from the ionizing radiations that would be
produced if they were not used One will be aligned with the linac axis and the other one will equip the
extraction area These shields will be installed as close as possible to the radiation source to absorb ndash or at
least to moderate ndash as much as possible of the produced radiations Their size and thickness will be
sufficiently large to reduce efficiently all secondary radiation (X-rays and neutrons) produced by the stopping
of the e- beam They will be made of a lined lead shield designed to moderate back-scattered radiation placed
in a cavity made of concrete blocks or high-density polyethylene Ne peut-on pas ecirctre plus preacutecis donner des
nombres sur les dimensions et preacuteciser les mateacuteriaux utiliseacutes avec une figure
97 Interlocks
Depending on the dysfunction origin the safety system will operate and break command circuits such as the
pre-amplifier trigger the modulator high voltage the electron gun trigger or its high voltage Feedback
information from the control systems may be gradually requested according to the anomaly
98 Accelerator emergency shutdown
Accelerator Emergency Shutdown (AES) are signal and alarm devices located in specific areas A sufficient
number of AES should be installed in prohibited zones namely the accelerator area the bunker basement and
the Optical hutch For the users to spot them easily AES are devices equipped with flashing lights which are
turned on whenever the local zone is closed
If people are still present in a dangerous zone when a machine run is about to start (ie when the access is
going to be forbidden after a short while) they should signal their presence by pushing an AES This action
prevents the accelerator operation The system reactivation must follow a specific procedure
99 Control console
Placed in the control room the control console collects all access keys The accelerator may be turned on if
and only if all keys are inserted in that console
The safety circuit validation (PLC safety hard wired logic) allows one to launch the accelerator running
procedure and triggers audible warning devices for 30 seconds before operation becomes possible
910 Evacuation of dangerous areas
A specific procedure must be followed for the evacuation of the bunker and its basement Operators should be
trained in methodical evacuations with the validation of all checkpoints (roundsman function) Non
compliance with the procedure will lead to the non validation of the facility operation
A sufficient number of roundsman functions should be located in the accelerator area Also an exit path
should be marked leading to those areas that provide a possibility to regress
911 Audio system and horns
Loud-speakers and horns will be installed in the Igloo basement (in the so-called lsquorotundarsquo) and pits and
inside the bunker The sound system must be of good quality speeches and broadcast messages must be
intelligible Horns must be loud enough 80 dBA at least
lsquo
153
912 Safety notices
Local notices Posters should be put up in lsquosupervisedrsquo and lsquocontrolled areasrsquo to remind people of the
regulation requirements Such posts maintain the workerrsquos awareness whenever they are in a radiological
environment Access doors to zones with multiple classifications should be equipped with highly visible
luminous warning panels Also measured radiation levels and messages could be flashed regularly Regarding
the radiation measured level in the Optical hutch an alarm symbol must be permanently displayed that
informs of this level
Signal boards at zone entrances and within the zones will specify the real-time safety conditions display
warnings (such as presence of live voltage ongoing operation mode etc) and the access permissions (such as
open access prohibited zone etc)
913 Radiological monitoring
The control of the radiological environment will comply with the ALARA principle It will be performed by
- using bimonthly exposure dosimeters based on radio-photo-luminescence (RPL) and neutron sensitive
dosimeters
- PCR controls with live measurements
- a centralized modular system which will continuously monitor exposures feeding a data acquisition system
that can handle up to 10 sensors
This centralized system will perform dose rate and integrated dose measurements manage alarms test
measurement channels check settings and calibrations It will allow a special device placed in the Optical
hutch to display in real time local safety information
The ALARA principle is a commitment to keep radiation exposures and doses at a level as low as reasonably
achievable (social and economic factors being taken into account)
lsquo
154
Chapter 10 MANAGEMENT OF THE THOMX PROJECT
Introduction
The management of the ThomX project will have a simple structure that takes into account the three
project phases 1) The construction 2) The exploitation 3) The technology transfer Three boards will
govern the ThomX activity covering all project aspects the machine design the purchasing and the
assembly the commissioning the usersrsquo access the communication and the relationships with the
industries This management board will comprise
1) A steering committee that will supervise the project activity and define the collaboration scientific
policy
2) A scientific committee composed of the scientific coordinators of the collaboration partners It will
assess the usersrsquo projects and prioritize the beam-time allocation This committee must also define the
scientific policy of the machine exploitation and RampD of the ThomX project
3) A partnership and valorisation office This body will evaluate the cultural and the economic
potential of the ThomX innovations It will stimulate transfer initiatives and check their outcome It will
work in close contact with the steering and the scientific committees
The composition of these three bodies is given in Fig 101
Figure 101 The ThomX project committees
In parallel the communication group (see Table 101) will be in charge of all activity concerning
communication and reach-out in the ThomX project It will organise events realise the publications
relating to the project status and progressrsquo and coordinate the official announcements of the
collaboration
LAL Orsay
Achille Stocchi
CELIA Bordeaux
Philippe Balcou
UMPC
Philippe Walter
Institut Neel
Alain Schuhl
INSERM Grenoble
Franccedilois Esteve
Synchrotron SOLEIL
Amor Nadji
ESRF
Jean Susini
THALES
Yves Trinquet
Alessandro Variola
LAL Orsay
Alberto Bravin
ESRF
Geraldine Le Duc
ESRF
Jean-Louis Hazemann
Institut Neel
Philippe Walter
UMPC
Alexandre Loulergue
Synchrotron SOLEIL
Francois Esteve
INSERM Grenoble
Eric Cormier
CELIA Bordeaux
Alessandro Variola
LAL Orsay
Pierre Bordet
Institut Neel
Philippe Deblay
Synchrotron SOLEIL
Yves Trinquet
THALES
Annabelle Alves
CNRS DR4
Edward Mitchell
ESRF
Fabian Zomer
LAL Orsay
lsquo
155
The project management is organized according to a matrix structure The ThomX facility has been
divided in five sub-systems Linac Ring and transfer lines X-ray line Laser and Fabry-Peacuterot cavity
There is a coordinator responsible for the design installation and operation of each sub-system All
these sub-systems involve various technologies which have been grouped in expertise domains
Mechanics and integration RF systems Magnets Diagnostics Power supplies and Pulsed elements
Vacuum Synchronisation Controls Beam Physics Alignment Cabling Nomenclature and
Infrastructures There is a coordinator for each one of these domains These coordinators will be in
charge of the activity harmonization within each technical field and of the maintenance during the
facility exploitation (or ldquoscientific phaserdquo)
The project leader his deputy and a project engineer are responsible for the project overall
coordination Fig 102 describes the project structure
Figure 102 The project structure
Project and Quality Engineer Technical
coordinator
AVermegraves Francois Wicek
LAL Orsay
Deputy Project leader
A Loulergue
Synchrotron SOLEIL
Project Leader
A Variola
LAL Orsay
Systems
LINAC RRouxLAL Orsay
Ring and Transfer Lines A Loulergue Synchrotron SOLEIL
X line M Jacquet LAL Orsay
Laser E Cormier CELIA Bordeaux
Fabry-Perot Cavity F Zomer LAL Orsay
Groups
Mechanics and Integration MLacroix LAL Orsay
Beam Physics ALoulergue Synchrotron SOLEIL
Alignment ALestrade Synchrotron SOLEIL
RF systems P Marchand Synchrotron SOLEIL
Magnets ME Couprie Synchrotron SOLEIL
Diagnostics L Cassinari Synchrotron SOLEIL
Infrastructures BPieyre MLanglet LAL Orsay
Control PGauron LAL Orsay
Vacuum C Preacutevost LAL Orsay
Power Supplies - Pulsed Magnets P Lebasque Synchrotron SOLEIL
Synchronisation N Delerue LAL Orsay
Cabling JN Cayla LAL Orsay
Nomenclature PCornebise LAL Orsay
lsquo
156
In the following Table the support structures are presented
Support Structures
Administration Communication Quality
Administration Brigitte Renard LAL
Orsay
Procurements Anngaeumllle Mardam
Beck LAL Orsay
EDMS Support Agnes Vermes LAL
Orsay
Secretary Chloe SevestreLAL Orsay
DR04 Support Edouard Dorion
Dominique Bony LAL Orsay
Marie-Pauline Gacoin
Synchrotron SOLEIL
Claus Habfast ESRF
Sandrine Agnes Vermes
LAL Orsay
Table 101
The ThomX collaboration needs an important support from all administrative services For this an
administrative cell has been set up which is headed by the person responsible for the administration the
purchasing office the project database management the secretariat and the contact with the regional
CNRS delegation support Last but not least a quality engineer plays a major role in the project
101 Partnersrsquo complementarity
The ThomX project requires a multidisciplinary environment given the complexity of the facility and of
the set-up operation Each partner contributes its competences to reach the final goal The ThomX
collaboration was organized so as to meet the requirements of a multicultural scientific environment
where different communities are called upon to share their know-how and their knowledge to develop a
common scientific program This was already the case ndash and still is the case ndash for the MightyLaser
project In this project LAL and CELIA teams have developed a fully integrated set-up composed of a
fibre laser and a high gain optical resonator which is presently installed at ATF (KEK Japan) There the
complementarity between the two teams was crucial LAL has a solid experience in developing optical
resonators in an accelerator environment while CELIA is among the leading laboratories in fibre
amplification technology This program aims at a progressive increase of the light beam power stored in
an optical cavity in order to maximize the Compton back-scattering rate The final goal of this RampD is
to demonstrate the feasibility of a γ factory based on Compton backscattering It started as a
demonstrator for Compton-based polarized positron sources needed for the next generation linear
colliders The interest shown by other scientific communities in this technology made it clear that a low
energy Compton source would be very useful in various research fields This conclusion led to the
launch of the ThomX collaboration
The ThomX CDR has already been published and the present TDR is about to be widely circulated For
the TDR writing it was mandatory to develop close connections between the various communities
lsquo
157
participating in the ThomX project in order to merge technological developments and usersrsquo
requirements in a single document All partners have contributed to the scientific case elaboration the
study of the facility feasibility and the definition of its main components
- LAL and SOLEIL contributed the description of the Compton backscattering characteristics and
carried out the study of the accelerators and the Fabry-Peacuterot resonator
- CELIA Bordeaux Lyon provided their expertise for the laser dimensioning
- THALES provided its experience in industrial components and sub-systems
- UMPC and ESRF enriched the scientific case
This task sharing results from the multidisciplinary approach to the ThomX project where half a dozen
institutes play an important role
The writing of this TDR provided a unique opportunity of collaborating for laboratories that belong to
different fields LAL specialized in fundamental high energy physics SOLEIL and CELIA in advanced
technology in laser and accelerators LC2RMF in social sciences and compact detectors ESRF in
medical sciences In the future the diversity of this set of teams will still increase with the participation
of the Neel Institute and the GIN-INSERM in Grenoble
After a prototyping phase ThomX will operate as a mini-synchrotron whose beam line can be accessed
not only by the facility partners but also by other communities Moreover the aim of the
industrialization phase is to provide the high technology scientific and industrial with compact and
reliable sources that can be operated with little or no accelerator and laser expertise This should open
the way to a large community of CBS source users that would benefit from the complementarity
between techniques developed at synchrotron sources and Compton back-scattering facilities Imaging
techniques biological tests on cells and animals metallurgy fast chemistry lithography are examples
of future possible applications of these unique instruments
102 ThomX larger framework
ThomX is one of the EQUIPEX projects submitted for approval to the French government by the P2IO
Consortium (Physique des deux Infinis et des Origines) of the Paris-Saclay Campus This Consortium is
composed of eight laboratories or institutes (CSNSM IAS IMNC IPNO IRFU LAL LLR LPT)
some of which are among the largest French laboratories All together their staffs comprise 2000
scientists engineers and technicians working in particle physics nuclear physics accelerator science
astroparticle physics astrophysics and their interfaces P2IO represents between 20 and 90 of the
national potential in the various sub-domains Although obviously not all of the eight laboratories
participate in each EQUIPEX project each project ndash including ThomX ndash is fully supported by P2IO and
will contribute to the very ambitious common technological platform that P2IO wants to create in the
coming years in the framework of the Paris-Saclay Campus initiative Once equipped with these
EQUIPEX facilities this campus will represent a unique opportunity for French researchers and
students
lsquo
158
References
11 Geoffrey A Krafft Gerd Priebe Compton Sources of Electromagnetic RadiationlowastReviews of Accelerator
Science and Technology Vol 3 (2010) 147ndash163
12 VBesson et al NIMA 608 (2009) S75-S77
13 T Akagi et al 2012 JINST 7 P01021
14 CDR ThomX LAL RT 0928 SOLEILSOU-RA-2678 editors AVariola FZomer ALoulergue
15 PRussbueldt et al Optics Express Vol 17 Issue 15 pp12230-12245 (2009)
16 IPupeza et al OPTICS LETTERS Vol 35 No 12 June 15 2010
17 M Y Shverdin et al 2009 OSAIPNRANLOSL 2009
18 CVaccarezza et al IPAC Conference proceedings New Orleans USA2012 TUOBB01
19 AVariola IPAC Conference proceedings San Sebastian Spain 2011 WEOAA01 1903-1905
21 S Doumlbert ldquoIntegration of the PHIN RF Gun into the CLIC Test Facilityrdquo Proceedings of EPAC 2006
Edinburgh Scotland 2006 MOPLS129
22 R Roux S Cavalier M Bernard G Bienvenu MJoreacuteBLeblondBMercier BMoutonCPrevost A
Variola ldquoPHIL a Test Beamline at LALrdquo Proceedings of EPAC08 Genoa Italy 2008 WEPP078
31 J Payet Beta Code CEA SACLAY
32 JB Murphy et al Longitudinal Wakefield for an Electron Moving on a Circular Orbit Vol 57
Part Acc 1997
33 K Bane M Sand The Short-Range Resistive Wall Wakefields SLAC-PUB 95-7074
34 E Forest Phys Tech Part Photon Beam
35 J Zang et al lsquoDipole fringe field effects in the Thom-X ringrsquo Proc IPAC13
36 R Nagaoka et al Recent Studies of Geometric impedance At SOLEIL PAC2006
37 Z Huang RD Ruth Phys Rev Lett 80976 (1998)
38 S K Mtingwa African Physical Review 2 (2008) 001
39 P Chen G Horton-Smith T Ohgaki AW Weidemann K Yokoya
NuclInstrumMethA355107-1101995
310 K Ohmi Phys Rev E 55 7550 (1997)
311 S Haifets D Teytelman Phys Rev Spec Top Acc and beams 8 064402 (2005)
312 E Weihreter Compact synchrotron light sources Series On Synchrotron Radiation Techniques
And Applications - Vol 3 World Scientific
313 P Lebasque et al Synchrotron SOLEIL lsquoEddy current septum magnets for booster injection and
extraction and storage ring injection at Synchrotron SOLEILrsquo EPAC 2006 Edinburgh Scotland
314 P Lebasque et al Synchrotron SOLEIL lsquoOptimisation of the coating thickness on the ceramic vacuum
chambers of SOLEIL storage ringrsquo EPAC 2006 Edinburgh Scotland
315 P Lebasque et al Synchrotron SOLEIL lsquoFour matched kicker systems for the SOLEIL storage ring
injection a full solid-state solution of pulsed power supplies working at high currentrsquo EPAC 2006
Edinburgh Scotland
316 F Marhauser E Weihreter (BESSY) ldquoImpedance Measurements of the HOM-Damped 500 MHz
Metrology Light Source Cavityrdquo BESSY Gmbh Germany 08 January 2007
317 M Langlois P Sanchez M Cornelis F Perez (ALBA) ldquoMeasurements on the RF cavity for the ALBA
storage ringrdquo Proceedings of EPAC08 Genoa Italy
318 R A Rimmer (Lawrence Bekeley Lab) et al ldquoDevelopment of a high-power RF cavity for the PEP-II B
factoryrdquo DE-AC03-76SF00098 (LBL)
lsquo
159
319 P Fernandes R Massarino R Parodi ATarditi (INFN Italy) A Massarotti (Sincrotrone Trieste Italy)
rdquoThe Design of the RF Cavities for ELETTRArdquo IEEE 1989
320 M Svandrlik A Fabris C Pasotti (Sincrotrone Trieste) rdquoImprovements in curing coupled bunch
instabilities at ELETTRA by mode shifting after the installation of the adjustable higher order mode frequency
shifter (HOMFS)rdquo IEEE 1998
321 W Bruns web site httpwwwgdfidlde
322 P Marchand et al Development of high RF power solid state amplifiers at SOLEIL Proceedings of the
IPAC2011 San Sebastian Spain p 376
323 C Pardine et al Status of the 476 MHz 50 kW solid state amplifier Proceedings of the IPAC2010
Kyoto Japan p 3972
41 YAG(CE) data sheet Saint Gobain Crystals httpwwwdetectorssaint-gobaincomYAG28Ce29aspx
42 SPI Suppliesreg YAGYAP Comparison Table
httpwww2spicomcatalogscintillsintillator_properties_comparison_tablehtml
43 K Nakamura et al (Particle Data Group) J Phys G 37 075021 (2010)
44 Wartski et al Detection of optical transition radiation and its application to beam diagnostics IEEE
TransNuclSci20544-5481973
45]Saturation of YAG Beam Profile Monitors Measured at ATF A Murokh J Rosenzweig V Yakimenko X
J Wang
46 Radiation Detection by Cerenkov Emission in Optical Fibers at TTF E Janata M Koumlrfer Tesla-Report
2000-27
47 CERN-2009-005 p273
48 CERN-2009-005 p272
51 J Bonis et al 2012 JINST 7 P01017 doi1010881748-0221701P01017
61 J Bonis et al Non-planar four-mirror optical cavity for high intensity g ray flux production by pulsed laser
beam Compton scattering off GeV-electrons JINST SISSA Vol 7 January 2012
62 R V Pound lsquolsquoElectronic Frequency Stabilization of Microwave Oscillatorsrsquorsquo Rev Sci Instrum 17 490ndash
505 (1946)
71 httpwwwjjxraydkhome
72 RW Alkire et al Nucl Inst and Meth A 350 (1994) 13-16
73 B Lengeler et al J Synchrotron Rad 6 (1999) 1153-1167
74 RL Owen et al J Synchrotron Rad 16 (2009) 143-151
75 MS Nogueira et al Radiation Protection Dosimetry 111 (2004) 105-110
76 M Krumrey et al AIP Conf Proc 705 (2004) 861-865
M Gerlach et al Metrologia 45 (2008) 577-585
77 httpwwwcanberracompdfProductsUltra-LEGe-SS-M3843pdf
78 K Sakaue et al Rev Sci Instrum 80 (2009) 123304
79 httpsaleshamamatsucomassetspdfparts_FF9892-31_-32_TMCP1039E03pdf
710 O Chubar et al Proc DIPAC (2001) CT09
711 D Paterson et al Opt Com 195 (2001) 79-84
712 M Mahesh RadioGraphics 21 (2001) 1033-1045
713 K Medjoubi et al JINST 6 C01080 (2011)
httpwwwvariancomusxrayproductsdigital_radiographypaxscan_productshtml
714 H Elleaume et al Physics in Medicine and Biology 47 (2002) 3369-3385
lsquo
160
715 Y Prezado et al Medical Physics 38 (2011) 1709
716 I Martiacutenez-Rovira and Y Prezadoa Medical Physics 38 (2011) 4430
717 L Samain Thesis Liege university (ULG) 2012
718 P Martinetto Thesis Joseph Fourier university (UJF) 2000
719 P Bleuet et al Analytical Chemistry 29 (2010) 518-527
720 P Bleuet et al Nature Materials 7 (2008) 468-472
81 httpwwwprofibuscomncdownloadspecifications-standardsdownloadsprofibus-standard-dp-
specificationdisplay
82 httpwwwtango-controlsorgDocumentstango-kernel
lsquo
161
Table of content
INTRODUCTION 2
1 The ThomX project 2
2 Scheme and options 3
3 General layout of the machine 4
4 Machine parameters 6
5 Summary of the TDR main technical points 8
Chapter 1 THE THOMX INTERACTION REGION 9
11 Electron beam and laser pulse characteristics in the interaction region 9
12 Expected flux and its characteristics 10
13 Position stability 11
14 Timing stability 12
15 Energy stability 13
Chapter 2 INJECTOR 14
21 Injector specifications 14
22 Technical description of the linac components 16
221 Photo‐injector 16
222 Solenoids 17
223 Laser 18
23 Acceleration section 20
231 The RF source 21
232 RF network 22
24 Beam dynamics 23
241 Nominal operation 23
242 Alternative operation modes 24
Chapter 3 TRANSFER LINE and RING 28
31 Transfer line 28
32 Extraction line 31
33 Storage ring 31
331 Optics and lattice 31
332 Non‐linear dynamics 35
333 Collective effects feedbacks 37
334 CBS and IBS effects 42
335 Scattering on the residual gas 46
336 Ion effects 47
337 Electron beam injection and extraction 48
338 Orbit corrections 50
341 Dipole magnetic design 53
342 Quadrupole magnetic design 54
343 Sextupole magnetic design 56
344 Corrector magnetic design 58
345 Injection dipole 59
lsquo
162
35 Power supplies 60
351 Pulsed magnets 60
36 Storage ring RF system 65
361 HOM impedance thresholds as regard to beam instabilities 66
362 Cures to the HOM induced instabilities 66
363 RF power source 70
364 Low level RF system and longitudinal feedback 71
365 RF equipment status 73
366 RF system conclusion 73
37 Transverse feedback 74
371 Specifications 74
372 Electronics 75
373 Kicker 76
38 Vacuum 77
381 Transfer line vacuum 77
382 Ring vacuum 78
Chapter 4 INJECTOR TRANSFER LINE AND RING DIAGNOSTICS 80
41 Charge measurement 80
411 Current transformers 80
412 Faraday cups 81
42 Transverse profile measurements 81
421 Diagnostics stations 81
43 Emittance measurement 84
44 Beam position measurement 85
441 Beam Position Monitors (BPM) 86
442 Orbit feedback system 90
443 Tune measurement and feedback 90
444 Energy measurement 90
45 Bunch length measurement 90
451 Cerenkov radiation 90
452 Synchrotron radiation 92
453 Transport of the Cerenkov and synchrotron radiations to the diagnostics hutch 93
454 Streak camera for the bunch length measurement 93
46 Bunch imaging using synchrotron radiation 94
47 Beam loss measurement 94
48 Beam dumps 94
Chapter 5 SYNCHRONIZATION 95
51 Overall scheme 95
52 Specifications 96
Chapter 6 OPTICAL SYSTEM DESIGN 97
61 Feedback system and results on a Fabry‐Perot cavity locking 97
611 Electron and FPC pulse synchronization 98
612 Laser parameters to be stabilized 99
lsquo
163
613 PDH Measurement technique 101
614 Actuators 103
615 The digital feedback system 107
62 High average power laser system and its coupling to a high gain FP cavity 110
621 The MightyLaser oscillator 111
623 Stretcher and compressor 115
624 ThomX development 115
63 Mechanical design 118
631 Optical cavity vacuum 123
Chapter 7 X‐RAY BEAM LINE AND EXPERIMENTS 124
Introduction 124
71 Available extracted X‐ray beam 124
72 Global view of the X‐ray lne 124
73 Chraracterization of the X‐ray beaugrave 126
731 Optical table 1 126
732 Beam transport 128
733 Optical table 2 128
74 Demonstration experiments 130
741 Experiments using the full X‐ry beam 130
742 Experiments using the central part of the beam 134
75 Summary 139
Chapter 8 CONTROL SYSTEM 140
Introduction 140
81 General principles 140
82 Technical considerations 141
83 The TANGO control system software 142
831 Software bus 142
832 A control system for accelerators 142
84 Control system architecture 143
841 Equipment to control 143
842 Inputsoutputs 144
843 Front‐end machine architecture PLCs and industrial PCs 145
85 IT infrastructure 146
Chapter 9 SAFETY AND RADIOPROTECTION 149
91 Introduction 149
92 PLC safety 149
93 Radiation monitoring 150
94 Radiation protection zoning 150
95 Access control 150
951 General statement 150
952 Particular requirements concerning the lsquorestricted areasrsquo 151
953 Access to the accelerator area and to the Igloo basement 151
954 Access to the X‐ray line and to the Compton room 151
lsquo
164
96 Beam dumps 152
97 Interlocks 152
98 Accelerator emergency shutdown 152
99 Control console 152
910 Evacuation of dangerous areas 152
911 Audio system and horns 152
912 Safety notices 153
913 Radiological monitoring 153
Chapter 10 MANAGEMENT OF THE THOMX PROJECT 154
Introduction 154
101 Partnersrsquo complementarity 156
102 ThomX larger framework 157
lsquo
3
electrons stored in the ring and the photons stored in the optical cavity These simulations lead to
precise predictions concerning the X-ray yield The last part of the TDR deals with the project structure
and its management Finally the updated cost estimate is presented
2 Scheme and options
X-ray sources based on Compton backscattering can be designed with various configurations
depending on the main performance goals (average flux peak flux brilliance monochromaticityhellip)
Once this choice is made one may envisage either a high or a low electron-photon collision frequency
In the first case the electron beam is recirculated in a storage ring or in an ERL in CW mode and
continuously interacts with the laser Another option is to operate directly a high duty cycle or CW
super-conducting Linac Correlatively the laser system has to provide a high average power in the CW
locked mode [15] In this case to amplify the laser pulse energy one may envisage the use of a passive
optical resonator with a high gain [16] If a high peak instantaneous flux in single bunch or multi-
bunch pulsed mode is required the laser system will be a high power laser with a low repetition
frequency Recirculation of the laser beam is possible for a few passages by means of passive or
regenerative cavities [17] As far as the accelerator technology is concerned the warm Linac
technology can be made to work in burst mode [18] This allows the production of short low emittance
bunches to increase the source brilliance Because of the Compton energy-angle correlation this
scheme can also provide an excellent performance as far as monochromaticity is concerned by using a
diaphragm which selects a fraction of the X-ray beam
The ThomX source is designed to maximize the average X-ray flux Another goal of this project is to
provide a compact reliable and tunable source which can be operated in hospitals or in museums in a
user-friendly way
These constraints imposed the choice of a high collision rate scheme and of the warm RF technology
Thus the ThomX accelerator system is based on a 50 Hz warm S band (2998 MHz) linac whose energy
is tunable up to 70 MeV an injection line and a compact electron storage ring whose RF cavity will be
warm and will operate at 500 MHz The ring revolution frequency is 178 MHz The laser is a fully
integrated fibre laser which provides a high average energy Its power is amplified by stacking the
pulses in a high gain four-mirror Fabry-Peacuterot resonator with a 30000 finesse The laser and the optical
cavity repetition frequency is twice the ring revolution frequency ensuring an electron-photon collision
every e- bunch revolution
The basic operating scheme of ThomX is illustrated in Fig 1 A 1 nC electron bunch is produced by an
RF gun and then accelerated up to the ring injection energy by an S band section A transport line
ensures the bunch transfer from the linac exit to the injection section of the storage ring After injection
the electron bunch is recirculated for 20 ms in the ring The ring optics and the RF peak value are such
that the beam size is very small (in all directions) at the interaction point (IP)
In parallel a high power fibre laser produces light pulses of ~ 14divide28 microJ energy at ~ 12 eV These
pulses are stacked in a four mirror optical resonator at twice the ring revolution frequency The
expected energy gain per pulse is 103 divide10
4 corresponding to a beam power of ~ 70divide700 kW in the
optical cavity
The synchronization system allows a near head-on collision (the crossing angle is two degrees) between
the electrons and the laser pulse every revolution of the electron bunch These collisions produce hard
X rays which are backward emitted in a cone whose angular opening is ~1γ (as a result of a relativistic
effect)
lsquo
4
Fig1 Schematic diagram of the ThomX source
At the end of a storage period of 20 ms the electron bunch is extracted from the ring and dumped in a
dedicated beam dump to avoid the background noise that would be produced by uncontrolled particle
losses At the same time a new lsquofresh beamrsquo is injected in the machine
3 General layout of the machine
As previously mentioned the ThomX design takes into account a strong constraint coming from the
goal of a small size machine Nevertheless this design allows for the possible integration of two
collision regions [19] Figure 2 shows the general layout of the machine The linac includes an electron
RF photogun (1) followed by a section dedicated to diagnostics (2) the acceleration structure (3) and a
quadrupole triplet (4) to allow for a three-gradient measurement in the direct line dump (5) where a
diagnostic chamber will be located The injection line consists of four 45 degree dipoles to steer the
beam four quadrupoles and two chambers (6 7) for energy spread measurements and beam
characterization before injection The injection is performed by a septum (8) and a fast kicker (9)
Another kicker (10) is inserted in the same line to provide together with the septum a fast extraction
Once extracted the beam is transported towards a dedicated beam dump (11)
lsquo
5
Figure 2 ThomX layout
The ring design ensures a four-fold symmetry based on a double bend achromat (DBA) optics It
includes 8 dipoles 24 quadrupoles and 12 sextupoles for chromatic corrections The bunch storage
time is too short (20 ms) and the beam energy is too low for synchrotron damping to play a role in
regard to the Compton back-scattering recoil effect nor in regard to collective effects such as CSR IBS
and ion instabilities For this reason an lsquoELETTRArsquo type RF cavity (12) will be used not to restore
energy but to bunch the electrons and to perform a longitudinal feedback Transverse position
diagnostics and feedback will be provided by a strip-line and by BPMs A synchrotron radiation
chamber will be used for transverse and longitudinal optical diagnostics The required ultra-vacuum
will be maintained by ion pumps distributed along the ring and by two dedicated pumps in the Fabry-
Peacuterot resonator
In the ring lattice two of the achromatic lines have been shortened by removing all the quadrupoles (see
Fig 3) so that the Fabry-Peacuterot cavity (13) can be easily integrated between two dipoles By minimizing
the beta functions it was possible to locate the collision region (14) in such a short section
Furthermore this design provides the possibility of implementing a second interaction region in the
future
lsquo
6
Fig 3 Layout of the interaction region and of the optical cavity integration
4 Machine parameters
The ThomX machine is composed of four main systems the injector the storage ring the laser and the
Fabry-Peacuterot resonator As mentioned previously we foresee some flexibility in the machine operation
so that the X-ray energy can be varied For this all four systems are dimensioned to withstand an
operating energy up to 70 MeV Nevertheless to characterize the machine a baseline configuration has
been chosen at 50 MeV In Tables 1 2 3 and 4 the parameter values which are given below correspond
to this baseline configuration
Table 1
Injector
Charge 1 nC
Laser wavelength and pulse energy 266 nm 100 microJ
Gun Q and Rs 14400 49 MWm
Gun accelerating gradient 80 MVm 5 MW
Normalized rms emittance 44 π mm mrad
Energy spread 04
Bunch length 43
lsquo
7
Table 2
Table 3
Laser and FP cavity
Laser wavelength 1030 nm
Laser and FP cavity Frep 3568 MHz
Laser power 50 ndash 100 W
Laser pulse energy 14 ndash 28 microJ
Fabry-Perot pulse energy 28 mJ
Fabry-Perot pulse length (rms) 5 ps
FP cavity finesseGain 3000-30000 1000- 10000
FP waist 70 microm
Power circulating in the FP cavity ~ 007 ndash 07 MW
Ring
Energy 50 MeV (70 MeV possible)
Circumference 168 m
Crossing-Angle (full) 2 degrees
βxy IP 01 m
εxy just after injection 5 10-8
m
Bunch length just after injection (rms) 4 ps
Bunch length at the end of a 20 ms storage
cycle
50 ps (rms)
Beam current 1784 mA
RF frequency 500 MHz
Transverselongitudinal damping time 1 s 05 s
RF Voltage 300 kV
Revolution frequency 1784 MHz
σx IP (just after injection) 70 microm
Tune xy 317 174
Momentum compaction factor αc 00136
InitialFinal relative energy spread (with IBS
and Compton back-scattering)
0406
lsquo
8
Table 4
Source
Photon energy cut-off 46 keV (50 MeV) 90 keV ( 70 MeV)
Total Flux 1011
-1013
photons
Bandwidth (with diaphragm) 1 - 10
Divergence 10 mrad (1γ) without diaphragm 50 MeV
5 Summary of the TDR main technical points
After the publication of the CDR much work has been put in simulations design and technical choices
The goal of this TDR is to provide a reference document in view of the machine construction as well as
a more precise planning and a more precise costing
The first chapter deals with the machine performances and the tolerances to be met so that the collisions
between the electron bunch and the laser pulse do take place in the conditions specified in the CDR
Then the main systems are described and our technical choices are presented This last section is
subdivided as follows
1 Linac
2 Beam transfer and injection section ring
3 Laser and Fabry-Peacuterot cavity
4 X-ray user line
A special attention is given to the integration of each system and to the global integration of the ThomX
machine The lsquoOptical systemrsquo chapter includes a short report on recent results obtained in the laser and
Fabry-Peacuterot cavity RampD program both in the laboratory and on a beam line at KEK-ATF (Japan)
Special chapters are dedicated to diagnostics the control of the main systems and their synchronization
safety and radioprotection The last chapter presents the project management
lsquo
9
Chapter 1 THE THOMX INTERACTION REGION
11 Electron beam and laser pulse characteristics in the interaction region
The ThomX injection frequency has been fixed at 50 Hz (20 ms period) by taking into account the
Compton scattering recoil effect which continuously deteriorates the beam longitudinal emittance This
effect is all the more important as synchrotron damping is strongly suppressed since the beam energy is
low Because of this lack of damping the Compton scattering recoil effect and eventual beam
instabilities must be controlled by longitudinal and transverse feedback systems
We stress the fact that in ThomX the beam does not reach a permanent state during a storage cycle its
properties are continuously varying To describe the Compton interaction dynamics we define a set of
nominal beam characteristics which are those prevailing just after the injection of a fresh e- bunch
Simulations have been carried out to characterize the X-rays emitted during a single crossing of the
electron bunch and the photon pulse (stored in the optical cavity) as a function of the collision
geometric parameters
These nominal collision parameters are as follows
Parameter value unit
Crossing angle 2 degree
Electrons energy 50 MeV
Electrons energy spread 03
Number of electrons 625 109
Fabry Perot pulse energy 28 mJ
Repetition rate 1784 MHz
Laser pulse transverse dimension (rms) 35 microm rms
Electron bunch transverse dimension (rms) 70 microm rms
Laser pulse duration (rms) 5 ps rms
Electron bunch length at injection 4 ps rms
Table 11 Nominal collision parameters
To extend these results to other geometric configurations one has to take into account the parametric
dependence of the Compton collision rate The X-ray flux is the product of the Compton cross-section
and the luminosity It is proportional to the electron bunch intensity and to the laser pulse power The
geometry dependence is given by the luminosity formula which involves the geometric 6D size and the
crossing angle Assuming Gaussian bunches and neglecting the hourglass effect the ring luminosity is
given by the following expression
Eq 11
L = NeNγ f cosφ
2π
1
σ ye
2 +σ yγ2 σ xγ
2 +σ xe
2( ) cos2φ + σ ze
2 +σ zγ2( ) sin2φ
lsquo
10
where σeγxyz represent respectively the two transverse dimensions and the longitudinal one of the
electron bunch and of the laser pulse f is the collision frequency Ne and Nγ are the number of
electrons per bunch and photons per pulse Φ is half the crossing angle
The X-ray energy cut-off is higher for head-on collisions and for photons which are exactly back-
scattered In this case the photon energy gain is 4 γ2
This calculation provides a useful reference to assess the efficiency of other configurations but it
remains an approximation since exact results must take into account the beam evolution during a
storage cycle and should be obtained from a full revolution by revolution simulation
12 Expected flux and its characteristics
In nominal conditions the expected flux is 78 1012
X-rayss whether one uses Eq 11 or the monte-
carlo simulation code CAIN Figure 11a shows the energy spectrum of the scattered photons and its
cut-off at 45 keV As shown in Fig 12b most photons are emitted very close to the interaction region
axis within a small cone with a 2 mrad opening angle They may be selected by using a small
diaphragm The energy spectrum of the X-rays emitted in a cone of 225 mrad opening angle is shown
in Fig 12a with a FWHM energy spread of 25 keV
Figure 11 Results of simulations performed with CAIN using the parameters listed above a)
Energy spectrum b) Transverse size of the source c) Angular emission distribution d) Horizontal (x-
xrsquo) phase space of the scattered X-rays
lsquo
11
The spot size at the source is dominated by the electron bunch of 7 10-5
m as illustrated in Fig 11b
The dimansions are assumed to be the same in the horizontal and vertical plane The total divergence
of the source is around 15 mrad given by the electron beam energy (ie the 1γ parameter) with an rms
size of 4 mrad (Fig 11c) The emittance of the photon source shown in Fig 11d is 03 mm mrad The
energy-angle correlation and the collimation effect are illustrated in Fig 12 As expected the high-
energy X-ray flux is peaked with a 1γ opening angle The diaphragm which is assumed for this
simulation selects emission angles and thus photon energies resulting in a ~ 10 monochromatic
photon beam
Figure 12 Results of simulations performed with CAIN using the parameters listed above
Left panel Energy spectrum of photons emitted in a cone with an opening angle of 225 mrad
Right panel Number of photonsmradsup2s versus energy and angle (the color bar indicates the photon
number)
13 Position stability
To optimize the X-ray flux a precise transverse alignment must be acieved in the interaction region It
can be performed either by steering the electron beam or by adjusting the optical table position These
procedures will be monitored by means of a double BPM which is integrated in the Fabry-Peacuterot cavity
vacuum chamber exactly at the interaction point The resolution of this technique is a few microns A
simulation has been performed to calculate the flux reduction resulting from a possible mismatch
between the electron bunch orbit and the laser pulse trajectory Figure 13 shows the number of X-rays
produced per bunch crossing and per keV as a function of a transverse displacement of the optical
cavity axis with respect to the electron orbit from zero to 140 microns
lsquo
12
Fig13 Number of back-scattered photons and their energy as a function of a transverse alignment
error This figure clearly shows that the losses entailed by a displacement comparable to the BPM
sensitivity are negligible The color bar indicates the number of X-rays produced at each crossing
14 Timing stability
A precise synchronization between the electron bunch and the laser pulse at the interaction point is
another important requirement A timing mismatch introduces a longitudinal shift between the two
bunches at the IP This can be due to an error in the location of the Fabry-Perot cavity with respect to
the e- storage ring or to a jitter in the synchronization system Fig 14 shows the variation of the X-ray
flux as a function of a longitudinal displacement
The flux is not so much reduced as long as the jitter is less than 10 ps This number fixes the
synchronization tolerance and thus the performance of the longitudinal feedback
Fig 14 Flux and energy spectrum of back-scattered photons as a function of the relative
synchronization of the beams (longitudinal displacement)
lsquo
13
15 Energy stability
The nominal ThomX injection energy is 50 MeV Phase rotation in the RF source or phase mismatch
will generate a jitter in the injection energy Such a jitter entails synchrotron oscillations which will be
damped by the longitudinal feedback However before this feedback has stabilized the longitudinal
oscillations the collision conditions will not be nominal This situation has been simulated the results
are illustrated in Fig15 One can see that it is in the higher part of the X-ray energy spectrum that the
loss is more important
Fig 15 Flux and spectrum of the back-scattered photons as a function of the e- energy
lsquo
14
Chapter 2 INJECTOR
The most important components of the linac injector are the electron gun and the acceleration section
which brings the beam to the desired energy Other components are the laser used for the e- gun the
magnetic elements used to transport the beam the diagnostics the vacuum system the waveguides and
the radio-frequency (RF) source Each component is detailed in the following First of all the injector
specifications are given since they lead to the choice of some of the linac components
21 Injector specifications
The project goal namely to produce a high flux of X-rays of energy 50 keV leads to the following
specifications for the linac
Energy 50 MeV
Charge 1 nC
Number of bunches per RF pulse 1
Emittance (rms normalised) lt5 mm mrad
Energy spread rms lt1
Bunch length rms lt5 ps
Average current 50 nA
Repetition frequency 50 Hz
Table 21 Nominal linac parameters
First of all one notices that the low average current 50 nA does not justify the use of cold technology
(ie of a superconductor) for the acceleration structure But there is still a large choice of possibilities
for an RF structure operating at room temperature For economic reasons and because of the long
experience accumulated in the field we have chosen an RF structure operating at 3 GHz This
frequency is a European standard
Furthermore the above specifications led us to choose a photo-injector for the electron source since a
low emittance beam is required Again to minimize risks we chose to build the same gun as the one we
constructed for the CLIC lsquoTest Facility 3rsquo at CERN [21] This gun has been running successfully for 4
years To increase the current per bunch we will use a metallic magnesium photocathode which can
deliver more than 1 nC with a laser pulse energy of a few tens of a microJ at a wavelength of 260 nm The
gun is made of 2 12 copper cells magnetically coupled to the waveguide To get 1 nC with an
emittance lower than 5 mmmrad an accelerating field of 80 MVm is required which means an RF
power of 5 MW in a 3 micros pulse The electron beam energy output by the gun is around 5 MeV
We will use an industrial laser to extract the electrons from the cathode by photoelectric effect The
main specifications of this laser are the following ones Wavelength 260 nm energy per pulse 100 microJ
pulse duration around 5 ps It must be synchronized with the RF frequency and triggered at 50 Hz This
laser will be installed in an optical hutch located outside the concrete shielding which will enclose the
accelerator Therefore its beam must be transported with dielectric mirrors and lenses over a distance of
about 20 meters
Concerning the acceleration section SOLEIL a partner of the project will lend us a lsquoLILrsquo section This
section was made by LAL for the pre-injector of LEP at CERN It has been kept as a spare for the
SOLEIL linac The ThomX project will provide an other section in case SOLEIL needs one The
section length is about 46 m To reach a final energy of 50 MeV the energy gain in the section must be
lsquo
15
45 MeV The latter value implies an electrical field of 125 MVm and an RF power of about 10 MW
However the accelerator should reach higher energies in order to produce X-rays beyond 50 keV Since
the maximum targeted X-ray energy is 90 keV the linac design should allow a beam energy of 70
MeV corresponding to an RF power of 20 MW Thus taking into account the RF system efficiency the
specification for the total RF power is set at 35 MW
This specification lead us to choose a klystron producing 35 MW in a 45 micros pulse To feed the
klystron a modulator is needed that provides high voltage pulses of typically 240 kV through a high
voltage transformer soaking in an oil tank The two technologies which can be used for this component
are discussed in detail below
For the electron beam steering two coils and two magnetic correctors are required The coils are placed
around the gun Opposite sign currents circulate in these coils (-310 A 336 A) so that the magnetic field
vanishes at the cathode location Its amplitude is 023 T roughly 7 cm downstream from the cathode
Concerning the magnetic horizontalvertical correctors once a good mechanical alignment is achieved
experience has shown that beam trajectory errors are not larger than a few mm between the gun and the
acceleration section Hence the correctors must produce a beam deflection of up to 10 mrad For the
model operating at LAL [22] two bipolar power supplies of 10 A per corrector are adequate
For the diagnostics three different instruments will be installed between the RF gun and the
acceleration section An Integated Current Transformer (ICT) will measure the charge per bunch
Beyond this device a beam position monitor (BPM) will measure the transverse positions (x y) of the
electron beam We will use a BPM based on striplines which was designed for SOLEIL The third item
comprises a vacuum chamber equipped with an arm which can be translated by compressed air in order
to introduce a YAG screen to intercept the beam The YAG crystal fluoresces when ionized by the e-
beam The emitted light shows the beam transverse shape A short optical transport line and a CCD
camera allow the measurement of the transverse dimensions LAL designed and constructed the
diagnostics chamber It has already been successfully tested in the PHIL facility A drawing showing
the linac layout is shown in Fig 21
lsquo
16
Figure 21 Linac layout In the upper figure the different components of the LINAC are illustrated
starting form the gun and subsequently passing trough the stripline the ICT for current measurements
the laser introduction chamber the diagnostics station the steerers and the accelerating section
22 Technical description of the linac components
221 Photo-injector
It is a copy of a gun built by LAL for the CLIC Test Facility 3 of CERN the so-called lsquoprobe beam
gunrsquo[22] Simulations have shown that the performances of this gun fulfill the ThomX specifications
We are thus benefiting from all the technical drawings which already exist and all the fabrication
processes are mastered by LAL from the machining to the brazing This RF gun is comprised of 2 12
cells The output cell is magnetically coupled and the other end is short-circuited The main parameters
are given in Table 22
Frequency 299855 MHz
Rs 50 MΩm
Q 14500
β 1
E 80 MVm
PRF 5 MW
Table 22 RF characteristics of the photo-injector at 30degC in vacuum
The photo-cathode will be made of magnesium It has the big advantage of producing the specified
charge without requiring an in-situ photo-cathode preparation chamber which is a heavy and expensive
lsquo
17
installation Preliminary tests performed at LAL have demonstrated the possibility to obtain good
quantum efficiency with such cathodes
The main difficulty expected in operating this device is the laser cleaning of the photocathode surface
when polluted by an exposure to air To perform this cleaning a laser intensity of 160 microJmm2 is
required together with an RF electric field of at least 50 MVm The magnesium disk holder is not
designed yet it could be press fitted screwed or brazed
222 Solenoids
The gun is equipped with two solenoids one is the focusing coil while the other is the bucking coil
which cancels the magnetic field at the photo-cathode location They have been modeled by usning
POISSON A schematic is shown in Fig 22
Figure 22 Schematic of the gun with its coils
Dimensions
For both solenoids the inner diameter is 160 mm and the outer one is 500 mm The copper part is 30
mm long for the bucking coil and 56 mm for the focusing coil
The conductor used for these magnets is a standard one with a 6x6 mm2 square shape It is cooled by
water circulating in a 4 mm diameter channel One can easily calculate the current the resistance the
dissipated power and the temperature rise assuming that the cooling water is supplied at 20degC and 8
bars Values are provided for each lsquopancakersquo since the conductors are wound by columns of two The
bucking coil Bbc has two pancakes and the focusing coil Bfoc four
Bbc Bfoc units
Number of coils 84 168 MHz
Number of pancakes 2 4
Current -274 326 A
P 23 33 kW
R 306 31 mΩ
Flow 164 164 lmn
ΔT +20 +28 degC
Table 23 Solenoid electric and cooling parameters lsquoNumber of coilsrsquo refers to one pancake
Two power supplies are required one for Bbc (400A 30 V) and one for Bfoc (400A 60 V)
lsquo
18
223 Laser
The laser extracts a bunch of electrons from the photo-cathode surface by photo-electric effect Table
24 specifies its characteristics
Parameter value unit
l 260‐270 nm
Emax 100 plusmn 05 microJ
Beam transverse quality M2 M2 lt 11
Gaussian Pulse width 5 ps
Transverse profile square
Pointing stability 10 of beam diameter
Oscillator frequency 74963750 externally triggered with jitter lt 1 ps MHz
Amplifier repletion rate 50 externally triggered Hz
Table 24 Laser specifications for the RF gun photocathode
The laser architecture is shown in Figure 23
Figure 23 Architecture of the laser proposed by industry
An oscillator provides a signal at the specified frequency It can be either a fiber or a TiSaphire laser
Then the pulse is stretched amplified (with one or two amplifiers) recompressed and converted in the
UV range
The laser will be installed on an optical table in a hutch whose temperature and hygrometry will be
regulated
224 Laser injection chamber
The laser injection chamber is located at a distance of 8011 mm downstream of the photo-cathode It
contains a metallic aluminum mirror placed 30 mm away from the accelerator axis It provides an 80
reflectivity The angle of incidence of the laser beam on the photocathode is 214deg This mirror is fixed
To adjust the laser beam position on the photo-cathode and its incident angle we will use a motorized
mirror with a remote control placed on an optical table just beside the laser injection chamber
The size of this optical table will be 1x1 m2 On it there will be
Oscillator Stretcher Preamplifier Amplifier Compressor
nJ microJ
mJ
Frequency
shifter
Clock
Syncro
Line syncro
Shaper
lsquo
19
- 2 dielectric mirrors
- 2 beam splitters
- 1 UV photo-diode to monitor the laser intensity
- 1 CCD camera to observe the laser spot on a screen (in paper or lanex) facing the photo-cathode
- An optical beam expander to adjust the laser spot size on the photo-cathode
This is the minimum number of optical items that will be put on the table some changes may be made
depending on the very final geometry of the accelerator and laser transport lines The set-up design can
be finalized during the last stage of the accelerator construction since it involves standard components
only
225 Beam steering coils
The e- beam trajectory will be adjusted by sets of two steering coils which produce dipolar magnetic
fields over short distances along the X and the Y axes One such device will be located at the gun exit
and the other about 800 mm downstream Only very small position and angle errors must be corrected
Figure 24 shows the LAL design for such a coil Experience with the PHIL photo-injector at LAL [22]
showed that an integral of Bmiddotdl of 52 G m is sufficient The latter figure has been validated by
calculations made by a private company With a current of 5 A a Bmiddotdl of 58 G m was obtained that
results in a 100 mrad deviation of a 5 MeV beam Thus a 100 mm lateral displacement of the beam can
be produced at the entrance of the acceleration section
Figure 24 Schematic of the beam steering coils Dimensions are in mm
All together the steering coils need 4 bipolar DC power supplies Their maximum current is 10 A
lsquo
20
23 Acceleration section
The acceleration section is shown in Fig 25 Its characteristics are given in Table 25
Figure 25 Picture of the SOLEIL LIL acceleration section
Length 45 m
Number of cells mode 135 2π3
Frequency 299855 at 30degC MHz
Rs 63divide74 MΩ
Q 14800
Attenuation 74 dB
Energy gain 15 MW 60 MeV
τ 135 micros
Ez max 182 MVm
Table 25 Characteristics of the acceleration section
In a second stage of the ThomX project a call of tender will be issued for a new acceleration section
that should be shorter in order to make the machine more compact
lsquo
21
231 The RF source
Figure 26 shows the RF source schematic
Figure 26 Schematic of the RF source
The RF timing and its synchronization are described in chapter 5 A clock signal is sent to the linac
which uses it to trigger a 3 GHz synthesizer Its output is fed to a preamplifier which brings its power to
350 W The preamplifier is triggered at 50 Hz Then the 3 GHz signal is amplified by a klystron whose
gain can reach 53 dB to produce a 35 MW RF wave as required by ThomX This power is distributed
via a waveguide network which is described below
To drive the klystron one needs a modulator delivering a pulsed high voltage (HV) whose
specifications are given in Table 26 The klystron cathode must be heated by a power supply that
provides a current rising slowly from 0 to 25 A and a voltage rising from 0 to 30 A in 20 minutes A
precision of 1 on the current value is required The klystron is generally equipped with focusing coils
and their power supply whose standard parameters are 40 A 120 V
Repetition rate 50 Hz
Plateau length 99 gt 3 micros
Plateau ripples lt plusmn 05
Pulse duration at 50 6 micros
Rise time 10 divide90 lt 05 micros
Fall time 90 divide10 lt 1 micros
Pulse to pulse stability lt 05
Voltage range 0divide300 kV
Peak current 285 kV 300 Hz
Table 26 Modulator specifications
The classical and well-proven modulator technology relies on the high voltage (40 kV) loading of a
delay line made of inductances and capacitances The delay line is switched by a thyratron A new
technology is presently emerging based on solid-state amplifiers Each amplifier produces a 100 V
lsquo
22
pulse Thus to produce a 40 kV pulse roughly 400 amplifiers are put in a rack and their output pulses
are added The performances of both technologies are similar
232 RF network
The RF power is transported and distributed to the electron gun and the acceleration section by a
waveguide network shown in Fig 27
Figure 27 Schematic of the RF waveguide network
The WR 284 waveguide network is pressurized at two bars with SF6 between ceramic windows at the
klystron output and at the acceleration structure entrance A 5 dB bidirectional coupler splits the power
in two channels In the branch which runs toward the electron gun there is a 10 dB attenuator and a
circulator to protect the klystron from the RF power reflected by the electron gun In the branch running
toward the acceleration structure there is a 10 dB attenuator and a phase shifter to adjust the relative
phase between the gun and the acceleration section Various RF loads (five in total) ensure the system
safety by absorbing of RF power of the reflected waves
lsquo
23
The network also includes three couplers to measure the incident and the reflected RF power One is
placed at the klystron power exit one is placed at the gun entrance and the last one is placed at the
acceleration section entrance The RF gun is equipped with a loop that measures the stored RF energy
All of these loops used for measurements are weakly coupled at -70 dB The signals are measured with
crystals or C boxes
24 Beam dynamics
241 Nominal operation
The beam dynamics was simulated with the PAMELA tracking code for the ThomX nominal
parameters ie a bunch charge of 1 nC at 50 MeV The 50 MeV can be obtained with an electric field
of 80 MVm in the RF gun and 125 MVm in the acceleration section It is clear that a different set of
field values can lead to the total of 50 MeV In particular one may wish to operate the gun at a higher
field in order to improve the beam quality We will operate the linac with this possible improvement in
view
The beam will be accelerated on crest Thus the coil magnetic fields are the only parameters savailable
to minimize the transveres and the longitudinal emittances Simulations have shown that for precise
magnetic field values (see Table 27) the e- beam emittances are optimized as illustrated in Fig 28
20
15
10
05
00
σxy
(m
m)
6005004003002001000
z (cm)
20
15
10
5
0
em
it (
mm
mra
d)
6005004003002001000
z (cm)
20
15
10
05
00
ΔE
E (
)
6005004003002001000
z (cm)
46
44
42
40
38
36
34
32
30
σz (p
s)
6005004003002001000z (cm)
lsquo
24
Figure 28 Beam characteristics (rms values) as a function of the distance along the accelerator (z
coordinate) Upper left panel Transverse size upper right panel normalized emittance lower left
panel Energy spread lower right panel Bunch length
The beam parameter values at the linac end are given in Table 27
Beam parameters value unit
Energy 50 MeV
Beam size 17 mm
Normalised Emittance 44 π mm mrad
Energy spread 04
Bunch length 43 ps
Table 27 Beam parameter rms values at the linac end
The accelerator parameters which should ensure the above beam characteristics are given in Table 28
RF structure parameters value unit
Ez in the gun 80 MVm
PRF in the gun 5 MW
Ez in the section 142 MVm
PRF in the section 14 MW
Solenoid parameters value unit
Bz max 2500 G
Ibc -274 A
Ifoc 326 A
Laser parameters value unit
Energy impinging on a Mg photocathode 10 microJ
Square transverse profile diameter 2 mm
Variation of the intensity from axis to border 14
Table 28 Summary of the linac parameters
242 Alternative operation modes
Concerning the injector other operation modes can be envisaged One at least will occur namely an
operation at 70 MeV in order to extend the X-ray energy range up to 90 keV Such an option has
attractive applications In this mode the electron beam performances should remain exactly the same as
the ones prevailing at 50 MeV (cf Table 27) For the accelerator parameters the only change is the RF
power required for the acceleration section it must reach 195 MW instead of 14 MW at 50 MeV
The operator can also be tempted to change the RF phase The one used in the simulations gives the
minimum emittance A phase shift of -10deg of the RF gun with respect to the laser could lead to an
energy spread reduction by a factor 2 at the expense of an emittance degradation Whatever the goals
the linac operator must be aware of the high sensitivity of the beam parameters to the RF phase This is
illustrated in Fig 29 An error of 5deg in the acceleration section phase results in almost a doubling of the
energy spread
lsquo
25
Figure 29 Energy spread at the linac end as a function of the RF phase in the acceleration section all
other parameters keeping their nominal values
The RF electrical field in the gun also plays an important role in the beam performances The results of
simulations of the emittance variation as a function of the gun electric field are presented in Fig 210 If
this field reaches 120 MVm the emittance is reduced by 20 with respect to the 80 MVm case
However the possibility to operate the gun at such a high gradient can only be assessed experimentally
Figure 210 Injector beam emittance as a function of the electric field in the electron gun (Gaussian
laser beam 1 nC e- bunches)
The injector performances as a function of the bunch charge are summarized in Table 29 Three modes
will be available in order to reduce the effect of collective instabilities at injection
Parameters Mode 1 Mode 2 Mode 3 Unit
Q 1 05 01 nC
σxy 165 15 08 mm
εxy 47 33 11 π mmmrad
σz 44 4 33 ps
σEE 04 03 02
Table 29 Linac beam performances (rms values) as a function of the bunch electric charge
As one can see in the above Table reducing the bunch charge by a factor 10 leads to an emittance
reduction by a factor 4 Although the X-ray flux is reduced the source brightness ie the ratio of the
flux to the divergence is improved by a factor 16 As a consequence a higher number of X-rays can be
20
18
16
14
12
10
08
06
04
02
δΕ
Ε (
)
151050-5-10
φ (deg)
10
9
8
7
6
5
ε (πmmmrad)
1401201008060
E0 (MVm)
lsquo
26
transported to a sample (number per unit area) Whatever the eventual benefits to the X-ray production
the advantage for the accelerator to operate at a lower charge per bunch is clear the reliability of the
accelerator is much enhanced the laser energy requirement is reduced and it becomes also possible to
use a copper photo-cathode instead of a magnesium one easier to handle But the main advantage lies
in the ring operation Indeed a 1 nC bunch whose length is only 4 ps induce a strong wakefield
stemming from the interaction between the beam and the vacuum pipe Such a wakefield perturbs the
beam and may cause oscillations of the bunch centroid possibly instabilities and thus a reduction of the
X-ray production rate
25 Photo-injector and acceleration section vacuum
Simulations of the photo-injector vacuum have been performed assuming a pressure of 3 10-8
Pa at the
cathode location and requiring a vacuum time constant (VS) that ensures an easy gun conditioning This
constraint must be respected to maintain a 10-7
Pa pressure at the acceleration section entrance where the
system will be baked to obtain an outgassing rate τ = 5x10-10 Pa m s-1
(N2)
Pressure in the line of injection
The use of a semi-conductor cathode (Cs2Te) puts severe constraints on the vacuum and requires a preparation
and a transfer chamber To avoid these complications some tests have been made on a metallic cathode (Mg)
The first attempt ran into difficulties (it took 12 h to reach a gradient of ~ 60 MV m-1
) we observed pressure
rises and sparks In presence of only the RF power we observed hydrogen CO and CH4 outgassing (as the
main components) A slight reduction in the quantity of water and oxygen was noticed due to chemical re-
composition When the laser beam was introduced to extract electrons a pressure increase by a factor of ten
was observed The 24th
25th
and the 26th
peaks also significantly increased for the HxCy compounds but no
Mg was measured
lsquo
27
Gas analyzes during the conditioning of cathode Mg
On the basis of simulations and experimental results we identified the need for the following items
‐ 2 ion pumps 40 l s-1
(upstream of the cathode amp injector) with their baking systems and power
supplies
‐ 2 ion pumps 55 l s-1
(for the RF network) with their baking systems and power supplies
‐ 1 ion pump 150 l s-1
(for the laser introduction chamber) with its baking system and power supply
‐ 1 ion pump 300 l s-1
(for the acceleration section) with its baking systems and power supply
‐ 1 gate valve manually actuated DN 100 (injector- upstream) and a baking valve
‐ 1 gate valve electro-pneumatically actuated DN 63 (injector - downstream) and a baking valve
‐ 1 gate valve electro-pneumatically actuated DN 40 (injector- downstream) and a baking valve
‐ 3 Pirani gauges (before the injector injector laser introduction acceleration section)
‐ 3 cold cathode gauges (after the injector injector laser introduction acceleration section)
‐ 2 power supply gauges
‐ 1 residual gas analyzer (RGA)
‐ 1 turbo pump (DN 63 ~ 80 l s-1
)
‐ 1 rotary vane pump (~ 12 m3 h
-1)
‐ 4 angle valves manually actuated for pre-pumping DN 63
‐ 2 T with RF grids
lsquo
28
Chapter 3 TRANSFER LINE and RING
31 Transfer line
The transfer line (TL) has to transport the beam from the linac to the ring and to match the optics of
these two devices In addition a set of beam diagnostics needed for an efficient injection in the ring are
localized along the line as sketched in Fig 31 The transfer line main characteristics are summarized in
Table 31
Figure 31 Layout of the injection and extraction transfer lines
A particular attention is paid to ensure a versatile optics that allows the matching of different linac
beams to different ring optics Simulations have been carried out using the BETA code [31] The
transfer line optics is optimized in order to accommodate some emittance degradation which may arise
from collective effects
Screen station + dump
Linac emit + pulse length diags
Screen + Scraper
Linac energy diags
Screen station
TL emit diags
Extraction line dump
lsquo
29
Figure 32 Transfer line optics including the ring The vertical line indicates the ring entrance point
The nominal transfer line optics is shown in Fig 32 from the linac to the ring together with the ring The
nominal input bunch Twiss parameters are x = z = 34 x = z = 42 for Q = 1 nC bunch length = 4
ps (rms) and an optimized normalized emittance of about 5 πmmmrad The strong bend produced by the four
dipoles of the transfer line increases the risk of emittance degradation under collective effects but simulations
show that the emittance increase is only of about 20 including coherent synchrotron radiation (CSR [2]) and
3D space charge effects (see Fig 33)
Figure 33 Emittance increase due to chromatic (cont lines) CSR (dashed) and space charge (dot dashed)
effects for 1 nC and 4 ps (rms) bunches at 50 MeV along the TL and the first revolution in the ring
The magnetic elements are identical to those of the ring
lsquo
30
Parameter value unit
Length 14 m
Nominal energy 50 MeV
Beta max x z 70 40 m
Maximum dispersion 1 m
Dipole numberfamily 4 1
Quadrupole number 7
Biplane corrector number 5
Table 31 Transfer line parameters and components
Besides ensuring the matching between the linac and the ring the transfer line must provide the measurement
of the beam emittances the beam energy and the energy spread Care has been taken to locate the diagnostic
stations in a way that ensures accurate measurements of these important parameters
The emittance measurements will be done by means of the lsquothree-gradientrsquo scheme (The pepper-pots
alternative is also foreseen) For instance with an adequate refocusing that produces a double waist (vertical
and horizontal) at the screen it is possible to scan both planes at the same time by varying only one
quadrupole gradient (see Fig 34) Other scan variations are also possible The following figures show the
optics retuning as well as the beam spot size variation at the screens of both stations the one at the linac end
and the one downstream of the U-turn
Figure 34 Optics retuning and beam spot size scan as observed on the screen (simulation)
The electron energy and its spread are measured in between the dipoles by a dedicated optics Note that it is
also possible to retrieve the bunch length by measuring the energy spread as a function of the linac phase
(three-phase scheme)
lsquo
31
32 Extraction line
To dump the residual beam at the end of the storage time (typically from 20 to 50 ms) an extraction line
composed of one small dipole and two quadrupoles is also foreseen This line will allow the measurement of
the beam properties after it has circulated in the ring The injectionextraction devices are symmetrically
implemented in the ring injection section
33 Storage ring
331 Optics and lattice
Thom-X must produce a high X-ray flux of 1011
divide1013
phs in a stable way The flux depends on the
characteristics of the electron bunch and of the laser pulse Assuming a 1 nC electron bunch charge at
50 MeV and a 20 to 30 mJ laser pulse ( 124 microm) the optics constraints at the interaction point (IP)
are as follows
- an rms size of the electron beam less than or equal to 70 microm in the transverse plane requiring
low beta functions of 01 m with an rms geometric emittance of 50 nmrad injected from the
linac (5 mmmrad normalized)
- a vanishing dispersion at the interaction point in order to avoid a beam size increase resulting
from an e- energy spread increase as well as to avoid potential synchro-betatron coupling
resonances
- a bunch length less than 20-30 ps to ensure a good overlap with the laser pulse (the crossing
angle will be 2deg) and long enough to reduce collective effects stemming from the pipe
impedance CSR bunch internal scattering etc
Besides these specific IP constraints there are two general layout constraints namely
- a ring circumference as short as possible with enough straight sections to accommodate the
interaction region the RF the injection and possible other equipment as RF and transverse
feedback systems
- the need for some flexibility as far as the optics and the working point are concerned
The ring layout and its optics are based on a two-fold symmetric lattice with four achromatic arcs and
four straight sections two short ones (02 m) and two long ones (16 m) This layout preserves the ring
compactness and provides an adequate interaction region in one of the two straight sections between
two adjacent dipoles (cf Fig 35) The main ring characteristics are summarized in Tables 32 and 33
lsquo
32
Figure 35 Ring layout
The laser pulse frequency is the harmonic 2 of the e- bunch revolution frequency in the ring ie 357
MHz This leaves open the possibility of storing two bunches in the ring in order to increase the
Compton flux but at present this operation mode is not taken into account because of injection
constraints The optical cavity mirrors are located outside the ring vacuum chamber thus enabling a
mirror substitution without affecting the ring In addition the extraction of the X-rays can be performed
closer to their source than in the case where the optical cavity is inserted in a long straight section
The two long free straight sections accommodate the injectionextraction devices for one and the RF
and feedbacks for the other
The ring is composed of 8 dipoles 24 quadrupoles and 12 sextupoles Its circumference is 168 m long
the revolution frequency is 1785 MHz The RF frequency is set at 500 MHz ie at the 28th
harmonic
of the ring revolution frequency The nominal optical functions have been optimized with the BETA
code [31] They are plotted in Fig 36 The dispersion vanishes in the long and in the short straight
sections (RF injection and interaction regions) The beta functions decrease down to 01 m at the IP to
provide a 70 microm focus assuming that the injected beam has an emittance of 50 nmrad (5 mmmrad
normalized) in both the vertical and the horizontal planes
RF and feedback section
Injection extraction section
IP 1
Interaction Point (IP)
lsquo
33
Ring parameters value unit
Circumference 1680 m
Nominal energy 50 MeV
Betatron tunes nx nz 317 174
Beta max xz 11 11 m
Dispersion max 08 m
Beta xz dispersion IP 01 01 0 m
Momentum Compaction Factor 0014
RF frequency 500 MHz
RF harmonic 28
RF voltage 300 kV
Synchrotron frequency 360 kHz
Period Revolution frequency 56 1784 nsMHz
Natural chromaticities -32 -82
Dipole nb family 8 1
Quadrupole nb families 24 6
Sextupole nb families 12 3
Corrector nb families 12 12
Table 32 Ring parameters
Max field 70 MeV Bore length Gap Diameter
Dipole 07 T 2765 mm
(R=352 mm)
40 mm
Quadrupole 5 Tm 150 mm 41 mm
Sextupole 15 Tm2 60 mm 42 mm
Correctors 510-4
Tm
Table 33 Main magnet characteristics
Taking into account the constraints imposed by the optical functions the chromaticity and the dynamic
aperture the operating point was chosen with tunes νx=317 and νz=174 There is still enough
flexibility to vary these tunes by plusmn 02 without spoiling the main ring performances The momentum
compaction factor can also be changed over a large range in order to scan the best working point
regarding the beam dynamics and the X-ray flux
Figure 36 Nominal beta functions and dispersion over one super-period
lsquo
34
The RF acceptance for the nominal momentum compaction factor (MCF) and a larger one including the
non-linear MCF and the vacuum pipe limit are plotted in Fig 37
Figure 37 RF acceptance Left panel nominal lattice Right panel for a MCF twice as large as the
nominal one VRF= 300 kV
RF acceptance nominal r56 = - 02 m r56 = - 04 m
Linear (RF bucket) 10 10
Non linear -32 +18 - 68 +4
Pipe limit 25 25
Table 34 RF acceptance for two different values of the momentum compaction factor
The two following plots (Fig 38) show (i) the vacuum chamber occupation by the beam at injection
when the emittance is 50 nmrad and the energy dispersion is 03 and (ii) at the end of the storage
cycle when the emittance reaches 300 nmrad and the energy dispersion reaches 06 (see section
438) The orbit components (misalignment errors) are not included since they are negligible once
corrected The choice of the pipe aperture (40 mm in H by 28 mm in V full aperture) takes into account
several constraints the beam pipe occupation the magnet bore radius the vacuum and the coherent
synchrotron shielding [32]
b a
lsquo
35
Figure 38 Transverse occupation (3 rms) of the vacuum chamber at injection (left) and at the end of
the storage cycle (right)
332 Non-linear dynamics
The natural horizontal and vertical chromaticities are respectively -32 and -68 Chromaticity
corrections are achieved by means of 12 sextupoles (3 families) located in the dispersive sections
Without performing a specific optimization the dynamic aperture is large enough about 50 times the
rms transverse size of the injected beam (see Fig 39) the quadrupole fringe fields being taken into
account Furthermore this dynamic aperture remains about the same for off-momentum particles (up to
plusmn3) compared to particles having the nominal energy The fringe field of the dipole magnets
introduces high order non-linear components Since the Thom-X dipoles are short with a large gap
their magnetic design (see section 34) has been carefully worked out in order to maintain the dynamic
aperture
Pole number 6 8 10 12 18 20 28 30
Inner dipoles +10 0 +3
Outer dipoles -16 -1 -6
Quadrupoles +10 +5 -5
Sextupoles +10 -10
Table 34 List of multipole tolerances for each kind of magnets expressed in 10-4
relative field variation
at 18 mm radial position
Figures 39 and 310 show the change of the dynamic aperture with and without considering the
multipole (the fringe field of the quadrupoles is also taken into account in these plots) These
Dynamical Apertures (DA) have been calculated with two different codes BETA and TRACYIII They
lsquo
36
both give very similar results In addition the TRACYIII code dedicated to long symplectic tracking
performs Frequency Map Analysis (FMA) giving more detailed information concerning the particle
diffusion process
Figure 39 On-momentum DA and FMA for the nominal ThomX lattice Beta code left and TracyIII
code (right) sextupoles only (top) and including magnet multipole tolerances (bottom)
lsquo
37
Figure 310 Off-momentum DA and FMA for the nominal ThomX parameters Beta code left and
TracyIII code (right) sextupoles only (top) and including magnet multipole tolerances (bottom)
333 Collective effects feedbacks
At low energy and thus low beam rigidity the electron bunch is sensitive to electromagnetic self-
perturbations that act revolution after revolution in the ring Several instability sources can be
identified
‐ Interaction with surrounding metallic items such as the resistive walls (RW) the RF cavity and
various discontinuities such as bellows BPMs and flanges HOMs etc
‐ Space charge forces
‐ Coherent synchrotron radiation (CSR)
These interactions affect the bunch energy and the bunch longitudinal time-profile leading to
distortions emittance growth bunch oscillations or even above some threshold instabilities and beam
losses Similar interactions may also affect the transverse beam dynamics but to a lower extent The
two relevant parameters for the instability threshold are the bunch charge and its length The strength of
the interaction is proportional to the bunch charge and varies inversely with the bunch length
The two plots in Fig 311 show the longitudinal wakefield over one revolution at 50 MeV generated by
a 1 nC Gaussian bunch with an rms length of 12 and 6 mm (4 and 20 ps) Five different contributions
are compared in this figure
‐ the shielded CSR following the Murphy and Skrinsky [32] formulation with a beam pipe full height
of 40 mm a dipole field of 05 T and a curvature of 0352 m
‐ the longitudinal space charge (LSC)
‐ the resistive wall (RW) following the K Banes and M Sands formulation [33] with a full beam
pipe height of 40 mm and an electrical conductivity of 14 106
‐ the vacuum vessel broadband component including 12 BPM 12 flanges and 12 bellows based on
the synchrotron SOLEIL type This potential has been calculated by means of the HFSS finite
elements code [34]
‐ the 500 MHz ELETTRA type cavity broadband component This potential has been calculated by
using the HFSS finite element code
They exhibit a very strong CSR wake field (up to few per mil) with the shorter bunch case
Figu
re
311
Lon
gitu
dina
l
wak
e
field
over
one
lsquo
38
turn generated by a 1 nC Gaussian bunch at 50 MeV for a bunch length of 4 ps (left panel) and 20 ps
(right panel)
The ThomX low energy operation (50 MeV) practically suppresses the natural damping effect of the
synchrotron radiation and complicates the bunch stabilization Each kind of perturbation grows
irreversibly implying that feedback systems in the three planes are essential to cope with the induced
oscillations of the bunch center-of-mass In addition to these collective effects the unavoidable
injection jitters have also to be taken into account To calculate these effects 6D tracking simulations
that include the longitudinal impedance as well as the large mismatch between the linac and the ring
optics in the longitudinal phase space have been performed (see Fig 312)
Large longitudinal oscillations
without feedback at injection
induced by collective effects
Longitudinal oscillations damped by the
RF phase feedback system operating at
F~360 kHz
Damping lt 50 micros
Figure 312 Tracking results (50 MeV
1 nC bunch) over the first 3000
revolutions just after injection without
and with the RF phase feedback system (VRF = 300 kV)
Large oscillations start very quickly after injection when the bunch is still short (4 ps) and are of the
same order of magnitude as the ones induced by the energy-phase jitter that affect the linac injection
This effect is the one to address in the design of the longitudinal feedback As far as the transverse plane
is concerned the growth times characterizing various instability sources are listed in Table 35 The
revolution-by-revolution feedback strength indicated for each instability source is the one needed to
suppress the oscillations
Source Type Growth time Revolution-by-
revolution kicker
strength
Beam pipe
geometry
TMCI
Head-Tail
-
160 micros
gt10 nrad
Resistive Wall
600 micros gt 2 nrad
Ions lt 100 micros gt20 nrad
Table 35 Transverse instability growth time and feedback kicker strength for a 1 nC bunch at 50 MeV
lsquo
39
In contrast with what prevails in the longitudinal phase space the main transverse oscillation source is
the injection position jitter Since the non-linear sextupole components are large this jitter may produce
a large emittance increase as a consequence of the phase space painting process (see Fig 313)
Figure 313 Sketch of the transverse phase space painting process
Simulations show that a very fast increase of the transverse emittance can result from steering errors at
injection in about 500 revolutions or 25 micros To overcome this effect a fast transverse feedback is
needed it has to damp the oscillation in about 100 revolutions or ~5 micros The strength of the revolution-
by-revolution feedback has to be on the order of 2 microrad This strength is far larger than the ones
estimated to cope with the transverse instabilities For this transverse feedback a dedicated strip-line
acting in both planes is foreseen in the RF section (see Fig 314)
Figure 314 Simulated transverse emittance increase induced by a 05 mm injection jitter with and
without feedback
After N
turns
lsquo
40
Phase space Specifications Device
Horizontal amp vertical Fr Frequency = 178 MHz (F0)
Strength = 2 microrad
Dedicated strip-line for
both planes
Longitudinal Frequency = 360 kHz (Fs)
Strength = 105 rads
Main RF phase loop
Table 36 Summary of the feedback specifications
Other collective effects such as the intra-beam scattering (IBS) which is important at low energy or the
Compton recoil effect play an important role during the 20 ms storage time Their dynamics differ from
an instability dynamics but they also increase the electron energy spread and the bunch emittance mdash
and thus reduce the X-ray flux
To summarize three different contributions to the e- bunch dynamics must be dealt with
‐ at injection jitters which may affect either the injected bunch energy or its trajectory or the precise
timing of its injection in the ring generate oscillations which may be large and which must be
quickly damped by feedbacks in all three dimensions (x y z)
‐ during a transition period corresponding to the first ~1000 revolutions strong collective effects
enhanced by a possible synchronization error between the linac and the ring may lead to a complex
turbulent regime in the longitudinal plane prior to ring matching (elliptical phase space density
profil)
‐ during the 20 ms storage period IBS and CBS produce a progressive increase of the bunch size in
all three directions This evolution cannot be stopped by a feedback system In contrast RF HOMs
and other perturbations may generate instabilities that should be controlled by feedbacks
To take into account collective effects a dedicated code has been developed including a collection of
wake field as well as non-linear dynamics It is based on a 6D multi-particle symplectic tracking
restricted to perfect hard edge drift quadrupoles and dipoles of finite length Other higher order
elements as sextupoles are treated in the thin length approximation The RF cavity is simply treated as
a longitudinal cosine kick The mapping through elements is given by the exact Hamiltonian solution
[34] for the drift sections and the dipoles (including their small curvature and dipole edge effects) The
quadrupole mapping is restricted to small radial angles It has been compared to various codes
(TRACYIII ELEGANT BETA and MADX) by performing simulations of the SOLEIL and the
ThomX rings with special care given to the dipole edge effects [35]
The transverse beam size at IP and the bunch length are plotted in Fig 315 over the first 10 000
revolutions or 056 ms with a 1 nC bunch charge This study shows that a small positive chromaticity in
the horizontal plane is necessary to stabilize the horizontal emittance During the very first revolutions
the tune dependent emittance behavior has not been fully investigated nor understood It appears that
this behavior follows from the bunch length mismatch between the linac and the ring The fact that the
linac produces very short bunches of 4-5 ps (rms) leads to a complex turbulent dynamics (see Fig
316) The effect of this mismatch disappears progressively over the first 1000 revolutions then the
bunch length reaches 20-25 ps (rms)
lsquo
41
Figure 315 Red RMS beam size and bunch length at IP Magenta bunch centroid position over the
first 10 000 revolutions (056 ms) for a 1 nC bunch The beam energy is 50 MeV
Figure 316 Turbulent dynamics in the longitudinal phase space that us taking place just at injection
These plots show the phase space density after 100 and 3000 turns
lsquo
42
334 CBS and IBS effects
The electron beam dynamics that prevails in a ring where Compton back-scattering is taking place is
similar to the one resulting from synchrotron radiation (SR) in a synchrotron light source CBS gives
rise to random excitation in the transverse and the longitudinal planes In the transverse plane the
emittance turns out to remain close to the one at injection [37] In contrast the CBS excitation strongly
affects the dynamics in the longitudinal phase space It leads to a substantial energy spread increase
over 50 ms storage time (see Fig 317a) resulting in a significant bunch lengthening
Based on the lattice parameters the calculation of the Touschek lifetime leads to a value on the order of
10 s this effect can thus be neglected
Figure 317 a) Energy spread increase and b) Radial (x xrsquo) relative emittance increase during a 100 ms
storage time Blue curves CBS effect only red curves IBS effect only black curves (almost on top of
the blue ones) CBS and IBS combined effect The IBS calculation is based on the Mtingwa formalism
[38]
In contrast the multiple Touschek effect (lsquoIBSrsquo) plays a significant role in the radial phase space (x xrsquo)
as shown in Fig 317b After 20 ms this horizontal emittance is increased by a factor ~ 4 from 50 to
200 nmrad and the relative energy spread by ~20 from 03 to 036 as can be seen in Fig 317a
Figure 317 shows that the Compton back-scattering process plays a role by far more important than the
IBS does It also shows that because of the large emittance increase produced by the CBS the bunch
density decreases by a large amount making the IBS much less effective (see Fig 317b where the
curve showing the combined effects lies below the curve illustrating the IBS effect by itself)
When the electron beam energy is increased up to 70 MeV the IBS effect on the horizontal emittance
becomes less important During a 20 ms storage cycle this effect is reduced by a factor 2 On the other
hand the energy spread degradation increases by more than a factor two compared to the 50 MeV case
This change is due to the higher energy of the back-scattered photons In contrast if the electron beam
energy is decreased down to 25 MeV the energy spread is reduced by a factor two while the horizontal
emittance increases by a factor seven compared to the 50 MeV case
Assuming an optimal overlap between the e- bunch and the laser pulse in the interaction region and
neglecting the e- bunch size variation across this region (the lsquohourglassrsquo effect) as well as the variation
of the laser beam envelope near its waist (characterized by the Rayleigh length) the X-ray flux is given
by the following formula (see Eq 11)
The nominal storage time is 20 ms only
lsquo
43
where σth is the Thomson cross section Nx is the number of photons per laser pulse f is the e- bunch
revolution frequency ϕ is half the collision angle σxyze σxyzγ are respectively the three dimensional
bunch rms sizes for the electron (e subscript) and for the laser pulse (γ subscript) In fact because of the
increase of the e- bunch size during the storage time the flux is reduced by roughly a factor two at the
end of a cycle as shown in Fig 318
Figure 318 X-ray flux as a function of the storage time These results take into account the energy
spread increase and the emittance degradation due to IBS only (blue curve) and to the combined CBS
and IBS effects (black curve)
To calculate more precisely the X-ray flux produced by Thom-X the code CAIN [39] (cf sect 12 above)
has been included in the previous 6D tracking code including the non linear effects of the lattice
magnets as well as collective effects The X-rays characteristics are computed at each ring revolution
The intra-beam scattering effect is calculated also at each e- bunch revolution based on the Bjorken and
Mtingwa formalism [38] To reduce the computing time the code extrapolates the IBS effect whenever
there is only a little change from one revolution to the next (this method reduces the computing time by
a factor 4) This code runs on linux OS Using the computer farm of the IN2P3 computing center in
Lyon it takes about 15 days to simulate a full Thom-X cycle (20 ms ie 400 000 revolutions) when the
e- bunch is modeled by 500 000 macro-particles
The way the total X-ray flux decreases during the first 10 000 revolutions is shown in Fig 319 This
decrease is due to the e-
beam quality degradation mainly due to the CBS effect Also shown in this
figure is the X-ray flux evolution as a function of the storage time for three different lower cut-offs in
the X-ray energy spectrum Figure 320 shows a typical angular distribution of the X-ray emission
Figure 321 plots the bunch longitudinal phase space for the first revolutions The macro-particles in
green are those which have been hit by photons at the last crossing while those in blue have been hit
before the others are in red
( ) ( ) φσ+σ+φσ+σσ+σ
φfNNσ=N
2
γzez
2
γxex
2
γyey
γeth
x22222
sincos
2πcos
lsquo
44
Cross comparison and benchmarking of this code against other codes developed for ThomX (6D
tracking without Compton effect or Compton + IBS without tracking) are in progress and indicate a
good agreement between the codes
Figure 319 Left panel Number of X-rays produced as a function of the X-ray energy at the end of the first
10 000 first revolutions (in red) and during the last 10 000 revolutions before extraction (in blue) Right
panel Number of X-rays produced summed over 2500 revolutions as a function of the revolution number
Figure 320 X-ray intensity as a function of the emission angle in mrad (horizontal axis) and their
energy in eV (vertical axis) The color bar indicates the number of X-rays
lsquo
45
Figure 321 Upper nine panels Evolution of the e- beam longitudinal phase-space after injection The macro-
particle positions are plotted along the horizontal axis while their energy deviations are plotted along the
vertical one Green electrons which have collided with photons at the last crossing blue electrons which
have collided before red electrons which have not yet undergone any collision These plots reveal a turbulent
dynamics Lower nine panels Time evolution of the beam longitudinal phase space as a result of a turbulent
regime from the initial lsquobanana shapersquo at injection to a Gaussian distribution after a few ms (colors indicate
the electron number-density red corresponds to the most populated region)
lsquo
46
Figure 322 Evolution of the X-ray energy spectrum when the e- beam energy is increased from 475 to 505
MeV Left panel The X-ray energy spectrum is plotted vertically for seven e- beam energies Colors
characterize the X-ray intensity Right panel Each curve shows the total number of X-rays above some lower
energy cut-off Green cut-off set at 20 KeV blue cut-off at 30 KeV red cut-off at 40 KeV
335 Scattering on the residual gas
Coulomb scattering of the electrons on the residual gas molecules induces an increase of the circulating
beam emittance The amplitude of this effect is proportional to the average of the beta functions and to
the gas partial pressure Moreover it varies as the square of the atomic number of the residual gas
atoms and is inversely proportional to the electron energy With a low beam energy of 50 MeV it is
therefore essential to keep a low pressure to minimize the time-averaged emittance Fig 323 shows the
growth of the transverse emittance during the 20 ms storage time in the nitrogen case and in the
hydrogen one Assuming that one deals with nitrogen at a pressure of 8 10-7
Pa the emittance increases
by a factor of two (see Fig 323) while the degradation is negligible in the dihydrogen case We
conclude that a pressure lower than 3 10-7
Pa must be maintained to keep the emittance growth at an
acceptable level
Figure 323 Emittance increase due to Coulomb scattering assuming that the residual gas is composed
of a) 100 nitrogen b) 100 dihydrogen
lsquo
47
336 Ion effects
The beam-ion instability [310 39 312] originates in the ionization of the residual gas molecules Ions
can subsequently be trapped by the electron beam electrostatic potential We recall that the on-axis
potential is inversely proportional to the beam size
When the trapped ion number density reaches some threshold beam instability is triggered Besides this
effect trapped ions damage the beam quality by shifting the tunes increassing the emittance and
lowering the beam lifetime since the local pressure is increased
For the Thom-X ring the critical mass above which ions are trapped is on the order of one in units of
the proton mass Thus all ions whatever their mass can be trapped in our case
Once trapped ions oscillate transversally in the potential well Their oscillation frequency depends on
the electron beam size and on the ion species (see Fig 324 1c) Because of the e- beam bunching
when the ion oscillation frequency becomes larger than the beam betatron frequency a lsquofast ion
instabilityrsquo may develop the e- bunch is submitted to the electric field of the oscillating ions eventually
resulting in an exponential growth of the oscillations of all particles (electrons and ions) This instability
arises above some value of the neutralization factor which depends on the lattice
The neutralization factor and the growth time of either the vertical or the horizontal oscillations are
shown in Fig 324 as a function of the horizontal beam size for several ion species For a horizontal
beam size averaged over the ring of 05 to 1 mm (which is about what is expected) the growth time is
very short (less than one micros) but for this would happen for a neutralization factor much larger than one
obviously this is excluded To keep under control the other ion harmful effects mentioned above one
may prevent ion accumulation over many storage cycles by introducing gaps between short series of
injections or by clearing the ions by an electrostatic potential that cancels out the potential well
produced by the e- beam A simple solution consists in polarizing the beam position monitors (BPMs)
thus avoiding the insertion of electrodes along the ring This scheme should be effective since ions will
drift longitudinally as the kicks they receive at each e- bunch revolution has a longitudinal component
The amplitude of these kicks along the z direction is shown in Fig 325 where the BPMs are
represented by circles They are placed near accumulation points where the longitudinal kick vanishes
ie where ions will converge A difficulty may arise at s =125 m This point is opposite to the IP in the
lattice at this point the e- beam has small transverse dimensions and a vacuum pump is placed Indeed
with a small beam size the neutralization factor threshold is very low (10-3
) and the growth time
constant is on the order of one micros This potential difficulty can be overcome by adding an electrode in
the short section between the two dipoles to clear the ions at this accumulation point
Ion clearing with polarized BPMs or gaps introduced in the injection sequence should keep other ion
effects at an acceptable level (more work is needed to quantify this statement)
Defined as the total number of ions divided by the total number of electrons
lsquo
48
Figure 324 a) Threshold neutralization factor b) growth time and c) ion oscillation frequency for the
following fractional becirctatron tunes 1) 064 and 2) 017 for different ion species The dashed lines show
the e- bunch betatron frequency σx is the mean beam size
Figure 325 Longitudinal kick given to on-axis ions at each e- bunch revolution
337 Electron beam injection and extraction
Originally the linac beam was injected in the ring by using only a septum followed by a fast kicker
Given the high repetition rate (50 Hz) radiation safety issues and a possible noise in the X-ray
experimentation area lead to set up an e- beam extraction line The symmetric septum magnet located in
lsquo
49
the middle of one of the long straight sections is used in a double pass mode for injection and
extraction It is flanked by two identical kickers one downstream for the injection and one upstream for
the extraction (Fig 326)
Figure 326 Sketch (top panel) implementation (middle panel) and simulation (lower panel) of the
injectionextraction process
The main characteristics of the septum and the kicker magnets are listed in Table 37 Since in both
processes the beam runs through quadrupoles some margins with respect to the nominal deflection
values must be provided to cope with the fact that quadrupole fields are not uniform
Kicker Septum
Deflection 15 mrad max (11 nominal) 170 mrad (40 mTm 70 MeV)
Pulse duration lt 56 ns half sine 130 micros full sine
Effective length 200 mm
250 mm
Overall length 450 mm 500 mm
Table 37 Main characteristics of the pulsed devices
Septum center
x =14 mm
xp=9 mrad + 170 mrad
Kicker center
x =0 mm
xp=0+11 mrad
2 fast kickers
Inj Ext
1 slow septum
lsquo
50
338 Orbit corrections
A set of 12 beam position monitors (BPMs) and 12 correctors in both planes are implemented along the
lattice in order to control and correct the orbit An additional BPM located at the IP is fixed on the
table that supports the laser cavity The main error sources are listed below Uncorrected and corrected
orbits are illustrated in Fig 327
rms value dBB dL dX dZ dS tilt
Dipoles 10-4 01 mm 02 mm 01 mm 02 mrad
Quadrupoles - - 02 mm - -
Dipoles 10-4 01 mm 02 mm 01 mm 02 mrad
Quadrupoles - - 02 mm - -
Table 38 Main magnet misalignment errors
Uncorrected orbit with typical magnet alignment errors
rms value xmax (mm) zmax (mm)
Max 7 3
IP 05 03
rms value xʹmax (mrad) zʹmax (mrad)
Max 12 4
IP 6 3
lsquo
51
Corrected orbit
rms value xmax (microm) zmax (microm)
Max 200 140
IP 200 30
rms value xʹmax (microrad) zʹmax (microrad)
Max 600 300
IP 350 60
Figure 327 Uncorrected and corrected closed orbits Red lines horizontal projection blue lines
vertical projection
Quadrupoles and sextupoles are fixed on girders with a maximum error of 50 microm The magnetic field
center error relative to the magnet yoke being of the same order the expected total alignment errors are
~100 microm in each plane The girders supporting all the elements will be aligned as solid blocks Since
lsquo
52
dipole misalignments have a large effect on the orbit dedicated additional support tools will allow the
readjusting of the dipole positions relative to the girder
One needs a maximum corrector strength of 05 mrad when this strength is computed by assuming just
one sigma alignment errors to cope with alignment errors which would be twice as large corrector
strengths of 1 mrad are thus necessary
Figure 328 Orbit bumps (red horizontal blue vertical) that can be produced at the IP
In addition to this standard orbit correction procedure a dedicated orbit scan at the IP will be very
useful to ensure an optimal overlap between the e- bunch and the laser pulse in the interaction region
(see Fig 328) To provide a scan range of up to 1 mm or 1 mrad the corrector strength must be further
increased by 1 mrad Thus the corrector maximum strength will be 2 mrad
The orbit correction will be performed by a slow feedback operating around 01 The correctors are in
the sextupoles magnets (combined magets) The orbit feedback correction will also control the RF
frequency in order to keep the orbit length at the ring design value A specific synchronization scheme
that ensures the proper injection timing and that maximizes the electron-photon collision rate is
described in chapter 5
Position amplitude scan at IP location up to 1 mm
Angle amplitude scan at the IP
location up to 1 mrad
lsquo
53
34 Magnets
Table 39 describes the magnets that will equip the transfer line the ring and the extraction line
Magnet type Dipoles Dipoles for inj Quad Sext Corr
Number 4+8+2 2 7+24+2 12 4+12+1
PS number 3 2 33 3 34
Strength 0180 T m 5 Tm 40 Tmsup2 05 mTm
Length 2765 mm 150 mm 60 mm 30 mm
Table 39 ThomX magnet parameters
341 Dipole magnetic design
The dipole bending angle is 45deg A magnetic field of 05 T is needed at 50 MeV Since their radius of
curvature is very small (0352 m) special attention has been paid to the field profile between the magnet poles
and to the pole edge shape where large multipole components are present Lateral shims and pole edge
specific profiles have been optimized with the TOSCA3D code to minimize the impact of the multipoles on
the beam dynamics
Characteristics unit value
Gap mm 40
Field at centre T 063
Overall dimension mm3 350 x 400 x 400
Current A 263
Voltage V 177
Pole width mm 100
Pole height mm 100
Yoke length mm 2765
NI Aturns 13000
Coil area mmsup2 67x40
Current density Ammsup2 116
Number of turns turns 48
Transverse size of the conductor mmsup2 6x6
Cooling channel mm 4
Water pressure drop bar 7
Temperature increase degC 47
Flow rate (per dipole) lmn 86
Electric power (per dipole) W 3100
Table 310 ThomX dipole parameters
lsquo
54
Figure 329 Drawing of a ThomX dipole
342 Quadrupole magnetic design
The quadrupole design (see Fig 330) was also performed with the TOSCA software The magnetic field
is produced by four coils carrying a DC current The conductor is wrapped around four poles which
concentrate and define the quadrupole magnetic field map
Figure 330 Quadrupole magnetic design
A quadrupole can be manufactured in four identical parts each one comprising one pole and one quarter
of the yoke or the yoke and the four poles can be manufactured separately The quadrupole will be
equipped with special shims which will be adjusted during the magnetic measurement to ensure a good
alignment of the magnetic axis and low harmonic components The ThomX quadrupoles are air-cooled
to reduce the infrastructure cost This is possible since the maximum current density is 15 Ammsup2
Figure 331 shows the magnetic field intensity versus the radial distance from the quadrupole axis The
maximum gradient is 5 Tm Figure 332 shows the magnetic field intensity at a distance of -20 mm
from the magnet axis versus the longitudinal coordinate z The maximum magnetic field is 01 T
lsquo
55
Figure 331 Magnetic field (in T) versus the distance (in mm to the magnet axis
Figure 332 Magnetic field intensity (in T) versus z (the longitudinal coordinate in mm)
lsquo
56
The main parameters of the ThomX quadrupoles are given in Table 311 Each quadrupole is powered
by a 10 A 32 V DC power supply thus the released heat power remains below 32 W A dedicated
magnetic bench equipped with a rotating coil will be used for the quadrupole magnetic measurements
This bench has been developed for the magnetic measurements of the SOLEIL storage ring
quadrupoles
Parameters unit value
Bore mm 41
Gradient Tm 5
Current density Ammsup2 15
Overall dimensions cm3 19x19x18
Current A 10
Voltage V 35
NI Aturns 810
Pole width mm 20
Pole height mm 50
Coil dimensions mmsup2 15x36
Number of turns turns 81
Wire dimensions mmsup2 5x125
Yoke length mm 150
Yoke thickness mm 20
Chamfer mm 5 (not optimised yet)
Table 311 Main parameters of the ThomX quadrupoles
343 Sextupole magnetic design
The ThomX sextupole design (see Fig 333) was also performed with the TOSCA software The magnetic
field is produced by six coils that carry a DC current The coils are wrapped around six iron poles which
concentrate the field and define the sextupole field map These magnets are combined function and are also
equipped with additional coils to produce the corrector dipole fields
Figure 333 Sextupole magnetic design
lsquo
57
Each sextupole yoke is manufactured as a single piece and the six poles are screwed on this yoke Then a coil
is wound around each pole During the design phase a chamfer was calculated with TOSCA to guaranty a low
harmonic content The sextupoles will be air-cooled to reduce the infrastructure cost The maximum current
density is 15 Ammsup2
The magnetic field intensity versus the distance to the magnet axis is shown in Fig 334 The maximum
sextupolar strength is 40 Tmsup2
Figure 334 The ThomX sextupole magnetic field versus the distance to the magnet axis
Fig 335 shows the variation of the magnetic field at a distance of -20 mm from the sextupole axis versus the
longitudinal coordinate z The maximum magnetic field is 0016 T
Figure 335 ThomX sextupole magnetic field versus z (the longitudinal coordinate)
Table 312 gives the main parameters of the ThomX sextupoles They will be powered by a 10 A 05 V DC
power supply
lsquo
58
Parameters unit value
Bore diameter mm 42
Sextupolar Strenght Tm2 40
Field 20 mm T 0016
Current density Amm2 25
Overall dimension cm3 19x19x8
Current A 10
Tension V 05
AmpereTurn At 100
Pole width mm 20
Pole height mm 40
Coil dimension mm2 6x40
Number of turns Tours 10
Conductor dimension mm2 2x2
Yoke length mm 60
Yoke thickness mm 20
Chamfer mm 15
Table 312 Main characteristics of the ThomX sextupoles
A dedicated bench equipped with a rotating coil will be used for the magnetic measurements of the sextupoles
This bench has been developed for the magnetic measurements of the SOLEIL storage ring sextupoles
344 Corrector magnetic design
Simple window frame correctors based on an already existing SOLEIL design will be used for the orbit
corrections of the two transfer line
Figure 336 Magnetic design of the window frame correctors
lsquo
59
345 Injection dipole
The two injectionextraction dipoles compensate the septum deflection with a 170 mrad deflection (cf sect 337
see Fig 326) They are also air-cooled Their design and their technical characteristics are illustrated in Fig
336 and Table 313
Figure 336 Magnetic design of the injection dipole
Parameters unit value
Gap mm 40
Field at the magnet centre T 0224
Current A
10
Voltage V 6
Pole width mm 60
Pole heigth mm 70
Yoke length mm 150
AI (per coil) At 3600
Coil cross section area mmsup2 60x60
Current density Ammsup2 09
Number of turns tours 360
Conductor size mmsup2 10 x 1
Pressure drop of the cooling water bar NC
Temperature increase degC NC
Power (per dipole) W 120
Table 313 Parameters of the injection and extraction dipoles
lsquo
60
35 Power supplies
In order to provide a high operation flexibility ThomX will be equipped with the following power supplies
ndash One power supply for the ring dipoles (the latter will be connected in series) one for the TL dipoles
and one for the DL
ndash Individual power supplies for all the other magnets (quadrupoles sextupoles and correctors)
All the power supplies will have a 16 bit-resolution and a plusmn20 ppm reproducibilitystability over 24 hours The
technology is a switched-mode one in order to achieve good efficiencies (gt 85)
Magnet Nb PS Nb mag current
(A)
voltage
(V)
type load
R
(mOhm)
L
(mH)
Dip ring 1 8 300 170 Unipol 565 48
Dip TL 1 4 300 85 Unipol 280 24
Dip DL 1 2 300 45 Unipol 150 12
Dip inj 2 1 12 18 Unipol 1330 180
Quad 33 1 12 6 Unipol 500 10
Sext 12 1 12 3 Unipol 250 08
Corr ring 24 1 plusmn12 plusmn3 Bipol 250 08
Corr TL DL 10 1 plusmn12 plusmn3 Bipol 250 08
Table 314 Overview of the ThomX power supply equipment
351 Pulsed magnets
3511 Layout
As discussed above the e- beam injection in the storage ring and its extraction will take place in the
same straight section (see Fig 337)
Figure 337 Injectionextraction section layout
Eddy current septum magnet
(under vacuum) Fast Kicker Fast Kicker
lsquo
61
The first injection step involves a thin septum magnet It bends the incoming bunch trajectory to bring it
inside the ring vacuum chamber close to the stored beam orbit Then a fast kicker compensates for the
residual trajectory angle and brings the injected bunch on-axis
Extraction is realized by another fast kicker followed by the same septum magnet as the one used for
injection This kicker deflects the bunch trajectory so that it enters into the septum channel in turn the
thin septum bends the bunch trajectory which enters then into the extraction channel that brings the
bunch into a beam dump
In this configuration the same thin septum magnet is used for both the bunch injection and the bunch
extraction This requires a symmetrical design with two flanges one dedicated to the injection and the
other to the extraction The magnetic gap is wide enough to accommodate the two bent trajectories
When operating at a 50 Hz injection rate the septum current pulse consists of a slow full sine whose
same positive half sine will be used successively for the lsquoold bunchrsquo extraction and the for the lsquonew
bunchrsquo injection with a suitable delay between the two steps
The two fast kicker magnets are identical their length has been reduced as much as possible because of
the very limited space available The current pulse in the kickers will have a half-sine shape One must
achieve a rise time (from zero current to peak current) and a decay time (from peak current to zero)
each less than a ring revolution period Thus the current pulse can have a maximum duration of two ring
revolution periods Consequently their charging voltage reaches high values even though most of the
deflection is performed by the septum magnet
The design of these three devices ensures that they can operate with a beam energy of 50 MeV
(nominal) or 70 MeV (maximum) and at a 50 Hz repetition rate
3512 Slow septum magnet and its pulsed power supply
The main constraints on the septum design are as follows This septum must produce a large trajectory
deflection (160 mrad nominal peak value 170 mrad maximum peak value) to bring the injected beam
very close to the ring straight section axis with no significant disturbance of the stored beam The
magnet mechanics should preserve the ring vacuum chamber geometry to keep its impedance as low as
possible Furthermore its stray field must remain very low within the vacuum chamber where the stored
beam circulates as the septum excitation is done by a full sine pulse whose width (130 micros) is very long
compared to the ring revolution time (56 ns)
An eddy current septum magnet installed in vacuum will fulfil these constraints The effective septum
blade thickness can be as small as 3 mm thus ensuring the closeness of the stored beam orbit and the
desired bunch trajectory at the output of the septum yoke An internal chamber connected to the magnet
yoke extends the ring vacuum chamber and preserves the impedance budget The injected bunch
deflected by the septum magnet field penetrates through a small hole in this internal chamber Such a
septum magnet is very asymmetric in the transverse plane it is driven by a copper loop that wraps the
steel yoke
In order to contain the magnetic flux the whole yoke is enclosed in a copper box which carries the eddy
current On the side facing the stored beam orbit the copper box thickness is reduced so that it
constitutes the thin septum blade As a result of the electric shielding provided by the eddy current
circulating in the copper septum such a structure is very effective to avoid a large magnetic stray field
outside the magnet gap The fact that the excitation pulse is a full sine reduces the duration and the
lsquo
62
amplitude of the stray field (the slow delayed stray field tends to be the sum of the two opposite half
current arcs this feature brings about a reduction of the stray field)
An additional magnetic shielding is required to strongly reduce the residual strayfield This shield is
provided by a 05 mm thick sheet of high permeability mumetal that will completely surround and
enclose the internal chamber (thus shielding the stored beam orbit from the strayfield)
Such a design that combines an eddy current shield a full sine pulse excitation and the use of a high
permeability magnetic material reduces the stray field to a very low level (a few 10 ppm of the main
pulsed field) [313]
Assuming a septum opening of H x V = 30 x 12 mm2 and an active length of 250 mm (the length was
chosen as short as possible because of constraints coming from the septum implementation in the
straight section) the septum magnet inductance is estimated to be 16 microH Connexions and cables
increase this inductance up to 23 microH
This design leads to the following parameters for the septum magnet and its power supply (values for a
70 MeV ring operation)
Septum deflection field integral field peak current charging voltage
170 mrad 40 mTm 160 mT 1500 A 226 V
Table 315 Septum main parameters
The pulsed power supply will be based on the lsquoclassicalrsquo resonant discharge circuit scheme composed
of a storage energy capacitor (190 microF) a fast high-current thyristor switch and a transmission line to
the magnet comprising a few low impedance coaxial cables
A 300V-20A power supply is foreseen It will be connected to the storage energy capacitor through a
current-limiting power resistor
3513 Fast kicker magnet and its pulsed power supply
The two fast kicker magnets must be identical Because of the very limited space available their length
has been reduced as much as possible The final numbers are 200 mm and 450 mm for the active length
and for the overall length respectively They will be located outside the vacuum vessel with their
lsquowindow framersquo rectangular ferrite yokes surrounding the ceramic vacuum chamber (see Fig 338)
Figure 338 Front view of the kickers
The two ceramic sections of the vacuum chamber will have a flange-to-flange length of 450 mm Their
aperture will be the same as the one in the ring standard sections but with a racetrack transverse shape
Both extremities of each ceramic chamber include a smooth shape transition and a short bellow that
lsquo
63
allows for RF finger contacts These bellows can accommodate thermal dilatations especially during
bake-out The ceramic chambers will be coated with a metallic deposit that will carry the bunch mirror
current and thus avoid an increase of the chamber impedance This coating needs to be very thin It will
consists of a titanium layer with a thickness le0050 microm The thickness has been calculated so as to
avoid the deterioration of the fast pulse shape and to avoid the temperature rise due to the mirror current
[314]
Figure 339 Time profile of the kicker pulse versus the titanium layer thickness
The kicker dimensions follow from
- the aperture needed by the stored beam H x V = 40 x 28 mm2
- the required deflection kick nominal 11 mrad maximum 15 mrad
- the beam energy nominal 50MeV maximum 70MeV
- the short revolution period of the stored e- bunch Trev = 56 ns
- the very short length of the injected bunch from 4 to 25 ps (rms)
With a ceramic thickness of 6 mm the effective magnet aperture must be H x V = 55 x 55 mm2 These
parameters determine the current and voltage pulse characteristics
The kicker magnets will be powered by a circuit delivering half-sine pulses [315] In fact because of
the e- bunch short revolution period (56 ns) the field pulse duration has to be less than twice the ring
revolution period (less than 112 ns) and thus must a have very fast rise time and decay time (both less
than a ring revolution period) On the other hand with such a very short bunch length (4 or 25 ps (rms)
whether one deals with the incoming bunch or the extracted bunch one may assume that the pulses do
not have to present a marked flat top
These considerations lead to the following kicker parameters
00000E
-2
17000E
-10
34000E
-10
51000E
-10
68000E
-10
85000E
-10
10200E
-9
11900E
-9
0000E+00
2000E-01
4000E-01
6000E-01
8000E-01
1000E+00
1200E+00
Thom-X injection kicker pulse field attenuation relating to Titanium thickness
Bext(t)
Bint(t) 002microm
Bint(t) 004microm
Bint(t) 005microm
Bint(t) 006microm
Bint(t) 008microm
Bint(t) 01microm
Bint(t) 02microm
Time
Fie
ld a
mp
litu
de
lsquo
64
Kicker beam energy deflection field integral
Operation at Enominal 50 MeV 15 mrad 25 mTm
Operation at Emax 70 MeV 15 mrad 35 mTm
field peak current estimated voltage
92 mT 406A 10 kV
175 mT 771 A 18 kV
A specific RampD program is necessary to guarantee that these constraints will be fulfilled by the power
supply and by the magnet Preliminary studies were based on the standard circuit used for such a pulsed
power supply (see Fig 340) involving a high voltage (HV) capacitor to store the energy in series with
a HV switch and a load whose inductance is close to the one of the kicker magnet In this circuit the
HV switch is a very fast MOSFET switch followed by a HV fast recovery diode module
Figure 340 Power supply circuit for the ThomX kickers
These first studies lead to unsatisfactory preliminary results
- The BEHLKE Ultra-Fast 17 kV 480 A MOSFET module can switch ON in a very short time less
than 8 ns Then the pulse shape is normally determined by the passive component parameters in the
circuit provided that the negative current is quickly interrupted either by HV diodes in series or by
a turn Off control of the HV switch
- But the different types of HV diodes used are not able to switch OFF the pulsed current quickly
enough Even the lsquofast recovery diodesrsquo have a reverse recovery time too much long to stop cleanly
the pulse at its zero crossing Thus it was necessary to work on other topologies and to find better
suited components
Further simulations were performed to see how the circuit could be improved It turned out that
controlling the switch both at the closing and at the opening can improves the shape of the current
pulse A prototype using the same Behlke MOSFET modules and including a coaxial transmission line
was built and tested A plot of the current pulse in the kicker coil that was obtained is shown below
lsquo
65
This plot was obtained with the following prototype circuit
As one can see more work is still needed to improve the shape of the current in the coil (namely one
should reduce the level of reflections improve the current pulse shape and work on the peak current to
meet the 70 MeV requirements) We are also looking for an other supplier who could provide controlled
HV switches ensuringr both ON and OFF fast operation
Mechanical and magnetic studies of the kicker magnets are ongoing
36 Storage ring RF system
The choice of 500 MHz for the RF frequency is a compromise between several constraints the cavity
fundamental and high order mode (HOM) impedances space requirements the availability of RF power
sources and other components The required accelerating voltage of 500 kV can be provided by only
one 500 MHz single-cell cavity with about 40 kW of RF power As shown in the following the choice
of the cavity design is mainly dictated by the need for a strong damping of the HOM
lsquo
66
361 HOM impedance thresholds as regard to beam instabilities
When a cavity mode is excited in resonance it generates beam oscillations with a rise time
approximately given by one of the two following expressions
(1τ)l = Ib α fm Rl (2 Qs Ee) in the longitudinal case and
(1τ)t = Ib βt fo Rt (2 Ee) in the transverse case
where fm is the HOM resonant frequency Rl [Ω] and Rt [Ωm] are respectively the longitudinal and the
transverse HOM impedances Qs = fs fo is the synchrotron tune (fs and fo are the synchrotron and the
revolution frequencies) βt is the beta function amplitude at the cavity location α is the momentum
compaction factor Ib is the average beam current and Ee is the beam energy in eV
Assuming typical values for the cavity HOM impedances (Rl fm = 01 to 1 MΩ GHz Rt = 1 to
10 MΩm) and the machine parameters listed in Table 315 the instability growth time is around 10 micros
in both the longitudinal and the transverse cases
Orbit length C 168 m
Electron energy E 50 MeV
RF frequency fRF 500 MHz
Revolution frequency fo 1786 MHz
Harmonic number h 28
Momentum compaction factor α 0015
β-function amplitude at the cavity location βxz 3 m 4 m
Relative energy spread σE E 40 10-3
Peak acceleration voltage VRF 300 (500) kV
RMS bunch length σs 27 (21) ps
Synchrotron frequency fs 358 (462) kHz
Bunch charge Qb 1 nC
Number of stored bunches Nb 1
Maximum current Ib 1786 mA
Table 315 Ring parameters used to estimate the rise time of the HOM induced instabilities
In a low energy ring like ThomX radiation damping is so weak (the damping time is ~ 1 s) that a
stationary state is not reached during the beam storage time (20 ms) On the other hand to ensure a
good ring operation it is sufficient to make all instability rise times much longer than this storage
period This can be achieved by attenuating the HOM impedances very strongly typically by a factor
104
362 Cures to the HOM induced instabilities
There are essentially two ways of coping with such HOM induced instabilities either by performing a
strong de-Qing of the HOM resonances or by tuning their frequencies away from the beam spectral
lines in order to prevent resonant excitation
In practice HOM de-Qing is achieved by extracting the cavity HOM power and transferring it towards
external dissipative loads But in this approach it is difficult to reach attenuation factors higher than
102 - 10
3 over a large frequency range and this technique requires heavy and cumbersome equipment
close to the cavity (coaxial lines with coupling antennas or ferrite loaded waveguides) The DAMPY
lsquo
67
cavity designed at BESSY [316] and which is used in ALBA [317] and the PEP II cavity developed
at LBNL [318] are two variants of this technique
The other method which consists in controlling the HOM frequencies is better suited to a small
circumference machine like ThomX where the beam spectral linesrsquo spacing (~ 18 MHz) is very large
compared to the HOM bandwidth (this bandwidth is of a few tens of kHz for Qo equal to a few 104) As
long as the HOM density is not too high and the modes can easily be tuned far enough from the beam
spectral lines (δf gtgt fm Qo) it should be possible to reduce their effective impedances (the ones lsquoseenrsquo
by the beam) down to tolerable levels ie Reff asymp Rs (2 Qo δf fm) 2 ltlt Rs
We are investigating the potential performance of this technique by applying it to an ELETTRA-type
cavity (see Fig 344) [319] which combines three frequency tuning means The HOM frequencies can
be precisely controlled by an appropriate choice of the temperature of the water cooling the cavity while
the fundamental frequency is maintained constant by a mechanism which deforms the cavity
longitudinally In addition a movable plunger tuner also called HOM frequency shifter (HFS) located
in the cavity equatorial plane provides an additional degree of freedom [320]
Tables 316 a and b list the characteristics of such a cavity fundamental and HOM resonances They
have been calculated with the 3D GdfidL code [321] One finds 18 significant HOM (9 monopoles and
9 dipoles) with resonant frequencies lower than the cavity cut-off frequency Figures 345 a and b
represent the corresponding HOM spectra fm Rl(f) and Rt(f) folded over a single base band of
18 MHz The horizontal dashed line in the graph represents the impedance threshold corresponding to
an instability rise time of 20 ms the beam storage time Although they remain somewhat critical these
results tend to confirm that it should be possible to find acceptable operating conditions as long as the
HOM frequencies can be shifted by a few MHz by combining temperature and HFS tuning
lsquo
68
Figure 344 Drawing and photo of the ELETTRA cavity
lsquo
69
Figure 345 (a) Longitudinal and (b) transverse HOM spectra folded over a single base band of
18 MHz for the ELETTRA-type cavity
τL = 20 ms
lb
os
I
eEQ
τα
2
ttob
o
fI
eE
τβ
2
τt = 20 ms
lsquo
70
a) Longitudinal modes fm [MHz] Q0 Rs Q0
[Ω]
Δfm (T)
[MHz]
Δfm (HFS)
[MHz]
L0 (fundamental) 499132 44164 765 000 000
L1 958618 48158 336 037 -100
L2 102781 57861 036 080 369
L3 14087 52562 220 174 021
L4 151528 69088 783 112 086
L5 155129 66375 850 240 721
L6 186912 55785 097 142 -092
L7 19495 81752 008 183 143
L8 203985 62729 086 400
L9 206896 87300 530 445 667
Table 316 Main characteristics of the monopole (a) and dipole (b) modes computed with GdfidL for
an ELETTRA-type cavity Δfm (T) and Δfm (HFS) are the effective temperature and HFS tuning ranges
assuming a constant fundamental frequency
It is worthwhile mentioning that in any event fast longitudinal and transverse feedback systems are
required to damp the beam oscillations resulting from injection transients (see section 333) They will
contribute to the HOM damping as well
363 RF power source
An RF power of about 40 kW is required to generate the desired voltage of 500 kV in a single 500 MHz
ELETTRA-type cavity For this frequency there exists a wide choice of power sources based either on
vacuum tubes (klystron IOT diacrode) or on solid-state technology The low operating voltage and
high modularity of the solid-state version offer significant advantages compared to vacuum tubes
Experience acquired at SOLEIL after few years of operation has definitely demonstrated all the benefits
of this technology [322] Figure 346 shows the 35 kW - 352 MHz solid-state amplifier used for the
SOLEIL booster synchrotron It consists of a combination of 147 modules Each one has a power of 330
W a gain of 11 dB and an efficiency of 62 This amplifier was commissioned about 7 years ago
During a running period of more than 35 000 hours it tripped only once and this trip was caused by a
human mistake
b) Transverse modes fm
MHz]
Qo Rsperp Qo
[Ω]
Rsperp[MΩm] Δfm(T)[MHz] Δfm(HFS)
[MHz]
T1 73701 50267 335 2599 04 075
T2 74606 52566 92 7557 155 285
T3 115202 47196 264 3006 049 -312
T4 125565 55386 51 7428 068 725
T5 130249 71674 36 7039 097
T6 169981 69325 103 2542 188
T7 173650 91039 165 5463 gt 2
T8 177337 59409 32 7061 gt 2
T9 183617 77697 55 1643 gt 2
T10 206142 88484 59 2254 gt 2
T11 222475 99092 11 5079 gt 2
T12 225933 56641 13 3484 gt 2
lsquo
71
This technology has already been extended to 476 MHz in the framework of a SOLEIL-LNLS
collaboration aimed at realizing two 50 kW amplifiers for the LNLS storage ring [323] More recently
the SOLEIL team has developed modules which can deliver up to 650 W at 500 MHz with a gain of
17 dB and an efficiency of 62 while still reducing the thermal stress [322] About a hundred of such
modules will be combined to obtain the 50 kW required by ThomX
Figure 346 35 kW - 352 MHz solid-state amplifier of the SOLEIL Booster (147 amplifier modules
and DCDC converters bolded on 8 water-cooled dissipaters)
364 Low level RF system and longitudinal feedback
The proposed low level RF (LLRF) system is conventional As shown by the synoptic diagram of Fig
347 it consists of three relatively slow loops (frequency amplitude and phase) aimed at controlling the
cavity voltage with an accuracy of about plusmn 02 in amplitude plusmn 02deg in phase and a response time
around 1 ms This scheme is suitable for compensating the pressure and temperature drifts as well as
the power supply ripples
Such a system is not fast enough to deal with the beam phase oscillations at the synchrotron frequency
fs on the order of 400 kHz that originate in injection transients These oscillations can produce an
undue emittance blow up and a phase jitter at the interaction point Beam dynamics simulations show
that preserving the beam quality requires a damping of these oscillations within a few tens of a micros (see
section 333) As the radiation damping is negligible during the storage time a longitudinal feedback
600 W 280 Vdc 28 Vdc converter
Power combination
330 W amplifier module
lsquo
72
(LFB) must be implemented It can be performed by a dedicated broad-band longitudinal kicker that
acts on the beam Since ThomX is operating with a single bunch we propose to use the main cavity as a
longitudinal kicker despite its narrow bandwidth of 25 kHz ltlt fs This difficulty can be overcome by
making the cavity effective bandwidth much larger than this value in fact larger than fs We will use a
high gain direct RF feedback (RFFB) that allows the operation of a fast phase feedback (ϕFB)
Assuming a cavity-to-amplifier distance of about 10 m the loop delay will be less than 150 ns Then
one expects an RFFB gain Go larger than 50 which leads to an effective bandwidth larger than 1 MHz
Simulations show that a LFB that combines such a RFFB with a ϕFB (see Fig 348) can damp injection
phase errors of up to 10 degrees within a few micros ie faster than the above specification
Figure 347 Basic LLRF system scheme with lsquoslowrsquo frequency amplitude and phase loops
lsquo
73
Figure 348 Schematics of the longitudinal feedback system combining a phase feedback with a direct
RF feedback
365 RF equipment status
For the ThomX commissioning we will use an ELETTRA cavity The design of all the components of
the 50 kW solid-state amplifier (600 W modules combiners power supplieshellip) was validated in 2011
The next two milestones are first the validation of a 9 kW unit (combining 16 modules) and secondly
the commissioning of the complete amplifier on a dummy load
Several versions of the LLRF system including LFB are under investigation either fully analogue (see
Fig 349) or mixed analogue for the fast feedback loops and digital for the slower ones (see Fig 350)
or even fully digital based on the new generation of fast ADC DAC and FPGA
366 RF system conclusion
For the RF system of the ThomX storage ring we plan to use a 500 MHz single-cell cavity of the
ELETTRA-type Maintaining at a tolerable level the impact of the RF cavity HOMs on the beam is a
challenge However this goal should be achievable with a proper control of the HOM frequencies It
can be done by combining three different means of tuning the cavity temperature control longitudinal
elastic deformation of the cavity and motion of a plunger The fast longitudinal and transverse feedback
systems - which in any event are required to damp the beam oscillations resulting from injection
transients - will contribute to the damping of the HOM excitations as well
In order to provide the required accelerating voltage of 500 kV the cavity will be powered by a solid-
state amplifier of the lsquoSOLEILrsquo type able to deliver a power of up to 50 kW The cavity voltage will be
controlled with an accuracy of plusmn 02 and plusmn 02 degree by means of conventional amplitude and phase
loops (either analogue or digital) with a few kHz bandwidths In addition a fast phase feedback system
combined with a high gain direct RF feedback will provide the longitudinal damping which is required
to preserve the beam quality
lsquo
74
Figure 349 Complete LLRF system longitudinal feedback included (analogue version with phase and
amplitude modulation ndash the option with IQ modulation is also considered)
Figure 350 Complete LLRF system longitudinal feedback included (mixed analogdigital version)
37 Transverse feedback
371 Specifications
Table 317 summarizes the instability growth time constants and the corresponding kicker strength
requirements for the main types of instability It shows that for Thom-X the betatron oscillations due to
the injection jitter in position and angle are the most critical ones They require a feedback damping
time constant of 5 micros or less (cf chapter 3)
lsquo
75
Source type growth time constant kicker strength
Beam pipe
geometry
TMCI
Head-Tail
-
160 micros
gt10 nrad
Resistive Wall 600 micros gt2 nrad
Ions lt 100 micros gt20 nrad
Injection Jitter 5 micros 2 microrad
Table 317 Overview of expected instabilities (1 nC 25 ps rms at 50 MeV)
372 Electronics
To cope with these instabilities we will use a SPring-8 based high performance digital transverse
feedback system which consists of a wideband detector (a BPM) an RF front-end an FPGA based
processor a power amplifier and a stripline kicker (see Fig 351) This system is capable of detecting a
coherent transverse motion and applying a counter kick to damp it bunch-by-bunch and turn-by-turn
with one or two bunches stored in the ring
Figure 351 Spring-8 based digital feedback system
Two BPMs (one for each plane) will provide the beam position information The RF front-end will
extract the betatron oscillation information which will be sampled by ADCs From the sampled data the
FPGA will compute a suitable kick to damp the oscillation The signal processor output will be
amplified by a 10 W wideband amplifiers before being fed to the kicker
In the RF front-end (see Fig 352) a baseband down-converter will be implemented in order to amplify
the beam signal and to minimize the jitter effect for the same reason the digital kick will also be
extended over four RF periods
lsquo
76
Figure 352 Front-end of the Thom-X RF electronics
For the commissioning phase a digital system (rather than an analog one) will be used to benefit from
the tuning flexibility provided by the digital parameters of the FIR filter Once the machine tune will be
fixed a more suitable low cost analog system will be installed
373 Kicker
The kicker is under development It consists of a stripline composed of four electrodes connected to
vacuum feedthroughs on each side (see Fig 353) The amplifiers will be connected to the downstream
ports whereas upstream ports will be used for signal detection and they will be equipped with 50 Ω
loads The same kicker will be used to act on the beam in both planes
Electrode line impedances must be adapted (50 Ω) Electromagnetic 3-D simulations are being
performed to optimize the bandwidth and shunt impedance (ie to maximize the stripline efficiency)
and also to minimize the kicker impedance to wakefields
Figure 353 Schematics of the Thom-X stripline kicker
lsquo
77
38 Vacuum
381 Transfer line vacuum
Concerning the design of the transfer line vacuum system three main constraints have been taken into
account
- Pressure of 10-7
Pa at the acceleration section output
- Ion pump operating at 10-6
Pa
- Ring pressure of 10-8
Pa
Simulations have been performed assuming that two 03 m3 s
-1 triode pumps will be installed one at the end of
the acceleration section and the other at the septum The vacuum chamber is a DN40 The septum output
conductance is 018 m3 s
-1 Without baking the outgassing rate would be σ τ = 2x10
-8 Pa m s
-1 and the
pressure at the ring entrance would be about 10-7
Pa Thus the septum and its environment must be baked to
bring τ down to 5x10-10
Pa m s-1
and obtain a pressure of 10-8
Pa at the ring entrance
Pressure in the line of transfer
The vacuum equipment envisaged for the transfer line is the following one
‐ 3 ion pumps 150 l s-1
with their baking systems and power supplies
‐ 1 ion pump 500 l s-1
with their baking system and power supply
‐ 4 Pirani gauges
‐ 4 cold cathode gauges
‐ 2 power supplies gauges
‐ 2 gate valves electro pneumatically actuated DN 40 (acceleration section output ring entrance beam
dump)
‐ 1 turbo pump (DN 63 ~ 80 l s-1
)
‐ 1 rotary vane pump (~ 12 m3 h
-1)
‐ 4 angle valves manually actuated for pre-pumping DN 63
lsquo
78
382 Ring vacuum
Concerning the ultra-vacuum to be maintained in the ThomX ring one must take into account first the
pipe relatively low specific conductance c = 452x10-3
m3 m s
-1 and secondly the pipe specific
outgassing rate θ τ = 108x10-10
Pa m2 s
-1 Also the pumpsrsquo effective pumping speed is reduced by the
T port with the grid Seff = 19x10-2
m3 s
-1 In a first stage the ionic pumps chosen for the simulations
are 0040 m3 s
-1 diode pumps This corresponds to S = 0030 m
3 s
-1 at 10
-8 Pa and with a grid pumping
port whose conductance is 00362 m3 s
-1 to an effective pumping speed Seff = 0019 m
3s
-1 The pressure
has been estimated by modeling a quarter of the ring including half of the non-baked optical cavity ie
in the most unfavorable case
Figure 354 Pressure distribution in the vacuum chamber
The calculated average pressure is 306x10-8
Pa It can be improved by increasing the grid pumping port
conductance by replacing the DN35 with a DN63 Monte-Carlo simulations show an improvement of ~
16 in efficiency
The entire ring must be baked Geometrical and temperature constraints (respectively a distance of only
07 mm between the magnet coils and the vacuum chamber a baking temperature of 150 degC for the
chamber while keeping the coilsrsquo temperature below 80deg C) are strict An ultra-thin baking system has
been constructed tested and validated
Figure 355 The vacuum chamber ultrathin baking set-up
lsquo
79
The following is the vacuum equipment needed for one quarter of the ring
- 3 ion pumps (40 l s-1
) with their baking equipment and their power supplies
- 1 residual gas analyzer (RGA)
- 2 turbo-pumps (DN 63 ~ 80 l s-1
)
- 2 rotary vane pumps (~ 12 m3 h
-1)
- 2 Pirani gauges
- 2 cold cathode gauges
- 1 gauge power supply
- 1 DN 63 gate valve electro-pneumatically actuated with an HF feed-through (one on each side of
the RF cavity and similarly for the septum) and the baking equipment
- the vacuum chamber baking equipment
- 3 angle valves manually actuated for pre-pumping DN63
- 3 T with the grid
lsquo
80
Chapter 4 INJECTOR TRANSFER LINE AND RING DIAGNOSTICS
Several diagnostic devices will be implemented in the ring to measure the electron beam parameters
listed in Table 41
Parameter unit value
Energy MeV 50
Average current mA 1784
Bunch charge nC 1
Energy dispersion 03 at injection to 06 at storage end
Emittance x mrad 5 e-8 at injection to 30 e-8 at storage end
Normalized emittance x mrad 5 e-6 at injection to 30 e-6 at storage end
Emittance y mrad 5 e-8
Normalized emittance y mrad 5 e-6
Bunch length ps-rms revolutions 1 ~ 4
revolutions 100-200 ~ 25
revolutions 200-300000 ~ 50
RF frequency MHz 500
Revolution frequency MHz 1784
Period ns 56
Injection repetition rate Hz 50
Ring circumference m 1680
(a) Ring parameters
Magnetic field T 05
Radius of curvature m 0352
R51 m 0 to -025
R52 m 0 to -045
R56 m 07 to -07
Beta x m 06
Beta y m 6
Eta x m 02 - 03
Transverse dimension x mm (rms) 06
Transverse divergence x mrad 2
Transverse dimension y mm (rms) 054
Transverse divergence y mrad 2
(b) Parameter values at the location of the bending magnets
Table 41 ThomX beam parameters
41 Charge measurement
411 Current transformers
The beam charge will be measured non-destructively with commercial in-flange high resolution
current transformers (ICT) Three devices will be installed
- one in the linac (LIDGICT010) located before the accelerating sections
- one at the transfer line entrance (TLDGICT010)
lsquo
81
- one at the transfer line exit (TLDGICT020)
In-flange ICTs are directly mounted in the beam line They are UHV compatible can be baked up to
150deg C (185deg C on request) and they are available for many pipe diameters (also with an elliptical
aperture or any other aperture shape) We will use the CF6rdquo model its internal diameter is 604 mm and
it is 60 mm long This device includes a ceramic gap a shield and a wall current bypass Bellows are
not required
The signal from each integrating current transformer will be processed by a BCM (beam charge
monitor) A built-in calibration generator provides an on-line check of the measurement reliability An
Integrate-Hold-Reset (BCM-IHR) device is suited for pulse repetition rates ranging from 10 kHz down
to single pulses and for bunch charges up to 2 nC The acquisition will be performed by using a 100
kHz 12 bit ADC
412 Faraday cups
Interceptive absolute beam charge measurements will be performed by Faraday cups There will be two
of them placed inside the two beam dumps (see Fig 2 of the Introduction) They should provide a
better resolution for small electric charges
42 Transverse profile measurements
421 Diagnostics stations
Five diagnostics stations will be installed to measure the transverse beam profile as shown in Fig 41
Diagnostics station 1 (DS1 ndash LIDGSST010) will be located after the RF gun at the beginning of the
linac Diagnostics station 2 (DS2 ndash TLDGSST010) will be located between the end of the linac and
the linac beam dump Diagnostics station 3 (DS3 ndash TLDGSST020) will be placed in the middle of the
transfer line Diagnostics station 4 (DS4 ndash TLDGSST030) will be located at the end of the transfer
line and Diagnostics station 5 (DS5 ndash ELDGSST010) will be located before the ring dump
lsquo
82
Figure 41 Location of the diagnostics stations (see text for the nomenclature)
The ThomX station design draws from the station built by LAL for PITZ (itself drawn from a SOLEIL
design) It is illustrated in Fig 42
Figure 42 3D view of a ThomX diagnostic station
Each station will have a motorized arm on which several screens will be mounted A stepper motor will
move this arm and insert the screens laterally The following screens will be available
- 1 YAG(Ce) screen in all stations
- 1 optical transition radiation (OTR) screen in all stations
lsquo
83
- 1 target to allow an online calibration of the camera in all stations
- one Cerenkov screen for bunch length measurements in station DS2 (described in more detail
below)
The YAG(Ce) and OTR screens and the calibration target will be tilted at 45deg with respect to the beam
axis so that the emitted or reflected light will travel downward and exit the vacuum through a DN60 CF
fused silica window A CCD camera will be located below the station to image the screens The
Cerenkov screen will be titled so that its light exits at the top of the diagnostic station Shutters will be
used to close either the Cerenkov window or the YAG(Ce)OTR window when not in use to prevent
measurement noise due to background photons Several LEDs located just after the bottom window will
be used to illuminate the inside of the chamber when needed
At the beginning of the Linac at station DS1 the beam size will be about 05-07mm (rms) in both
planes at the end of the Linac at DS2 the beam size will be 15 mm (rms) in both planes In the middle
of the transfer line at DS3 the beam size is expected to reach 25 mm (rms) in x and slightly less than
1mm in y At the end of the transfer line at DS4 it will be about 1 mm in both planes After the
extraction at DS5 the beam size will be about 05-07mm (rms) in both planes Given these beam sizes
we chose a screen diameter of 25 mm for all stations
Screen photon yield
The YAG(Ce) screen will be 100 microm thick Such screens have a light yield of 8 photons per deposited
keV [41] The YAG radiation length is 35 mm [42] assuming the usual model for energy deposition in
matter [43] each electron will deposit about 143 keV in the screen this corresponds to the emission of
~1100 photons Thus the total number of photons produced by an e- bunch crossing the screen will be
71 1012
We recall that YAG(Ce) emission occurs in all directions
The second screen will be of the OTR type The screen support design allows either the use of a 100 microm
aluminized silicon wafer or of an ultra-thin aluminized mylar film The silicon wafer has the advantage
of giving a better image quality while the mylar film allows a beam injection through the screen without
significantly affecting its quality The OTR yield within a cone of half-aperture θ for photons whose
wavelengths range between λ1 and λ2 is given by [44]
Thus for a standard CCD camera whose sensitivity ranges between 400 nm and 800 nm and assuming
an acceptance cone of 003 radian this expression gives ~0002 photon per electron at 50 MeV (station
DS2-5) A similar yield is reached at 5 MeV with an acceptance of 03 radians (station DS1)
To image the screens a lens will be located 500 mm away from them A small telescope made of two
lenses will be used to focus the light on the CCD camera At least one of the telescope lenses will be
movable to provide the needed dynamic range A Gigabit Ethernet camera with at least 600 x 400 pixels
will record the images The camerarsquos square pixels are about 74 x 74 microm2
With such a configuration one expects that a 50 MeV bunch passing through the YAG(Ce) screen will
emit 71x1012
photons and assuming that the CCD chip has an average efficiency of 50 1010
counts
will be recorded by the camera taking into account the solid angle covered by the lens
lsquo
84
The same bunch passing through the OTR screen will emit 82 107 photons within the lens acceptance
Thus 3 106 counts will be recorded by the camera
For a 5 MeV bunch (station DS1) the YAG(Ce) screen will emit 2 1012
photons and 109 counts will be
recorded by the camera The OTR screen will emit 82 107 photons within the lens acceptance and 3 10
6
counts will be recorded by the camera
Assuming a transverse beam size of 15 mm one expects 8 106 counts per pixels with a YAG(Ce)
screen and 25 105 counts per pixel with an OTR
If the 50 MeV beam is defocused (for example during a quadrupole scan) to produce a 5 mm spot size
these numbers become 8 105 for the YAG(Ce) screen and 2 10
4 for the OTR
Number of OTR
photons emitted
Number of
OTR
photons
recorded by
the camera
Number of
YAG
photon
emitted
Number of
YAG
photons
recorded
DS1 82 107 3 10
6 2 10
12 10
9
DS2-5 82 107 3 10
6 71x10
12 10
10
Assuming a YAG Cerium dopant density of 018 we get a Cerium density of 25 1019
cm-3
[45] and
a 15 scintillation efficiency It follows that the scintillating site density is 37 1018
cm-3
Therefore the
100 microm YAG(Ce) screen will saturate when the photon density reaches 36 1014
mm-2
This last figure
translates into an electron density of 32 1011
mm-2
for an e- beam energy of 50 MeV In these
conditions the YAG(Ce) screen will saturate if the beam is focussed on a spot smaller than 20 microm This
is five times less than the smallest beam foreseen at ThomX
Additional targets will be used to calibrate the cameras when necessary They will be made of stainless
steel plates with etched grids Each square of the grid will be 2 by 2 mm2 In some of these squares
smaller lines of known thickness will be etched Images of these targets will allow one to check the
whole optics
To minimize a possible radiation deterioration of the cameras lead-glass windows will be inserted
between the vacuum windows and the focusing lenses
43 Emittance measurement
The beam emittance will be measured with the quadrupole scan technique In this technique a triplet
(set of three quadrupoles) is used to find the minimum transverse beam size in each plane A fit of the
beam size variation versus the quadrupole strength enables one to retrieve the projected transverse
emittance
The sets of triplets and the diagnostic stations implemented along the ring allow an emittance
measurement at three different locations at the exit of the linac with DS2 in the transfer line with DS4
and in the ring with DS5 by using the extraction kicker The beam emittance measurement with DS5
lsquo
85
will provide a check of the turn-by-turn emittance growth thus bringing precious information on the
transverse beam dynamics
Pepper-pot measurements are also foreseen The advantage of this technique with respect to a
quadrupole scan is that it consists in a single shot measurement independent of machine operation
fluctuations
A pepper-pot set-up comprises a plate with evenly distributed holes Such a plate is placed in the beam
path together with a scintillating screen located downstream and an imaging system A pepper-pot
mounted on a movable arm could be located at the end of the linac between the last dipole and the
diagnostic station DS2 and another one could be located just before station DS5
At station DS2 the beam size will be about 15 mm (rms) and the geometric emittance about 5 10-8
m
rad Therefore the maximum beam divergence will be about 50 microrad By adjusting the quadrupoles one
may focus the beam down to a 20 microm spot and increase its divergence up to 250 microrad To measure the
transverse emittance one would have to position this focal point two meters before DS2 (or DS5) and
to position a pepper-pot mask with 100 microm holes or slits one meter upstream of DS2 (or DS5)
44 Beam position measurement
The beam position will be measured by a set of BPMs placed all along the ring Their locations are
shown in Fig 43
lsquo
86
Figure 43 Location of the BPMs along the ring (note the BPM located at the IP has been suppressed
since this drawing was made)
441 Beam Position Monitors (BPM)
Two types of BPMs will be used short circuited striplines in the linac and the transfer line and buttons
in the storage ring
Compared to button BPMs striplines generate a higher signal but need more space and contribute more
to the wake-field generating impedance For these reasons striplines are preferred for the linac and the
transfer line where we need single pass measurements and where wakefield is not an issue In the
storage ring button BPMs will be chosen for their small size and small contribution to the ring
impedance The lower signal produced by the small electrodes will be compensated by the high
repetition rate of the stored beam (averaging is possible)
BPM electronics detection frequency on the storage ring is usually equal to the RF frequency as it is the
only common frequency component present in the BPM data whatever the filling pattern is Even
though only one bunch will be stored in ThomX the same approach is chosen Therefore the storage
ring BPM electronics detection frequency is 500 MHz
Since it is more convenient to adopt the same electronics for all the BPMs (linac transfer line and
storage ring) stripline BPMs on the Linac and transfer line are optimized to give a signal with the
highest possible component at 500 MHz Their electrode length is thus adapted to this frequency which
means that it has to be equal to λ4 (150 mm 500 MHz) where λ is the RF wavelength
4411 BPM for the linac
The linac BPM equipment will be as follows
- Number of BPM 1
- Type short circuited stripline
- Electrode length 100 mm
- Required resolution 100 microm for a single pass measurement and a 1 nC bunch charge
Figure 44 Linac BPM
lsquo
87
Due to lack of space in the linac section where a stripline must be installed the electrode length cannot
be 150 mm but must be reduced to 100 mm Transfer impedance calculations show that at 500 MHz
the signal amplitude will be reduced by 13 compared to the optimal length of 150 mm (see Fig 45)
This amplitude reduction is acceptable
Feedthroughs will be soldered on CF16 flanges
Figure 45 Stripline transfer impedance versus frequency for various electrode lengths With an
electrode length of 100 mm the signal is reduced by 13 compared to its maximum value for a
frequency of 500 MHz
4412 BPMs for the transfer line
The transfer line will be equipped as follows
- Number of BPM 4
- Type short circuited stripline
- Electrode length 150 mm
- Required resolution 100 microm for a single pass measurement and a 1 nC bunch charge
lsquo
88
Figure 46 Drawing of the BPMs that will equip the electron transfer line
Feedthroughs will be soldered to CF16 flanges
4413 BPMs for the storage ring
The specifications relevant to the ring BPMs are as follows
- Number of BPM 12
- Type button
- Button diameter 107mm
- Required resolution ~1 microm 50 Hz and for a bunch charge of 1 nC
- Absolute precision plusmn 250 microm
In order to provide additional BPM signals for other applications (see below) 4 BPM blocks will be
equipped with a double set of electrodes In that case only 4 electrodes (among 8) will be used for
position monitoring The button size and the beam pipe size remain unchanged
Figure 47 Simple and double BPM blocks for the ThomX storage ring
The button positions and dimensions have been determined by simulations with Poisson Superfish (an
electrostatic simulation software) An evaluation of the impedance due to the button discontinuities has
shown that their effect will be negligible
Feedthroughs will be soldered directly to the BPM block They are bakable Connectors will be SMA
males to prevent any deformation during baking
lsquo
89
Alignment
BPM blocks will have an external mechanical reference This reference will be used for BPM alignment
during the installation phase and to position precisely the block in a calibration bench
Calibration
Storage ring BPMs will be calibrated by using a dedicated bench An antenna inside the block will be
fed with a 500 MHz wave of constant amplitude Calibration is done by measuring the signal output by
the 4 buttons A precise position of the antenna is not needed as position errors can be eliminated by
repeating the measurement with the BPM block turned by 180deg and averaging the results
The second halves of the blocks equipped with two sets of buttons will be dedicated to
‐ longitudinal (beam phase) feedback (one set of BPMs)
‐ transverse feedback (one set of BPMs)
‐ ion clearing To suppress ions which may generate beam instabilities BPM buttons will be
polarized with a high voltage (max plusmn200V) This electric field will deflect the ion trajectories
toward the vacuum chamber
Electronics
Electronics will be the same for all BPMs (whether equipping the linac the transfer line or the storage
ring)
Data rate
Revolution-by-revolution beam position measurements would be best But because of the high
revolution frequency (178 MHz) it might be difficult to get electronic devices that handle the
corresponding data rate Then one may work with averages over two turns This data will be provided
on demand and the acquisition will be triggered The buffer size should accommodate data collected
during at least 20 ms to cover full storage cycles Revolution-by-revolution data will be used for
machine physics studies and tune measurement The position measurement resolution must be better
than 100 microm
Slow acquisition
Slow data acquisition will be used for slow orbit correction and slow orbit feedback It will be obtained
by averaging the fast data flow with a sampling rate of ~10 Hz A data rate of 50 Hz would provide just
one measurement per storage cycle The resolution aimed at for this measurement is one micron or
better
Acquisition must be synchronous with the machine revolution frequency Moreover modules must be
synchronized together
Connections and cables
The four cables that carry the signals of each BPM should be put together in a strand Their electrical
lengths will be matched so that phase differences are kept below 13deg 500 MHz The cable attenuation
should not differ much from one BPM to another If the cable lengths cannot be the same for all BPMs
cable sections will be chosen in order to ensure the same attenuation for all cables To minimize both
lsquo
90
attenuation and cost cable lengths below will be kept below 30 m The LMR240 cable is an example
of what could be used its attenuation is ~6 dBm 500 MHz over a distance of 33 m
As radiation close to the machine can damage the cable insulator short (~1 m) pigtails (that can be
easily replaced if needed) will be inserted between the BPMs and the long cables
442 Orbit feedback system
A feedback system will keep the beam orbit stable within plusmn1 microm It will use the position measurements
provided by the 12 ring BPMs The correction rate will be comprised between 01 Hz and 10 Hz
Correction signals will be applied to the 12 correctors located along the ring The orbit feedback system
will be a matlab based application
443 Tune measurement and feedback
The tune will be obtained from the revolution-by-revolution data provided by the BPMs There is no
need for any kicker to excite betatron oscillations as the beam transverse position will naturally oscillate
after each injection (ie every 20 ms) If necessary a tune feedback will be implemented to keep the
tunes constant
444 Energy measurement
In the middle of the transfer line ie downstream of the dipole located at the end of the linac the
dispersive function is non-zero Therefore the beam energy and its spread can be deduced from the
electron horizontal distribution shown by the YAG screen in DS3 The same measurement will be
performed with DS4 using the dispersion induced by the extraction magnet
45 Bunch length measurement
The bunch length will be measured at two different places at the end of the linac and in the ring
Depending on the observation point two different physical processes are used to produce visible light
pulses which are then transported by dedicated optical lines through the ThomX shielding wall to a
double-sweep streak camera located in the experimental hall
451 Cerenkov radiation
Cerenkov radiation (CR) produced in a target placed at the end of the linac will be used to measure the
e- bunch length Cerenkov radiation is emitted when a charged particle passes through a medium at
velocity greater than the speed of light in that medium It is emitted at an angle θcr ndash with respect to the
particle momentum ndash determined by n(λ) the medium index of refraction and β the particle velocity as
follows
Above 10 MeV the emission angle is almost independent of the particle velocity Ncr the number of
radiated photons whose wavelengths range between λ1 and λ2 summed over a path of length d along the
charged particle trajectory is
( )
minus
minus
λnβλλπαd=N
cr 22
21
11
112
( )
minus
minus
λnβλλπαd=N
cr 22
21
11
112
( )( )λβn
=θcr
1cos
lsquo
91
where α=73 10-3
is the fine structure constant
Two types of radiator are usually used to produce Cerenkov radiation an aerogel or a solid target
Aerogels dramatically spoil the vacuum and therefore have to be mounted in a separate vacuum vessel
to maintain ultra-high vacuum conditions Solid targets are UHV compatible This is why they are
preferred
At the end of the linac when the bending magnet is off the beam goes straight into a beam dump
Before the dump several types of targets can be inserted in the beam trajectory for diagnostic purposes
For the production of Cerenkov radiation a sapphire target is adequate This materialrsquos index of
refraction is 177 With a 50 MeV beam the Cerenkov angle is θcr=5563deg Assuming that the window
thickness is 025 mm 16 photonse- are created with a wavelength ranging between λ1 = 300 nm and λ2
= 800 nm Their spectrum is given in Fig 48 For a 1 nC electron bunch 1011
photons are created The
energy deposited in such a target is negligible
Figure 48 Spectrum of the Cerenkov radiation emitted by a 50 MeV e- beam passing through of
sapphire window 025 mm thick
For simplicity the sapphire target is oriented so that the Cerenkov radiation exits in the vertical
direction For a Cerenkov angle θcr = 5563deg the window angle with respect to the beam axis must be θP
= 11475deg (see Fig 49)
8x10-8
6
4
2
0
In
ten
sity
[au
]
800700600500400300200
Wavelength [nm]
lsquo
92
Figure 49 Orientation of the sapphire target used to observe the Cerenkov light emitted by the e- beam
at the linac end
At the target location the beam dimensions will be σxy= 14 mm (rms) It will produce a light spot
whose dimensions will be σx = 14 mm (rms) and σy = 154 mm (rms) in the sapphire To intercept the
whole light beam and to leave some margin for the alignment we will use a target whose diameter will
be equal to 2 x 6 x σy ~ 20 mm
The Cerenkov radiation will exit the vacuum chamber via a UHV viewport made of fused silica (FuSi)
with a 90 transmission over the 300-800 nm wavelength range The viewport is mounted on a CF63
flange that provides a 68 mm clear aperture At the exit of the viewport a FuSi lens is used to focus the
light This collimated beam is then transported via a set of planar mirrors across the shielded area and
then through the ThomX shield to a dedicated hutch in the experimental hall (see sect 453)
452 Synchrotron radiation
Most of the synchrotron radiation (SR) emitted when the electrons circulate in a bending magnet (BM)
will be in the far infrared but visible light will also be present The cut-off wavelength of this radiation
corresponds to ~077 eV Thus the X-ray content is negligible This last feature makes the extraction of
the visible part of the spectrum easy The spectral flux per unit bandwidth per unit surface ΦBM
[Photons01bwmmsup2] is shown in Figure 410
Figure 410 On-axis spectral flux 1 m downstream of a bending magnet Photon energy in eV The
visible spectral range corresponds to [400 nm - 800 nm] or [155 eV - 31 eV]
The radiation is extracted at the exit of the BM ndeg5 through a porthole which provides a 63 mrad x 32
mrad aperture at 15deg with respect to the 0deg incident angle in the dipole The maximum power density is
lsquo
93
4 microWmmsup2 in the horizontal plane of this extraction porthole thus no dedicated absorber is required
(But for the porthole region the radiated power will be dissipated in the vacuum chamber wall) The
radiation will exit the vacuum chamber through a UHV viewport made of MgF2 (this material offers
the highest transmission coefficient over the 200-800 nm range) The synchrotron radiation distribution
at 5 eV one meter downstream of the bending magnet is around 6 mm (FWHM) So the spectral flux
integrated over a collecting area of 6 mm x 63 mm is about (around 5 eV)
Φe [Phs] = ΦBM [Phs01bwmmsup2] x 6 mm x 63 mm x 1000
Since photons of lower energy thus with a higher divergence will also contribute to the total spectral
flux this value gives a low limit estimate of the real total spectral flux Figure 411 shows the Φe
spectral flux
As was the case for the Cerenkov radiation a FuSi lens will be used at the exit of the viewport to focus
the beam The collimated radiation will then be transported via a set of planar mirrors across the
shielded area and then through the ThomX shield to a dedicated hutch in the experimental hall (see next
section)
Figure 411 Spectral flux Φe in a 63 x 32 mrad2 aperture 1 m downstream of a bending magnet
453 Transport of the Cerenkov and synchrotron radiations to the diagnostics hutch
Both the Cerenkov light and the synchrotron radiation have to be transported to the diagnostics hutch
located in the experimental hall Starting from different locations both radiations will be transported via
a set of planar mirrors to a lsquomeeting pointrsquo inside the shielded area A flip mirror (ie a double-
orientation mirror) will enable the selection of one of the radiations The one selected will be further
transported again with a set of planar mirrors through the shielding wall to the diagnostics hutch
Inside the diagnostics hutch the last lens will focus the radiation on the streak camera entrance The
lens aperture and its focal length will be adapted to the selected incoming radiation All the line
components and their parameters are chosen to minimize spatial aberrations and to maximize the
number of photons entering the streak camera
454 Streak camera for the bunch length measurement
We plan to use a double-sweep streak camera to measure the bunch length and to visualize its evolution
over short time scales Since the bunch length may vary from 4 to 20 ps (rms) a 2 ps (FWHM)
resolution streak camera is required
lsquo
94
The time resolution of the bunch length measurement depends on the radiation type (CR or SR) which
is used and involves several contributions the electron transverse beam size at the light emission point
chromatic effects etc All these contributions will be minimized as much as possible to maintain the
effective resolution as close as possible to the one of the streak camera
46 Bunch imaging using synchrotron radiation
The synchrotron radiation transported to perform the bunch length measurement (see section 45) can
also be used to image the bunch while it circulates For this purpose a beam splitter could be installed
at the radiation exit port the main part of the radiation would be sent to the streak camera while a small
fraction would be directed to a CCD camera mounted nearby A focusing lens placed between the
viewport and the CCD would enable the imaging of the stored beam
In the orbit section where the bunch emits the synchrotron radiation used for the above measurements
the e- beam transverse size is about 05 mm (rms) Since the size of a CCD chip is typically 5 mm the
lens gain should be about 10 Such a set-up would enable one to follow the evolution of the beam size
in real time (we will use a CCD whose acquisition rate is about 10 Hz) It would be a non-destructive
and cheap diagnostic Similar setups are currently used on storage rings to measure the electron beam
emittance assuming that the beta function is known
47 Beam loss measurement
Particle losses will be measured by using optical fibers [46] When a charged particle crosses a glass
fiber Cerenkov radiation is emitted within the fiber and propagates along the fiber It can be detected
by a photomultiplier that collects the light at the fiber end The measurement eg with an oscilloscope
of the pulse production (in the fiber) time enables one to recover the particle loss location
Two fibers will be installed One will be used to monitor e- losses along the linac and the transfer line
The second fiber will be used to monitor particle losses along the ring Each one of these fibers will be
connected to a fast photomultiplier (PMT) The PMT time resolution will determine the accuracy with
which the losses can be localized
A fused silica fiber has a yield of 227 photons per cm of a relativistic electron track [47 48] Therefore
a 5 mm thick fiber will have a yield of ~100 photons per relativistic electron which crosses it Such a
signal should be easily detected by a PMT
As it may be difficult to place the fiber close to the beam pipe in some parts of the ring the fiber may
have to be located at a distance d away from the beam axis Then the number of lost particles which will
cross the fiber will be reduced by a factor dsup2
48 Beam dumps
The ThomX set-up will include two particle dumps to stop the e- beam in such conditions that the
radioactivity then generated is minimal The first dump will be located at the linac exit The second
dump will be located downstream of the injectionextraction section The latter dump can be used to
stop the beam either at the exit of the transfer line or after any number of bunch revolutions in the ring
Both dumps are based on the SOLEIL design and can handle the 35 W power deposition (1 nC 70
MeV 50 Hz repetition rate)
lsquo
95
Chapter 5 SYNCHRONIZATION
51 Overall scheme
An ultra-low noise 500 MHz synthesizer will be used as the main clock for ThomX The 10 MHz
output of this synthesizer will be used to drive a second synthesizer (the so-called lsquolinac synthesizerrsquo)
operating at 3 GHz The RF output of the latter synthesizer will be used for the linac operation It will
be split in two lines The first one will be used to synchronize the gun laser oscillator (after appropriate
down conversion of the frequency) The second line will be used to drive the gun and the linac RF
The RF output of the main synthesizer (500 MHz) will be used for the ring RF it will be split in three
lines The first line will drive the ring RF (after appropriate phase shifting) The frequency of the second
line will be divided by 14 this line will synchronize the Compton cavity As in the MightyLaser
experiment [51] the laser used to inject power in the Compton cavity will be synchronized with that
cavity by using the PDH method The frequency of the third line will be divided by 28 and will
therefore be synchronous with the electron revolution frequency This line will be used to synchronize
programmable delays delivering turn-by-turn triggers and injectionextraction triggers To ensure that
injection is always taking place at a zero crossing of the mains the programmable delays will also be
synchronized with the mains A counter will reduce the mains frequency to the operations frequency
(when lower than 50Hz) The gun trigger will be generated when the mains will be at a zero crossing
and the ring RF at a preset phase that takes into account the time it will take for the electrons to travel
from the linac gun to the ring accelerating cavity
To ensure that the e- bunches will always be injected at a triple zero (50 Hz 500 MHz and 3 GHz) we
need to specify the tolerance on the 50 Hz zero crossing We may take 1 (ie 02 ms) for this
tolerance this means that the coincidence between the ring RF (~500 MHz) and the linac RF (~3 GHz)
must occur at a frequency greater than 5 kHz Therefore by choosing a heterodyne frequency (ie the
frequency difference between the linac RF divided by 6 and the ring RF) greater than 5 kHz we ensure
that there will always be a common zero crossing between the ring RF and the linac RF within 1 of
the mains zero crossing This sets a constraint on the ring radius of 10-5
ie 168 micrometres on a
168 m long orbit (see Table 2)
The RF bucket must be positioned with an accuracy of 3 ps in the ring RF Thus the phase noise
between the linac RF and the ring RF must be lower than 3 ps when integrated between 50 Hz and
500 MHz Most good-quality synthesizers achieve such a performance
The information collected by the ring diagnostics will be used to adjust the exact value of the frequency
(around 500 MHz) generated by the main synthesizer so that it matches the exact ring length This
information will also be used to adjust the phase between the injection trigger and the ring
To reach the correct signal level each RF line may have to be amplified An isolator will be added to
avoid reflections that may degrade the overall performance of the system
To ensure long-term low frequency synchronization a Network Time Protocol (NTP) server will be
available to allow all computers working at ThomX to synchronize their internal clock
lsquo
96
52 Specifications
Synthesizer
The main synthesizer will be used to generate a 500 MHz adjustable signal Its phase noise jitter
integrated between 50 Hz and 20 MHz must be less than a few picoseconds It must support a frequency
modulation of at least 100 kHz Typical mid-range synthesizers meet such specifications
Measurements done at LAL in collaboration with SOLEIL have shown that the addition of an external
frequency reference does not necessarily improve the phase noise of these synthesizers in the frequency
range of interest
The linac synthesizer specifications are the same ones except that it must be able to generate a
frequency of 3 GHz
Programmable delay
Two low jitters units with 8 channels each will be used
Phase shifter
The phase shifters must be voltage controlled and cover 360deg (two 180deg units)
Figure 51 Synchronization layout
lsquo
97
Chapter 6 OPTICAL SYSTEM DESIGN
Since the writing of the ThomX CDR three directions have been pursued improvement of the feedback
system improvement of the laser amplification and mechanical design of the optical resonator The
results are described in the following three sections
In the first section the ThomX feedback system is described During the past year a strong RampD
program has been developed on the locking of a TiSapph picosecond laser beam on a two-mirror high
finesse cavity at OrsayLAL Very good results in operating for the first time a 30 000 finesse cavity
in the picosecond regime were obtained The feedback techniques used for this task are directly usable
for ThomX and are reported in this section A 3000 finesse four-mirror cavity at KEK was also installed
and operated and we succeeded in producing γ rays off the ATF electron bunches (Mightylaser project
[61]) Hardware damages were however caused by the Japanese 2011 earthquake At the time of
writing this document we are recovering all our equipment to reach our final goal locking a 30000
finesse four-mirror cavity to produce a high γ ray flux at ATF Thanks to a contact with the SYRTE
laboratory we have acquired a new laser oscillator with a 1030 nm wavelength that we are testing and
that we plan to use for ThomX With this oscillator by embedding an intra-cavity EOM we are able to
increase our feedback bandwidth by more than one order of magnitude
The second section illustrates the laser amplification developments done at CELIA First a non-all-
connected fiber amplifier for the Mightylaser project was set up that allows us to extract an average
power of 50 W This amplifier was used to produce γ rays at ATF A new setup has been designed and
built to reach both the reliability and the power level required by ThomX An important activity is
devoted to the construction of a fully all-connected amplification chain As mentioned above the new
laser oscillator and the new laser amplification chain are being integrated in order to fill a 30000 finesse
high finesse cavity to conclude the ANR Mightylaser project
In the third section the details of the implementation of the cavity in the electron ring and the cavity
vacuum system are described with the opto-mechanical system that will be used for a precise alignment
of the cavity mirrors
61 Feedback system and results on a Fabry-Perot cavity locking
To produce a high X-ray flux the ThomX facility produces high intensity laser pulses and collides them
with electrons stored in a ring Hence the locking system (see figure below) has a double goal
- Stabilize the synchronization of the Fabry-Perot cavity (FPC) photon pulse with the electron bunch
- Stabilize the synchronization of the laser pulses with the FPC to obtain amplification
For this last point as will be seen in section 62 firstly the laser beam will be amplified with a CPA
technique using optically pumped fibers and then it will be amplified again by accumulation during
many round trips into a high finesse FPC (the target is a gain of 10000 which leads to a finesse of
30000)
This section describes the following topics related to the stabilization of the laser pulses relative to the
FPC
- Parameters to be stabilized repetition rate and carrier-envelop phase (CEP)
- Error signals the Pound-Drever-Hall (PDH) measurement technique
lsquo
98
- Actuators dynamic range and bandwidth
- Behavior of the digital feedback system (DFS)
General scheme for the locking system Detection and actuation elements are shown in red
The results on the locking of a TiSapph laser oscillator on a 30000 finesse cavity will also be
illustrated
Figure 61 Locking scheme for the synchronization of the electron bunch and the FPC pulse for the
ATF experiment at KEK
611 Electron and FPC pulse synchronization
This kind of stabilization has already been achieved for the MightyLaser experiment at KEK (ATF) a
1785MHz repetition rate A digital feedback system was used (i) to acquire two clocks one from the
laser and one from the master oscillator of the electron ring (ii) to perform a digital phase detection
(iii) to build a digital integrator filter to remove phase drifts and (iv) to react on a PZT inside the FPC
to change its length according to the measured phase drift (see Fig 61)
lsquo
99
612 Laser parameters to be stabilized
We will choose a laser with a repetition rate corresponding to twice the e- bunch revolution frequency
in the ring (two laser pulses per electron bunch revolution) around 500MHz14 = 357 MHz A phase
noise measurement on a similar product operating 1785 MHz was already carried out to characterize
the repetition rate phase noise and the drift (see Fig 62 and Fig 63)
Figure 62 Repetition rate phase noise spectrum Figure 63 Repetition rate drift measured
X-axis offset frequency from the carrier in Hz during about one hour X-axis repetition
Y-axis single side band phase noise power rate drift in Hz Y-axis time duration in
density in dBcHz seconds
The global shape of the phase noise in 1f will permit to use three feedback actuarurs a high bandwidth
low dynamic one (EOM for repetition rate and AOM or pump current for CEP) a middle
bandwidthmiddle dynamic one (PZT for repetition rate) and a high dynamiclow bandwidth one
(mirror motor and isochronic wedge motor)
For this kind of laser the harmonic content (index m) is well known and can be represented either in the
time domain or in the spectral domain (see Fig 63 and Fig 64)
( )2
CEL REP CE REPf m m f f m f
φ
π
Δ = + = +
lsquo
100
Figure 64 Time domain and spectral representation of the laser pulses
Since one is dealing with two parameters (the repetition rate and the CEP) one needs two different
sources of information to measure them The standard method to measure these two parameters consists
in using the Pound-Drever-Hall (PDH) technique [62] which gives the position of the laser harmonics
relative to the FPC resonances
( )FPC CAVf m m f=
One of the parameters is deduced from the whole beam signal using a central photodiode the second
one is derived from a part of the spectrum selected by a grating and a slit and measured by the grating
photodiode
If the repetition rate and the FPC round trip frequency are close enough the error signal coming from
the central photodiode (index m0) is proportional to the difference between the laser harmonics and the
corresponding resonances of the FPC
1 0 0
2
CEREP CAVm f m f
φε
π
Δ prop + minus
Frequency
domain
lsquo
101
As the locking system cancels the error the central laser harmonic m0 is perfectly aligned on the FPC
resonance but all other laser harmonics (m=m0+Δm) are slightly detuned from the corresponding FPC
resonances This introduces a correlation between the CEP and the repetition rate
( )( )
1 0 0
2 0
0 0
02
1 12
CEREP CAV
CEREP CAV REP CE
m f m f
m mm m f f f f
m m
φε
π
φε
π
Δ = rArr + =
Δprop minus minus = minus = minus
As a result of these equations the signal received by the PDH system located after the grating (ε2) is
directly proportional to the offset frequency (fCE) and then to the CEP Thus if we change the CEP of
the laser we should clearly see on the grating photodiode PDH signal ndash which measures the laser-FPC
detuning on the border of the spectrum (for an m index far from m0) ndash a linear relation between the error
signal and the CEP
This method well known for femtosecond lasers has been tested in our laboratory in the picosecond
regime Figure 65 shows our results (submitted to Applied Optics) on the locking of a 76MHz
TiSapph picosecond laser oscillator on a 30000 finesse cavity
Figure 65 Top experimental measurement of the CEP value of a TiSapph laser oscillator Bottom
measurement of the corresponding grating photodiode signal
In Fig 65 one sees that when the CEP is measured close enough to zero the grating photodiode
exhibits a linear relation with the CEP otherwise the photodiode is saturated (due to the PDH signal
saturation) In our experiment the ratio between the cavity reflected power and the incident beam
power was ~20 which leads to a value of 80 for the coupling of the incident beam power to the
cavity
These results demonstrate for the first time that a picosecond laser oscillator can be locked to a 30000
finesse cavity (implying a power enhancement factor of 10000) with a power coupling of 80
613 PDH Measurement technique
Having shown that we are able to measure independently both parameters of interest (the repetition rate
and the CEP) we will now describe the apparatus to transform these measurements into error signals by
lsquo
102
the PDH technique (see Fig 66) Side-bands to the laser optical harmonics are produced by an electro-
optic modulator (EOM) modulated by a generator coming from the digital system Side-bands and laser
harmonics are reflected by the FPC the first ones without acquiring any phase the second ones shifted
in relation with the discrepancy between the laser and the FPC frequencies All the harmonics and the
side-bands of the reflected laser are combined into the photodiodes and the front-end electronics giving
an error signal at the modulation frequency
Figure 66 Pound-Drever-Hall scheme with digital phase detection
The photodiode front-end (see Fig 67) amplifies these signals at a level compatible with the digital
feedback system (DFS)
Figure 67 Electronic scheme and picture of the PDH front-end
Inside the DFS the PDH signals are filtered by a band-pass filter to remove unwanted frequencies and
multiplied by the same modulating signal with a phase shift to allow for the phase detection A low-pass
filter at the end of the chain removes the image signal at twice the modulation frequency Figure 68
shows an example of the input and the output signals
lsquo
103
Figure 68 Modulated input (blue) error signal (yellow) FPC transmitted signal (pink)
614 Actuators
In the previous sections we described how the two parameters (the repetition rate and the CEP) of a
pulsed laser have to be controlled in order that all the laser harmonics match the FPC resonances and
we also detailed how to measure the variations of these two parameters We will now describe the
devices inside or around the laser on which we need to react to correct the variations of the laser
harmonics relative to the FPC resonant frequencies
Figure 69 Typical scheme of a laser with all its actuators
The actuators will produce an action on the two parameters to be controlled with different dynamic
ranges and different bandwidths (see chart in Fig 610)
Figure 610 Dynamic ranges and bandwidths of the laser actuators
lsquo
104
All of the actuators are located inside the laser except the AOM (see Fig 610) They are the key items
to achieve a good stabilization
All the actuators measurements have been made on the 1785MHz laser version used at KEK
- Mirror motor dynamic range precision and bandwidth
The motor is a step motor with a precision of 140 nm on the round trip variation (equivalent to 15 Hz on
Frep) Figure 611 shows the linearity and the maximum dynamic range available with the motor stage
while Fig 612 shows a step response which indicates the maximum bandwidth we can obtain
Figure 611 Mirror motor range Figure 612 Time response of Frep for
1k motor steps X-axis repetition rate
offset (Hz) Y-axis repetition rate
The repetition rate range is 178033 - 179244 MHz (gt 1MHz)
- Piezoelectric sensitivity dynamic range and bandwidth
The PZT sensitivity is approximately 3Hz V 28 nmV (length variation to be compared to the
length of a round trip of the FPC pulse)
The PZT can be operated up to 150V providing a total dynamic range of 42 microm (round trip length
change) Its first peak resonance has been measured and found above 40 kHz
lsquo
105
Figure 613 Repetition rate change produced by a Figure 614 PZT frequency response X-
PZT voltage variation from 0 to 28 V frequency range 01-5 kHz Y-axis
Attenuation (dB) ndash arbitrary offset
Figures 613 and 614 show the PZT sensitivity and its transfer function
- EOM sensitivity measurement
Measuring the sensitivity of the EOM is not direct thus we chose to apply a 10 V amplitude ndash 10 Hz
frequency modulation ndash to the EOM and to measure the power of the harmonics (see Fig 616)
Assuming a pure frequency modulation the power of the harmonics fis a Bessel series that depends on
the sensitivity Thus taking the ratio of two consecutive harmonics gives directly the sensitivity
Figure 615 EOM modulation spectrum used for sensitivity measurement X-axis frequency centered
on the repetition rate Y-axis relative optical power around the repetition rate due to the modulation
The table on the left gives the frequency and the power of each harmonic
0 5 10 15 20 25 300
10
20
30
40
50
60
70
80
90
100
PZT voltage (V)
Fre
p v
ariation (
Hz)
Frep variation vs PZT voltage
AJ0(z)
AJ1(z)
AJ2(z)
lsquo
106
We now check that the sensitivity calculated from the ratio of the first harmonic to the fundamental and
the one obtained from the ratio of the second harmonic to the fundamental are the same
∆
∆
~1
2
∆
10~ minus 2909 rarr ∆~07
∆
∆
~1
8
∆
10~ minus 6398 rarr ∆~07
Since the ratio is 07 Hz for an applied sinusoidal signal of 10 V the sensitivity is around 007 HzV
- Isochronic wedge
It is made of two glass prisms with different refraction indexes chosen to maintain a constant group
delay in order to avoid repetition rate variation
We measured that the maximum variation of the repetition rate for a full CEP range (several times 2π)
is about 300 Hz for a repetition rate of 1785 MHz (see Fig 616 which shows the measured repetition
rate shift plotted along the X-axis versus the date of the measurement plotted along the Y-axis) Thus
the influence of this actuator on the repetition rate is about 210-6
Figure 616 Measurement of the influence of wedge displacement on Frep
- Acousto-Optic Modulator (AOM)
Such an actuator has already been used with a laser which is not the same as the one used for the other
actuators namely a 76 MHz laser locked on a two-mirror FPC
In Fig 617 we present the closed loop measurement The lowest frequency part (lt10 kHz) is
attenuated by the loop gain If the loop were open this part of the transfer function would be quite flat
To estimate the AOM bandwidth limits we can use the frequencies where the gain starts to drop
significantly this bandwidth is about 300 kHz
300 Hz
lsquo
107
Figure 67 Frequency response of the AOM in closed loop
615 The digital feedback system
The feedback system will deal with several coupled inputs and outputs To ensure flexibility we have
chosen a digital system Since we also want to work with a large bandwidth in closed loop (gt 1MHz) a
low latency system (lt 100 ns) is needed A digital system featuring such performance does not exist on
the shelf Thus we decided to design and build such a system at LAL (see Fig 618)
Figure 618 Scheme of the DFS and picture of the DAC daughterboard
This system will perform several functions (see Fig 619)
103
104
105
106
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
Frequency (Hz)
Magnitude (
dB
)
Bode plot
lsquo
108
Figure 619 Organization of the digital feedback functions
- The first important function is to produce the error signal from the modulated signal coming from the
photodiodes It is a phase detection block (Digital PDH DPDH) the main problem being to reduce the
latency as much as possible This will be achieved by optimizing the low-pass filter and the modulation
frequency (see Fig 620 for a digital implementation)
Figure 620 Digital phase detection (DPDH) to produce the PDH error signals
- The second important function is to provide Infinite Impulse Response (IIR) filters to build
Proportional-Integrator (PI) filters in order to cancel the system drifts
lsquo
109
Figure 621 Digital IIR filter implementation with adders and multipliers
These filters can be programmed by the user to change the filter coefficients and it can be cascaded to
realize complex filters They have been cut in Second Order Sections (SOS) to minimize the
propagation of fixed precision errors due to quantized data and coefficients (see Fig 621)
- The last important function integrated in the digital system is performed by an identification block
which works like a network analyzer by (i) sending a small harmonic signal (ii) looking at the system
response in closed loop at this frequency and (iii) sweeping this small harmonic signal frequency to
cover all the range to be measured (see Fog 622)
Figure 622 Identification system
lsquo
110
This kind of identification system can provide information on (i) the feedback system itself (ii) the
system to be stabilized or (iii) the open and closed loop transfer functions by producing Bode or
BlackNichols diagrams (see Fig 623) These informations are very useful for system analysis and for
tuning or designing the feedback filters
Figure 623 Typical Bode (top-left magnitude bottom-left phase) and Black-Nichols (right) diagrams
produced by the identification system
62 High average power laser system and its coupling to a high gain FP cavity
The all-fibered laser system will draw from the system developed in the MightyLaser project It will
consist in an oscillator associated with three amplifying stages involving single and double clad Yb-
doped fibers (see Fig 625) At present the amplifiers are based on commercial components to facilitate
a potential replacement of damaged parts The architecture is designed to provide the mandatory
diffraction-limited beam This relatively simple and compact architecture will implement the lsquoChirp
Pulse Amplificationrsquo (CPA) scheme In order to limit non-linearity effects in the fiber the signal is
initially stretched in time thus reducing its intensity in the waveguide then amplified in the fiber and
finally recompressed to its minimal duration The difference between the MightyLaser and the ThomX
repetition frequencies (respectively 1785 MHz and ~357 MHz imposed by the different repetition rates
of the two accelerators) requires an adjustment of the laser design
Figure 625 Sketch of the all-fiber laser system
lsquo
111
621 The MightyLaser oscillator
This oscillator is an all-normal dispersion mode-locked fiber laser based on the nonlinear polarization
evolution technique equipped with a fibered output It uses an ytterbium doped fiber which allows an
emission centered around 1030 nm with a full width at half maximum of 10 nm (see Fig 626) It
delivers short pulses at a repetition rate of 1785 MHz with a low average output power (~10 mW) in
order to minimize the noise level (see Fig 628) The main advantage of the all-normal dispersion
regime resides in the significant peak power reduction provided by the pulse stretching during the
propagation along the cavity Non-linear effects are thus reduced allowing a high-energy extraction
The output pulses are naturally chirped with an estimated duration of 2 ps
Figure 626 Emission spectrum of the oscillator
The oscillator is passively mode-locked by taking advantage of the non-linear evolution of the
polarization This technique is based on the optical Kerr effect The polarization of a pulse propagating
in a fiber evolves non-linearly as a function of intensity Thus by combining polarization controllers
and a polarizer one may generate losses that depend on the intensity (see Fig 627) Beyond the
polarizer all the pulse components are linearly polarized The wave plates transform the linear
polarization into an elliptic one The latter polarization state then evolves in a non-linear way while it
propagates along the fiber This evolution results from the combined effect of the self-phase and the
cross-phase modulations induced on the two polarization components The polarization rotation of the
more intense central part of the pulse is larger than the polarization rotation of the pulse wings Since
the pulse center crosses the analyzer with a reduced loss compared to the wingsrsquo loss the pulse duration
is shortened
Figure 627 Principle of mode locking based on the non-linear polarization evolution
lsquo
112
Figure 628 Spectrum of the oscillator phase noise noise level (dBcHz) versus offset frequency (Hz)
622 Amplification stages
The amplification chain is divided in three stages in order to distribute the gain and to saturate the
second and the third amplifying stages The first and the second stages are based on the same
architecture It is an all-fibered amplifier whose central component is a polarization maintaining single
mode using an Ytterbium doped fiber with a core diameter of 6 microm and a clad diameter of 125 microm (see
Fig 629) A wavelength division multiplexer (WDM) is spliced on the doped fiber to transfer the pump
power into the gain medium and to couple the signal of the picosecond oscillator to the amplifier The
first stage is pumped by a single mode diode centered at 976 nm and delivering 200 mW The second
one is pumped at a higher average power namely600 mW The two amplifying stages are separated by
an optical isolator to avoid any parasitic feedback from one amplifier to the other
Figure 629 Schematic of the first and the second amplification stages
The average power level at the input of the first amplifying stage is about 1 mW A limitation is
imposed on the output of this stage which should remain below the damage threshold of the isolator
fixed at 100 mW Consequently we adjust the pump level in order to obtain 80 mW at the output of the
ytterbium-doped fiber The 3 dB losses of the isolator allow one to inject 40 mW into the second
amplifying stage The evolution of the average output power versus the pump level is shown in Fig
630 For the maximum pump level of 600 mW the second amplifying stage delivers 330 mW
lsquo
113
Figure 630 Second stage output power plotted versus the pump power
Beyond the second isolator the third amplifying stage (see Fig 632) is built by using as its central
component a polarization maintaining double-clad Ytterbium-doped fiber with a core diameter of 25
microm and a clad diameter of 250 microm The amplifier is clad-pumped through a special combiner 6+11
(see Fig 631) In practice this component has an input signal fiber 6125 microm directly spliced to the
previous amplification stages an output signal fiber 25250 microm spliced to the double-clad Ytterbium
doped fiber and six multimode fibers of 105 microm to couple the pump diodes Here only three pump
ports are spliced to multimode diodes emitting up to 55 W at 976 nm corresponding to a pump power of
at most 165 W The development of a mode field adapter was necessary to splice the pump combiner to
the double-clad Ytterbium-doped fiber because their numerical apertures are different Losses entailed
by this component are compensated by the last amplification stage
Figure 631 Pump combiner Figure 632 Schematic of the third amplification stage
Figure 633 is a plot of the average output power versus the pump level For a pump power level of
76W the last amplification stage delivers 52W This was the first objective The fiber laser power
stability was measured during 1 12 hour at this power level (see Fig 634) The residual pump power
the pump and signal feedbacks into the pump diodes are also plotted in this figure
lsquo
114
Figure 633 Third stage output power Figure 634 Laser stability at 52 W
versus the pump power
New measurements were made after optimizing the central wavelength of the multi-mode pump diodes
The efficiency was increased to reach 787 for an output power of 24 W (see Fig 635) The fiber
laser power has been tested at this level during two hours (see Fig 636) The observed power variation
is less than 01
Figure 635 Output power versus pump power Figure 636 Laser stability at 24 W
after wavelength optimization
The all-fibered amplifier setup has been integrated in a 19rsquorsquo rack as shown in Fig 637 An FCPC
connector placed in the front panel allows the connection of the fibered oscillator to the master
amplifier module Several 955 optical fiber couplers have been inserted in our setup at strategic
locations to monitor the power level in real time by using photodiodes they may also signal potential
problems The output fiber comes out of the module through its front panel The back-side is dedicated
to all the electronic controls (diode pump photodiodes and modulators for noise management described
elsewhere)
lsquo
115
Figure 637 Pictures of the 52 W all-fibered setup integrated in a 19rsquorsquo rack
623 Stretcher and compressor
A chirp volume Bragg grating (CVBG) is used in our setup to stretch and compress the pulse with a
very good efficiency of 94 The CVBG transmission bandwidth is around 2 nm (see Fig 638) leading
to a pulse duration ranging from 200 to 250 ps depending on the grating dispersion It is possible to
recompress the pulse after amplification by using a second CVBG Then the pulse length ranges
between 20 and 87 ps depending on the second CVBG dispersion (see Fig 639)
Figure 638 Transmission bandwidth of the CVBG
1‐2 couple 3‐2 couple 3‐1 couple
Figure 639 Time profile of the recompressed pulses for different grating pairs
624 ThomX development
Some changes must be made to the all-fibered lsquoMightylaserrsquo design in order to use it in the ThomX
project The oscillator repetition rate needs to be scaled down to match the frequency imposed by the
accelerator As far as the laser design is concerned the stretchercompressor couple must be modified to
limit non-linear effects The increase of the output power from 50 to 150 W imposes an upgrade of the
second amplification stage and eventually of the third one
lsquo
116
6241 Oscillator
The oscillator for ThomX will be based on the same principle as the one used for Mightylaser The only
difference is the repetition frequency imposed by the accelerator (357 MHz for ThomX) The low
average output power around 5 mW permits to limit the phase noise A particular attention will be paid
to the noise level of such a long cavity as no commercial products have been tested at this repetition
rate
6242 Amplification stages
For the ThomX project the lower repetition rate and the higher average output power require a change
of the second amplification stage architecture in order to limit non-linear effects and to saturate the last
amplification stage Consequently the second pre-amplifier will be based on a polarization-maintaining
double-clad Ytterbium doped fiber with a core diameter of 15 microm and a clad diameter of 130 microm (see
Fig 640) The amplifier will be clad-pumped through a special combiner 2+11 with an input signal
fiber 6125 microm directly spliced to the previous amplification stages an output signal fiber of 15130
microm spliced to the double-clad Ytterbium doped fiber and two multimode fibers of 105 microm to couple
the 9 W pump diodes
Figure 640 Schematic of the second amplification stage of the ThomX laser
Concerning the final amplification stage various fibers are available (see below) Micro-structured
fibers can be engineered and tailored but suffer from the lack of splicing techniques Hence their use in
an all-fibered setup will require additional developments dedicated to splicing (see Fig 641) and
integration If splicing is not possible we still have the possibility of proceeding with air propagation
and injection However this approach requires precise mechanics control and advanced engineering
The lsquocoherent beam combining techniquersquo will also be considered at this point In such a configuration
amplification would be supported by two parallel amplification stages dividing by two the energy level
in the gain medium Yet a first version will be developed with commercially spliceable flexible fibers
and preliminary tests of cavity locking with high power from these fibers will be performed to validate
or invalidate operating with a high level of non-linear effects
lsquo
117
Fig641 A micro-structured fiber
6243 Stretcher and compressor
Because of the high average output power required by ThomX we must modify the stretcher size Two
techniques can be considered First we may continue to use a CVBG which can stretch the pulse up to
450 ps with a maximum size of 25 cm (see Fig 642) It would be possible to stretch the pulse up to
900 ps with an efficiency of 88 through a double pass in a CVBG or a single pass in cascaded CVBG
The second way involves a chirp fiber brag grating (CFBG) as depicted in Fig 643 When associated
with an optical circulator it allows a pulse stretching with up to 800 ps in an all-fibered setup The
efficiency is around 70 (92 for the CFBG and 75 for the circulator) in this configuration This
solution is less efficient than the CVBG but the all-fibered configuration ensures the set-up robustness
and stability and the losses can be easily recovered by the amplifier chain However dispersion issues
have been reported for these devices and a detailed investigation is needed prior to a definitive
integration
Fig 642 Examples of CVBG Fig643 Example of CFBG
The high output average power forces us to use a CVBG to compress the pulse one or two units
depending on the stretcher configuration The final pulse duration can be varied between 1 ps and 50 ps
thus leaving the possibility to optimize the X-ray production by adjusting the pulse duration
We also performed simulations to evaluate the influence of the couple stretchingfiber core diameter on
the non-linearity with 3 different fiber cores 25 microm 30 microm and 40 microm Our results are summarized in
Table 61
lsquo
118
Fiber
dimensions
mode field
diameter stretching B-Integral
25250 20 microm 250 ps 47 rad
25250 20 microm 500 ps 263 rad
25250 20 microm 1 ns 118 rad
30150 22 microm 1 ns 09 rad
40200 30 microm 1 ns 0525 rad
Table 61 Pulse stretching as a function of the fiber core diameter (simulations)
A first configuration will be developed with a stretching of 500 ps If the non-linearity effects remain
too high to allow for an efficient locking on the Fabry-Perot cavity we will modify the setup to induce
a larger stretching factor or insert a fiber with a larger core diameter in the last amplification stage or
both
63 Mechanical design
Collisions between the laser pulse and the stored electrons will take place between two dipoles (see Fig
643) The constraints on the optical cavity mechanical design are as follows
- High stability level (vibrations)
- Good accessibility to the cavity mirrors
- Weak impact on the electron beam dynamics
- Ultra high vacuum (UHV) system with baking capability
- Motorized granite table for micrometric scan oof the electron beam
lsquo
119
Figure 643 Implementation of the cavity in the electron ring
The cavity mirror system is mounted on a massive granite table which can be driven to perform a
micrometric electron beam scan (see Fig 643)
lsquo
120
Figure 644 Scheme of the Fabry-Perot cavity mounted on the optical table
The Fabry-Perot cavity is composed of two cylindrical UHV vessels linked by a dipole chamber
Bellows are placed between each element to absorb the dilatation due to the backing (see Fig 644)
e‐
e‐
X‐Ray
1rsquorsquoMirrors
IP
Bellows
1rsquorsquoMirrors
Vessel
Vessel
Dipole
chamber
lsquo
121
Figure 645 On the left detail of the standard mirror assembly and on the right detail of the piezo
electric mirror assembly
The Fabry-Perot cavity is composed of two spherical and one flat mirror mounted in a standard
assembly and one flat mirror mounted on a special assembly with a piezo electric actuator for very
small longitudinal displacements (see Fig 645)
Figure 646 Detail of a truck system
Mirrors
Piezo
electric
actuator
Mirror
Gimbal
Actuators
lsquo
122
Figure 647 Cut of an UHV actuator
All the mirrors are mounted on a truck system that porvides three degrees of freedom (two rotations and
one translation) driven by three micrometric actuators These actuators are specially designed to be
compatible with UHV and backing constraints (see Fig 646 and Fig 647)
Figure 648 Cut of a vacuum vessel
Trucks (two trucks per vacuum vessel) and plugs with special cables composed of ceramic connectors
oxygen- free copper cables and ceramic pearls and spacers for electrical isolation are placed in a UHV
vacuum vessel One of the vacuum vessels has a beryllium window to extract the X-ray beam (see Fig 648)
Micrometric Actuator
Capsule
Trucks
Beryllium
window
lsquo
123
631 Optical cavity vacuum
A simulation was carried out in the most unfavorable case when the whole optical vacuum chamber is not
baked The area of one half of the optical cavity is about 2 m2 the conductance of the rectangular chamber
connecting the two cavity halves is C =70810-1
m3s
-1 (value for one half of that rectangular chamber) The
conductances of the slits connecting the optical cavity to the ring are respectively C = 45210-2
m3s
-1 (9 x 48)
and C = 16010-2
m3s
-1 (6 x 30) The outgassing rate of one optical cavity half without any baking (σ = 210
-8
Pams-1
) is Q = 410-8
Pam3s
-1 The outgassing rate of one rectangular chamber half (σ=10
-9 Pams
-1) is Q =
7610-10
Pam3s
-1 The outgassing rates of the two slits (σ=10
-9 Pams
-1) are respectively Q = 22810
-13
Pam3s
-1 (9x48) and Q=14410
-13 Pam
3s
-1 (6x30) The diode ion pump used for the optical cavity has a
pumping speed of 05 m3s
-1 DN 150 S = 0320 m
3s
-1 at ~10
-10 mbar The conductance of the pumping port
being C = 136 m3s
-1 the effective pumping speed is Seff= 0278 m
3s
-1 The diode ion pump used for one half
of the rectangular chamber has a pumping speed of 015 m3s
-1 DN 100 S = 0090 m
3s
-1 at ~10
-10 mbar The
conductance of the pumping port being C = 0046 m3s
-1 the effective pumping speed is Seff= 0075 m
3s
-1
The outgassing rate of the optical cavity without baking is very high 2 x 410-8
Pam3s
-1 (which is
approximately 40 times the outgassing rate of the whole ring) With such a rate one would need a pumping
speed of 8 m3s
-1 to reach the required pressure level of 10
-8 Pa (the present pumping speed is 07 m
3s
-1) The
conductance of the rectangular chamber that connects the two halves of the optical cavity (710-1
m3s
-1) and
the conductance of the two slits between the optical cavity and the ring (610-2
m3s
-1) are high These figures
do not allow a differential pumping between the cavity and the ring Simulations of the ring vacuum level
show that it is mandatory to bake the whole optical cavity
Figure 649 Location of the cavity pumps
The vacuum equipment needed for the optical cavity is as follows
‐ 2 ion pumps 150 l s-1
with their baking equipment and their power supplies
‐ 2 ion pumps 500 l s-1
with their baking equipment and their power supplies
‐ 2 gate valves manually actuated (pipe diameter DN100) and their baking equipment
‐ 4 Pirani gauges
‐ 4 cold cathode gauges
‐ 2 power supplies for the gauges
‐ 4 residual gas analyzers (RGA)
‐ 1 baking equipment for the vacuum chamber
‐ 4 angle valves manually actuated for pre-pumping (pipe diameter DN63)
lsquo
124
Chapter 7 X-RAY BEAM LINE AND EXPERIMENTS
Introduction
The equipment of the X-ray beam line should firstly ensure a continuous detection and monitoring of
the beam (mainly in the first optical hutch) and secondly allow a complete characterization of the
beam (in the experimental hutch) This characterization consists in measuring the beam absolute
intensity its total energy range its spectral and angular distributions and its degree of transverse
coherence Time analyses of the bunched X-ray beam will also be performed This line also aims at
confirming the ThomX assets in such fields as cultural heritage biomedical and materials sciences by
performing demonstrative experiments
71 Available extracted X-ray beam
The X-ray beam is extracted through a 50 mm diameter beryllium window as schematized in Fig 71
Because of limitations due to the dimensions and the location of the Fabry-Peacuterot cavity mirrors the X-
ray beam available for characterization and experimentation is a cone with a half-aperture of 16 mrad
Figure 71 The X-ray beam extracted from ThomX through a beryllium window The beam fills a cone
whose total aperture is 2 x16 mrad
In the case of a lsquoperfectrsquo beam (ie without taking into account the electron beam emittance) the X-ray
spectral and energy distributions are shown in Fig 72 as a function of the scattering angle for two
electron beam energies 50 and 70 MeV and a laser wavelength of 106 microm In Fig 73 the energy
bandwidth and the flux per unit solid angle are shown for on-axis Compton photons produced by either
a 50 MeV or a 70 MeV electron beam
72 Global view of the X-ray lne
As can be seen in Fig 74 the positioning of the machine in the Igloo imposes the splitting of the X-ray
line into two sections a first optical table (3x2 m2) located just after the beryllium exit window and a
second table (4x2 m2) located outside the Igloo in hall D1 Along its path and until the optical table 2
the beam is transported in the primary vacuum inside stainless steel pipes with the exception of a short
upstream section of a few tens of cm between the beryllium window and the optical table this
insulating section will prevent the transmission of harmful vibrations to the Fabry-Peacuterot cavity This
primary zone is kept oxygen free by a nitrogen flow that prevents ozone production
lsquo
125
The beam diameter at the entry of the first optical table is 45 cm After this first optical table the beam
reaches the second table via a 13 cm diameter beam pipe inserted in a hole which was drilled long ago
in the Igloo wall About 8 mrad beam angular divergence are preserved at the entry of the second
optical table where the beam diameter is 13 cm
Figure 72 X-ray spectral distributions (left panel) and X-ray energy (right panel) versus the scattering
angle for an e- beam of 50 MeV (black) and 70 MeV (red) The laser wavelength is 106 microm Grey
areas correspond to emission angles larger than 16 mrad
Figure 73 X-ray flux (left panel) and X-ray energy bandwidth (right panel) of on-axis photons per unit
solid angle versus the emission angle for an e- beam of 50 MeV (black) and 70 MeV (red) The laser
wavelength is 106 microm
lsquo
126
Figure 74 Global view of the X-ray line in ThomX
73 Chraracterization of the X-ray beam
731 Optical table 1
A global view of the optical table 1 design is presented in Fig 75 Just upstream of this optical table a
60 mm diameter beam shutter activated when one needs to take background data will allow stopping the
beam A few millimeter thick tungsten is sufficient to stop the full beam (tungsten attenuation mass
factor microen ~ 4043 cm-1
for 100 KeV photons) This shutter does not have to be cooled since the ThomX
radiated power will not exceed 100 mW A drawing of the shutter mechanical design is shown in Figure
76 (a)
Figure 75 Global view of the optical table 1 showing the location of the main devices placed on it
lsquo
127
Beam monitoring
The optical table 1 equipment (see Fig 75) is mainly devoted to the detection and the continuous
monitoring of the X-ray beam The first device monitoring the beam presence is a fluorescent screen
mounted on a two-position motor (up or down) as shown in Fig 76 (b) It is inserted in the beam path
and viewed by a CCD camera Its screen absorbs almost the totality of the beam and thus has to be put
in the up position when not in use Downstream of the fluorescent screen a device composed of two Si
PIN diodes measures the beam scattered by a thin Kapton foil inserted in the beam path This setup
shown in Fig 76 (c) will monitor the beam flux variations and beam position displacements The
Kapton foil is fixed at an inclination angle of about 60o with respect to the beam path The sum and the
difference of the two diode currents will provide a relative measurement of the beam flux and of the
beam position respectively This set-up which is nearly transparent to 20-90 keV radiations is a non-
intrusive monitor and can be used continuously Then a high precision double slit [71] intercepts the
X-ray beam and allows one to let through only the desired part of the Compton cone (see Fig 76 (d))
The minimum dimensions of the slit aperture are 10 microm in the horizontal and the vertical directions
Beyond the double slit a wire monitor [72] shown in Fig 76 (e) is inserted in the beam This device
consists in two tungsten wires stretched horizontally and spaced by a few tens of a millimeter This
monitor will be driven vertically through the beam The difference of the two wire signals allows the
measurement of the absolute beam position
Figure 76 Drawings of the X-ray line devices located on the optical table 1 (a) Beam shutter
(b) Motorized two-position fluorescent screen (c) Kapton foil and Si diodes (d) Double slit (e) Wire
profiler
lsquo
128
Focusing device
Downstream of the beam monitoring instruments a focalizing device will be installed (see Fig 75) to
make the X-ray beam parallel andor to focus it on a sample It can be either a device based on
compound refractive lenses [73] (a so-called lsquotransfocatorrsquo) or two bendable mirrors in the Kirkpatrick-
Baez configuration (KB Optics) or a capillary optical device These three techniques will be
investigated to evaluate the advantages and disadvantages (aperture efficiency focusing strength etc)
of their eventual implementation in the ThomX facility Such a focusing apparatus has to be located as
close as possible to the X-ray source because of the rapid growth of the beam transverse dimension
Finally at the end of the optical table 1 some space is kept free for any specific study which might
concern optics detectors or samples
732 Beam transport
As shown schematically in Fig 77 beyond the optical table 1 the X-ray beam crosses the Igloo wall
inside a beam pipe that runs through a 15 cm diameter hole and reaches the D1 hall where the second
optical table is installed in the lsquoexperimental hutchrsquo At the entrance of the wall hole a second beam
shutter in lead around 15 cm in diameter and 25cm thickness is installed This shutter is connected to
an interlock system that ensures radioprotection It absorbs radiations coming from the electron
machine which propagate through the Igloo cylindrical hole and allows X-ray users to work in the
experimental hutch without stopping the electron machine
Figure 77 Schematic of the X-ray beam transport across the Igloo wall from the optical table 1 located
in the Igloo until the optical table 2 located in the D1 hall
733 Optical table 2
Several kinds of instruments will be placed on the optical table 2 The goal is to study in detail the X-
ray beam produced by ThomX firstly to determine how the beam characteristics depend upon the
machine operation parameters and secondly to prepare the demonstration experiments A global view
of the optical and detection instruments placed on that table is provided by Fig 78
lsquo
129
Figure 78 Global view of the optical table 2 indicating the main instruments that will be used to
characterize the beam
Absolute and spectral flux measurement
The measurement of the X-ray beam absolute flux and of its energy spectrum will be performed by
using two detectors a calibrated Si diode [74] and a cooled high-purity Ge solid-state detector (SSD)
[75] The Si diode will be bought already calibrated [76] A lsquogoodrsquo Ge SSD has an energy resolution
(ΔEE) around 1 at 20 keV 06 at 45 keV and 045 at 90 keV [77] The energy calibration of the
Ge SSD will be carried out in the X-ray beam line by using the Kα fluorescence lines of reference
materials (see Table 71)
Element Ge Mo Sn Cs Ce
EKα1 (keV) 9886 17479 25271 30973 34719
EKα2 (keV) 9855 17374 25044 30625 34279
Element Eu Er W Pt Pb
EKα1 (keV) 41542 49128 59318 66832 74969
EKα2 (keV) 40902 48221 75981 65112 72804
Table 71 Kα fluorescence lines of chemical elements used to calibrate the Ge SSD in the 10-90 keV
energy range
The two detectors will be mounted alternately on a (X Y) translation stand to measure a large number
of points of the emission spectrum
- the mean emission energy E (Ge SSD)
- the energy spread ΔE (Ge SSD)
- the absolute flux (photon numbers) (Si diode)
These beam characteristics will be measured as a function of the size of the selected X-ray sample the
selection being carried out with a double slit device (see Fig 76 (d)) Our goal is to check
lsquo
130
experimentally the theoretical angularspectral distributions shown in Fig 12 of the Introduction by
measuring the number of Compton photons per second per mrad2 and per 01 of energy bandwidth
Time analysis of the bunch structure
The X-ray source has a time structure which involves two main scales a 50 ns period which is the
interval between two successive collisions between the laser pulse and the electron bunch and the 20
ms e- storage cycle To perform a pulse by pulse analysis we will use a high resolution time detector
(~1-5 ns) with a high repetition frequency (~10-100 MHz) eg a micro-channel plate (MPC) detector
[78] or an avalanche photo-detector (APD) Such time measurements have already been performed at a
similar facility by Sakaue et al [79] MCP Hamamatsu devices can reach a 07 ns (FWHM) time
resolution which is good enough not only to resolve each one of the collisions between the laser pulse
and the electron bunch but also to measure their relative intensity accurately from the beginning of the
e- storage cycle to the end These measurements will be performed on the X-ray beam axis They will
have to be synchronized with the ThomX clock
Transverse coherence of the beam
Some simple diffraction and interference measurements will be performed in order to analyze the
source spatial coherence The diffraction of the X-ray beam by a slit of a known aperture or the
interference effect produced by the insertion of a phase-shifting object in the beam allows one to
quantify the transverse coherence of the beam [710] In these experiments the on-axis photon intensity
distribution will be recorded with a 2D high resolution detector (such as for instance an XPAD CdTe
camera with 130 microm pixels) This measurement will be repeated at several positions along the beam
The pattern analysis will inform us about the degree of coherence of the source A monochromator will
be inserted upstream of the diffractive slit or the phase-shifting object to select a well defined energy
Later on Young double-slit experiments could also be carried out [711] to measure the beam
coherence length beam
74 Demonstration experiments
In parallel with the complete characterization of the ThomX beam described in the previous
sections some simple experiments will be performed in order to demonstrate the potentialities
of the ThomX source In this phase we will focus on medical and materials science
applications An important aim of the ThomX project is to perform medical and materials
imaging exploiting the high and tunable energy of its beam A second aim of the ThomX
project is to show its capability in the structural investigation of materials using X-ray
diffractionscatteringspectroscopy techniques
741 Experiments using the full X-ry beam
Three experiments issued from the expertise of the ESRF-INSERM teams in medical applications of
synchrotron radiation have been chosen to be carried out as demonstrators on the ThomX prototype
The goal of setting up such experiments is to demonstrate (i) the suitability of ThomX of performing
both imaging and therapy experiments (ii) the tenability of the source adjustable at energies
corresponding to the K edge of high Z elements routinely used in medicine (Gd I) or of those of new
experimental radiotherapy adjuvants (Au Gd nanoparticles CisPt) (iii) suitability of the source for
phase contrast imaging (iv) exploitation of the advantage of using a 2D beam with respect to the typical
laminar synchrotron X-ray beam Some imaging and therapy experiments in the biomedical field
require a wide 2D beam Using the natural divergence of the ThomX beam a sample of a few
lsquo
131
centimeters in diameter located at 10-20 meters from the ThomX interaction region can be fully
illuminated by the a conic beam with no need of optical elements Calculations have been performed in
order to show the potential of using the ThomX source in comparison with a hospital X-ray generator
(fluoroscopy unit type) as well as in comparison with the beam from the biomedical line ID17 at ESRF
Results of these comparisons are given below for each medical experiment we intend to perform with
ThomX
An imaging experiment
For imaging experiments the energy range of the beam has to around 60-90 keV According to Fig 73
the corresponding half-aperture will be 5 mrad leading to a photon flux of 61012
phs When the beam
reaches the sample stand placed on Table 2 at 20 m from the ring interaction region it will have a
diameter of 20 cm This conical beam will then have a cross section of ~300 cm2 and permit
tomographic acquisitions of a full human head using a 2D detector This wide 2D beam is a major
advantage of the ThomX source for biomedical imaging with respect to the laminar synchrotron
radiation beam The dose rate (DR) in Gys can be expressed as
(71) DR = 16 10-13
Ф[phscm2] E[keV] (microenρ)[cm
2g]
where Ф is the photon flux per cm2 E is the mean energy of the beam and (microenρ) is the mass
attenuation coefficient For an X-ray beam of 75 keV (ie the average of our 60-90 keV energy range)
the mass attenuation coefficient microenρ for soft tissues is 0028 cm2g leading to a dose rate of 67 mGys
delivered to a 300 cm2 area The corresponding expected lsquodose area productrsquo (DAP) of the ThomX
source for such imaging experiments is DAP = 20 Gyscm2 For comparison at the ESRF ID17 line a
dose rate of 800 mGys is delivered to an area of 150x2 mm2 corresponding to a DAP of 24 Gyscm
2
As far as conventional sources are concerned typical laboratory devices used in 2D fluoroscopy deliver
a DAP of 06 Gyscm2 [712]
A feasibility experiment will be performed by using a standard human head phantom (see Fig 79)
which will be mounted on a rotating stand that allows a 360deg rotation It will be possible to translate this
stand in three directions (along the X-ray beam axis and in the two other perpendicular directions) to
optimize the sample position in the beam Acquisition will be performed using the 60-90 keV pink
beam before and after the insertion of a high-Z element solution (typically iodine or gadolinium) in a
slot available in the phantom head Several images will be acquired using different concentrations of
high-Z element chosen in the range of concentrations that can be typically reached into a tumor (in
humans the high Z element is injected intravenously) The study will also aim at finding for each high-
Z element the lowest concentration that can be measured with the ThomX imaging system
Figure 79 Typical phantom featuring a human skull from X-ray absorption point of view
lsquo
132
The detector will be placed downstream of the sample A typical detector that may be used is a large
field of view 2D detector equipped with Si or CdTe sensors with ~100-200 microm pixels [713] Finally a
standard computed tomography (CT) algorithm will be used to reconstruct the tomography images and
to calculate the concentrations (in mgml) of the given high Z element in the skull The results will be
compared to previous work done at the ESRF [714]
A radiotherapy experiment
For this experiment the targets will be rats implanted in their right brain hemisphere with rodent brain
tumors (F98 9L) at an appropriate tumor stage (typically at D10 or D14 after implantation) For the
irradiation we need a pink beam with a 10 keV energy bandwidth and a median energy above the K-
edge of the high-Z element to be injected as a radiosensitizer (ie in the 45-80 keV range for both
iodine and gadolinium) The beam size will be a few centimeters in diameter Figure 710 shows the X-
ray beam energy and beam flux generated by a 69 MeV electron beam versus the photon back-
scattering angle X-rays of energy 80 keV within a 10 keV bandwidth are contained in a cone of half-
aperture 27 mrad and their flux is ~31012
phs At the sample location on optical Table 2 at 9 m from
the ring interaction region the beam has a diameter of 5 cm ie a cross section of ~20 cm2
The center of the ratrsquos right hemisphere will be placed on the rotation axis of the irradiation system and
slits will limit the beam transverse extension in order to define a precise field of irradiation Assuming
an X-ray flux (Ф) of 31012
20 phscm2 with an average energy (E) of 80 keV a mass attenuation
coefficient of 0025 cm2g (soft tissue 80 keV X-rays) Eq (71) leads to a dose rate DRe of 48 mGys
close to the tissue region where the X-ray beam penetrates Before reaching the tumor the radiation is
attenuated by the bone (~ 02 cm thickness (microρ)att = 022 cm2g ρ=192 gcm
3) and by the skull healthy
tissues (~ 15 cm thickness (microρ)att = 018 cm2g ρ = 10 gcm
3) Thus the dose rate DRt delivered at
the tumor is DRt = TDRe (where T = exp (-02219202) exp (-0181015) = 070) and is
expected to be 337 mGys The dose will be delivered while the rat undergoes a 360deg rotation The total
dose to be delivered to a tumor ranges between 10 to 20 Gy Therefore according to the expected rate
of 337 mGys the irradiation will take typically from 5 to 10 minutes with the big advantage of not
having to move the rat A dosimetry protocol (such as the one used at ID17 [715]) will be set to check
the homogeneity of the irradiation and to measure the absolute dose with a 2 to 5 uncertainty
Figure 710 X-ray energy (left panel) and X-ray flux (right panel) versus the photon back-scattering
angle for stored electrons of 69 MeV The laser wavelength is 106 microm
For humans and a tumor placed at 8 cm into the skull the transmission coefficient becomes T= 019
Therefore the expected dose rate DRt delivered by the ThomX source at the tumor is 91 mGys One
may compare with the ESRF ID17 beam-line where 800 mGys can be delivered for irradiation on a
lsquo
133
surface of 2x50 mm2 ie with T=019 a dose rate DRt of 015 Gys on 2 mm in height at the tumor
which is equivalent to a rate of 6 mGys to cover 5 cm of height with a scanning rate of 50 mms
ThomX could therefore deliver doses for human skull irradiation larger than those available at
synchrotron radiation facilities and also perform the irradiation without having to move the irradiated
sample Concerning the conventional hospital devices they deliver low doses deposited by X-rays
whose energy spectrum is broad and moreover they are not made to operate continuously for such long
time irradiation
Also the tunability of the ThomX beam will allow testing several high Z drugs containing iodine
gadolinium platinum or gold proven to be promising radiosensitizers [716]
Conformal radiotherapy
A final step in this sequence is performing conformal radiotherapy experiments ie doing both imaging
and therapy at the same time In fact one major issue of radiotherapy is to spare as much as possible
healthy tissues located around the tumor and at its border In this regard it is a key issue to adapt the
field of irradiation to tumor geometry size and positioning by performing first a tomography
Phase contrast imaging
The absorption medical imaging technique is based on the fact that different parts of a sample absorb X-
rays differently But the fact that X-ray absorption by biological material is weak constitutes also the
limitation of this technique To enhance or replace absorption-based imaging refraction diffraction and
interference of X-rays can also be used A better visibility at the edge of different regions of a specimen
can therefore be obtained by using the information contained in the phase of an X-ray wave For this a
spatially coherent X-ray source is required and was available only since the development of modern
synchrotron sources Experimentally the simplest way to visualize phase-contrast imaging is provided
by the propagation-based imaging (PBI) method In this method a spatially quasi-coherent radiation
illuminates an object which gives rise to a spatially varying phase shift in areas with spatial variations
of the refractive index As the radiation propagates beyond the sample parts of the wavefront which
have experienced different deflections on both sides of an edge of the sample produce a serie of fringes
due to the classical Fresnel-edge diffraction phenomena [717]) This gives rise to a characteristic phase
contrast pattern which is then recorded by a 2D detector placed at a proper distance downstream of the
object (see Fig 711 left panel) The actual phase-contrast signal is convolved with the detector point
spread function which has to be sufficiently narrow to avoid the smearing of the interference pattern
Additionally the phase contrast signal can be visualized if and only if the transverse coherence is
sufficient ie if Lcoh = λRs is larger than the details to be imaged (here R is the source-sample distance
and s is the source size) In contrast the requirements in terms of longitudinal coherence
(monochromaticity) are rather weak even a beam with a bandwidth of several keV does not lead to a
significant smearing of the phase contrast signal [717]
Assuming s = 70 microm R = 15 m a sample-detector distance of 5 m and a detector point spread function
of 50 microm the phase contrast signal expected when the ThomX beam illuminates a nylon wire of 300
microm diameter ranges from 10 to 3 when the X-ray energy increases from 20 to 80 keV (see Fig
711 right panel)
lsquo
134
Figure 711 Left panel The set-up for propagation-based phase contrast imaging Right panel The
simulated signal produced by a nylon wire of 300 microm diameter illuminated by the ThomX source
(simulation)
The experiment with ThomX will comprise two parts a planar radiography and a computed
tomography For the radiography three kinds of samples will be used (i) a custom-made phantom
composed of nylon wires of different diameters (ii) the ACR (American College of Radiology)
accredited phantom used in mammography quality (iii) a 4 cm thick formalin-fixed human breast tissue
obtained from a hospital bank The X-ray energy will be chosen in the 20-80 keV range For each
sample and for several energy values images will be acquired at two different sample-detector
distances in the absorption regime for comparison purposes (the detector will be in contact with the
sample) and at 5 m A silicium detector with 50 microm pixels will be used Images will be acquired for
different doses (measured with a standard PTW dosimeter) in order to obtain the signal-to-noise ratio as
a function of the dose These measurements will allow one to qualify the phase contrast imaging set-up
at ThomX
Computed tomography will be applied to (i) a meacutethyl polymeacutethacrylate (PMMA) custom-made
cylindrical phantom containing insets of different compositions and densities (water ethanol paraffin
etc) (ii) a formalin-fixed human breast tissue (5 cm in diameter) Images will be acquired using the
same parameter values as the ones chosen for the radiography Samples will be installed on a rotating
stand positioned on a double cradle that will permit an alignment with the beam and with the detector
Images will be taken during the sample rotation over 360 degrees
The final step will consist in applying standard algorithms to the images in order to calculate the phase
and to extract quantitative information about the samples This data will be compared with equivalent
images that will be acquired at the ESRF
742 Experiments using the central part of the beam
Most specimens from cultural heritage and functional materials are heterogeneous these lsquorealrsquo
materials often exhibit complex hierarchical structures involving various levels of order and
heterogeneity To understand the properties of such materials better one needs at least a description of
their structure at the atomic scale and at the macroscopicmesoscopic scale which takes into account
weak structural signals (relating to disorder minor constituents microstructure hierarchical orderhellip)
lsquo
135
This requires much probing as already demonstrated with synchrotron radiation sources A major goal
of materials science is to link global properties to local atomic features (orientation strain structural
distortion grain size) of some components in relation with neighboring domains and to deduce from
that the macroscopic properties As the functionalities of these materials often stem from the association
of different phases or constituents or of specific inter-relation between them (microstructure) highly
selective structural probes need to be used to characterize them Also in the particular domain of
cultural heritage many specimens require non-destructive methods and probes In order to understand
how they were made and to preserve them from degradation over time their physical and chemical
properties must be determined There is an increasing demand for material probing with complementary
techniques Fluorescence emission techniques excel in revealing the elemental composition of the top-
most layers of samples Diffraction is suited for studying quantitatively the phase composition the
structural details of the constituent phases and the characteristic distribution of sizes and orientations of
crystallites Tomography techniques excel in revealing the 3D mapping of the chemical andor the
density heterogeneities within a sample This is why the cultural heritage community is today strongly
involved in the development of new instruments that may characterize patrimonial artifacts by mapping
not only the chemical composition of a variety of samples but also their physical phases
Experiments described in section 741 require a large 2D beam and exploit the natural divergence of
the ThomX beam In this section diffraction scattering and spectroscopy experiments require a
focalized or a parallel beam and it should also be monochromatic or lsquopinkrsquo The optics will therefore
select the central part of the beam ie its higher energy part Let us give an order of magnitude
assuming a perfect 50 MeV electron beam a collimator of 2 mm diameter located at 2 meter from the
IP will cut the X-ray conic beam at 05 mrad leading to a flux of ~8 1010
phs with an energy of 45 keV
and an energy bandwidth ΔEE on the order of 10-3
(see Fig 73) In practice this bandwidth will be
broadened and will reach a few percent because of the actual e- beam divergence and energy spread For
experiments requiring a narrower energy bandwidth an additional selection will be performed with a
monochromator For instance selecting an energy bandwidth of 10-4
would lead to a flux on the order
of 109 phs
Since the goal of the first series of demonstration experiments is to qualify ThomX as an X-ray source
these experiments will be performed with samples previously characterized with other sources such as
synchrotron radiation sources
Diffraction and scattering
Diffraction and scattering experiments are major ways of performing materials structural and micro-
structural analyses This concerns highly ordered samples as well as powders or ill-ordered samples
Complete scattering analyses are based not only on Bragg diffraction but also on diffuse scattering in
order to look beyond the average structure and to investigate the local structure or short-range order
The crystalline quality of powders depends on the preparation methods Grain sizes vary from about 10
nm to several hundreds of microm In the case of nano-materials Bragg peaks are strongly broadened and
X-ray diffraction performed with standard laboratory sources does not provide suitable data for
structural investigations As illustrated by Fig 712 laboratory X-ray sources do not allow the
identification of minor phases in a poorly crystallized powder Figure 713 shows an example of a
diffraction pattern obtained with an excellent resolution The sample used for this analysis is an
Egyptian cosmetic powder dated to 2000-1200 BC The patterns were obtained with the ESRF
synchrotron source (bottom panel) and with a classical laboratory source (top panel)
lsquo
136
Figure 712 Powder X-ray diffraction patterns obtained by using a laboratory X-ray source Top panel
Poorly crystallized Prussian blue sample Bottom panel Prussian blue crystals From [718]
Figure 713 Powder X-ray diffraction patterns obtained by using a laboratory X-ray source (top panel)
and the ESRF synchrotron source (bottom panel) Arrays indicate minor phases From [719]
We intend to use the ThomX source to record diffraction patterns and scattering plots in order to
characterize short-range ordering and identify minor phases in such heterogeneous samples
For the first demonstration diffractionscattering experiments we will study ill-ordered Prussian blue
and white lead samples Pigments used for paintings or frescos have various compositions Since their
usage followed a definite chronology such pigments can sign an era Pigment characterization can also
help the undertaking of a restoration while a thorough knowledge of pigment synthesis through
centuries can help to apprehend the degradation mechanisms better
lsquo
137
For ordered samples diffraction will be performed with a single crystal by using a monochromatic
beam but also by using a lsquopink beamrsquo as done in the Laue method For ill-ordered samples the whole
scattering pattern will be recorded and analyzed with the Pair Distribution Function (PDF) method This
analysis technique (often called the ldquototal scattering methodrdquo) can be used alone or associated with
diffraction For experiments using the lsquopink beamrsquo Laue method the beam will be focused directly on
the sample (by using convergent refractive lenses (CRLs) or a capillary optic system as mentioned in
Section 731) To determine powder crystalline parameters diffraction analyses require the
monochromatization of the beam in order to reach ΔEE ~ 10-4
In the optic hutch (optical Table 1) the
diverging beam will first be made parallel by the CRLs or the capillary optic method and then
monochromatized in the experimental hutch (optical Table 2) by a monochromator The set-up for these
measurements needs a goniometer to align the sample and a low noise 2D-detector sensitive to high
energy photons
Fluorescent spectroscopy
X-ray fluorescence spectroscopy experiments allow one to determine the chemical composition of
materials and the mass concentrations of elements Indeed depending on the atomic number of a given
chemical element energies of the fluorescence lines emitted after an X-ray excitation are a signature of
the element An illustration of this approach is presented in Figure 714 where the fluorescence
tomographic images of a Globorotalia inflata specimen are shown and compared with a classical
radiography image
Figure 714 Fluorescence tomographic imaging (a) Set-up (b) Radiography (c) Fluorescence
spectrum measured at a given angleposition of the sample (d) 3D tomography reconstruction From
[720]
We plan to carry out fluorescence experiments since the energy of the ThomX beam can reach the
excitation energy of most chemical elements usually involved in material science for instance the
critical excitation energy of lead regarded as a heavy element is 88 keV) For this experiment the
excellent energy resolution of cooled Ge detectors will be an asset to detect the fluorescence light (the
detectors will be oriented at 90o with respect to the incident beam)
lsquo
138
K-edge and XANES spectroscopy
In K-edge experiments two images of a given element present in some sample are performed one at an X-ray
energy just below the K edge of the chosen element and the other at an energy just above Then they are
subtracted (see Fig 715 left panel) The K-edge imaging technique is used in particular in the medical and
the cultural heritage domains In XANES (X-ray Absorption Near Edge Structure) the absorption by a
material is measured over an energy range of about 100 eV on both sides of the absorption edge This
technique provides information about the local structure prevailing close to a particular chemical element (see
Fig 715 right panel)
Figure 715 Left panel K-edge subtraction imaging Right panel Principle of the XANES method
The first XANES spectroscopy experiments with ThomX could exploit the K-edge of heavy elements
such as W Pt or Pb By first selecting the central part of the ThomX beam with an energy spread of a
few 10-2
a monochromator could then provide a beam with an energy spread of 10-4
Samples will be
aligned by using a goniometer and the transmitted intensity will be collected by a low noise 2D-
detector
Combined diffractionabsorptionfluorescence tomography
A major limitation of diffraction methods comes from the fact that only average properties can be
obtained while most lsquorealrsquo materials are heterogeneous and complex Thus structural studies of such
materials are difficult to carry out A new tool for structure mapping of materials by means of X-ray
diffractionabsorptionfluorescence combined with tomography allows one to perform non-destructive
3D structure selective mappings of heterogeneous samples This way one may identify minor phases
and determine crystallite orientation and grain size Furthermore the selectivity of the method allows
one to separate and locate the origin of the diffuse or diffracted patterns to identify amorphous phases
In archaeology such complementary micro-structural analyses could shed light on the mechanical
history of artifacts
A recent experiment carried out with a micrometer beam at synchrotron radiation sources have
established the validity of such a local probe by analyzing a mixture of chalcedony and iron pigments
containing haematite [721] This experiment illustrated in Fig 716 consists in (i) performing a
structure tomography mapping directly from the diffraction data (top panel) (ii) obtaining selective
structure patterns of unknown minor phases from these maps (bottom panel) This study was realized by
the Neacuteel institute and the C2RMF and was one of the first diffraction tomography experiments
performed with a micro-beam
With the ThomX source we will use poorly crystallized samples and combine in the same experiment
and simultaneously tomography diffraction with fluorescence and absorption measurements Diffraction
lsquo
139
tomography is beam-time consuming it will be performed to demonstrate its feasibility at ThomX The
experimental set-up will be similar to the one used for multimodal analyses (cf the top panel of Fig
716) The sample will rotate and this rotation will be combined with a translation to provide a 2Θ scan
of 2D diffraction patterns obtained with a 2D detector sensitive to high energy photons The diffracted
2D fluorescence patterns will be recorded with our Ge detector while the sample absorption will be
recorded with our calibrated Si diode
Figure 716 Top panel Experimental set-up for multimodal spectroscopydiffractiontomography
analysis Insets Images of the chalcedonyndashFe mixed-pigment sample are reconstructed by combining
the various signals X-ray absorption (bottom) Fe X-ray fluorescence (top left) and chalcedony
diffraction (top right) Bottom left panel Reconstructed diffraction image based on the whole
diffraction diagram (black) compared with the calculated pattern for pure chalcedony (red) Bottom
right panel Reconstructed image that only uses the iron pigment scattering contribution (black)
compared with the calculated pattern for hematite and siderite grains (red) From [721]
75 Summary
As shown in this chapter two types of beams will be available at the ThomX compact X-ray source
(i) wide beams well adapted for imaging (conventional or phase contrast imaging) and therapy in the
biomedical field and (ii) almost monochromatic beams that can be either focused or made parallel for
diffraction and spectroscopy experiments in the field of materials science
The X-ray line is designed
- to ensure a continuous monitoring of the X-ray beam
- to measure the beam brightness and its transverse coherence
- to demonstrate the potential of this new type of compact source and identify the benefits it can
bring to the currently available X-ray sources
lsquo
140
Chapter 8 CONTROL SYSTEM
Introduction
The ThomX control system combines those hardware and software elements involved in the
management of all the facility components But more than a system dedicated to carrying out operating
processes (processes launched from the X-ray beam control room in order to undertake some specific
task such as a performing an imaging experiment or a material chemical analysis) it must also be an
advanced tool for various studies (data processing new equipment integration operating mode
evolution) and scientific experiments which will be performed with the X-ray beam line Hence the
control system has to be a versatile tool easily scalable by various users (scientists operators IT
technicians)
In the following a lsquoSub-systemrsquo is a physically connected set of hardware items sharing a global logical
function The ThomX sub-systems are as follows the linac (LINAC) the transfer line (TL) the ring
cell 1 (RI-C1) the ring celle 2 (RI-C2) the optical cavity (OC) the extraction line (EL) the section
of the X-ray line located in the Igloo (XLI) and the section of the X-ray line located in the X-ray hall
(XLH) A lsquoDomainrsquo is a set of hardware items related by a common purpose The ThomX domains are
as follows alignmentmetrology (AM) control-command (CC) diagnostics (DG) infrastruture (IF)
magnetic elements (ME)n optics (OP) pulsed elements (PE) power supplies (PS) radio-frequency
(RF) safety (SA) synchronization (SY) and vacuum (VA)
81 General principles
Some general principles drive the design of such a control system In broad terms the following criteria
will serve as guidelines
- Since monitoring is a major role of a control system comprehensive monitoring interfaces must be
provided to all users Moreover the monitoring system must take into account sub-systems specificities
dedicated GUIs must be designed for every need for instance in synoptic form to give a global system
status overview or as charts to show the time evolution of some specific parameters of a particular
device (eg pressure readings on TPG300)
- Operating processes management a control system also aims at the prioritization and sequencing
of processes It has to provide high-level functions in order to remotely launch complex operations (eg
machine startstop interlockshellip) Computing and data analysis tools which could launch such particular
procedures will be integrated or developed to maximize the automation of the machine
- Default management the facility control system must be able to manage operation defaults It will
be necessary to design graphical tools to quickly identify defaults when they occur Color display of
warnings and errors according to a criticality-based convention will be implemented in addition to the
recording of running process logs
- Safety for obvious reasons the ThomX radioprotection and safety system must standalone It should
not use any data generated by the control system Nevertheless each end-user must be able to make
sure that the ThomX facility does not present any danger whenever the accelerator is running This
information must be available in every control room through a continuous display of all the safety
equipment status (radioprotection sensors door openingshellip) A full historical record of safety-related
lsquo
141
events will be made available for analysis and a procedure of detailed acknowledgement (operatorrsquos
name action to close an incident) will be defined in order to guarantee safe operations
- Ease of use The ThomX control system is intended for various usages therefore it will concern
various users including operators who are not familiar with software development Hence it is crucial
to make it modular through relevant to sub-systems GUIs with various visualization interfaces control
buttons graphs sub-system overviews in synopsis or detailed display of subset status The definition of
new interfaces will be possible through visual development frameworks to fit new needs
82 Technical considerations
In order to guarantee a proper accelerator operation to make its use as easy as possible and to limit
costs some technical points have to be taken into account
- Distributed architecture an accelerator is a complex structure which results from various designs in
different fields The hardware part of the control system relies on dedicated IT devices (industrial PCs
programmable logic controllers) whose role is to drive sub-systems Each of these nodes will be fully
and remotely controlled by the system software from the control rooms (or from any authorized user
terminal in the ThomX network) independently of possible other issues All of these components must
communicate with each other when required by the operation needs Moreover the supervision part of
the control system must be available in different places (main control room X-ray line control room
perhaps others) The chosen system must meet these requirements and must run on several terminal
types
- Standardization of the hardware items and of the software codes is necessary in order to simplify
their integration in the control system (eg same brandmodel for all the stepper motors) Then it will be
possible to mutualize high-level controlling tools whether pre-installed or homemade Additional
benefits of this approach concern the overall maintenance the deployment of new releases and the
replacement of hardware parts
- Reliability The service continuity of the control system must be guaranteed A spare strategy must be
set up to make sure that the replacement of defective parts will be done on short notice In particular
storage memory redundancy will guarantee that none of the machine parameters or of the collected data
will be lost Periodic backup of main software systems (device databaseshellip) will be performed right
from the start Uninterruptible power supplies are foreseen to protect major elements of the control
system from electrical failures
- Upgradability Special attention will be paid to communication features when choosing hardware
devices Ethernet is now one of the major protocols and must be preferred when possible For the PLC
driven devices the fieldbus Profibus [81] is recommended since one single master interface can
manage more than 100 slave devices without data losses This product is also valuable as far as cabling
cost is concerned
- Safety Since several users may wish to perform various machine-related tasks (integration
maintenance measurementshellip) a policy must be established to control the access to the ThomX
network and its control system The management of these access permissions will be provided It will
take into account the role and the field of expertise of each project member
lsquo
142
83 The TANGO control system software
Tango [82] is the software chosen to be the backbone of the ThomX control system It is an open-
source software initially developed at the ESRF It is now used in several Institutes to control huge
accelerator facilities like the SOLEIL synchrotron In fact the SOLEIL staff is playing a major role in
maintaining and developing new Tango features
831 Software bus
An accelerator equipment is usually highly heterogeneous Thus its control system must enable various
devices to communicate with each other even if they use different protocols or communication
standards like Ethernet RS232 USB PCI DAQ Cardshellip
The first asset of Tango is the communication standardization that it provides between any type of
devices Tango implements a CORBA-based communication protocol so that a common network is
virtually created for the whole hardware If needed the parameters of a given device are thus available
to every other element of the control system This is achieved by adding an extra Tango-compliant
software layer above the hardware low-level libraries This layer is specific to a device brand and model
and is a simple executable file which can be instantiated as many times as required by the number of
devices This key element of the Tango system is called a lsquoDevice serverrsquo
832 A control system for accelerators
For basic reasons the Tango core and many of the tools developed around this software bus are open
source tools It is therefore an accessible system fully customizable for specific ThomX needs and a
system which can be maintained Since Tango is Linux and Windows compliant issues originating in
driver availability for the Linux OS can be avoided while Tango enables the use of scientific
applications which are often developed for Linux
Furthermore Tango is developed and maintained by a community which involves several accelerator
Institutes Even if Tango is not an accelerator-dedicated tool a number of developments have been
carried out or are being carried out that deal with devices currently used at such facilities Thus a large
Tango library of device servers is presently available to any new user
Finally more than a simple tool which provides a network access to a set of devices Tango is also a
framework that provides high level operational services such as configuration tool logging archiving
services and alarms permission management For developers specific tools are provided for the
generation of codes that fit in the common structure required by all device servers Other tools allow
easy GUI development As a result the integration of new devices or the upgrade of control and
supervision windows do not need in-depth coding As far as the compatibility with commercial software
is concerned dedicated bindings can be freely used eg applications developped for Matlab or
Labview can be used to perform a variety of tasks involving the ThomX devices
lsquo
143
84 Control system architecture
841 Equipment to control
On a complex machine like ThomX a wide range of devices and control signals must be taken into
account motors cameras scopes diodes or valves will deal with analog and digital inputsoutputs
complex signals transmitted through GPIB USB Ethernet or profibus interfaces
The inventory of devices to be controlled is presented in the following table Safety equipment is not
mentioned as it is a fully standalone sub-system
LINAC TL Ring OC XL
Power Supplies
Coils 2
Steerers 2 4 24 2
Dipoles 5 4 8 3
Dipole chicane 1
Quadrupoles 7 24 3
Sextupoles 12
Pulsed elements
Septum 1
Kickers 2
Synchronization
3GHz synthesizer 1
500 MHz synthesizer 1
Phase shifter 5
Programmable delays 2
Counter 1
Vacuum
Ion pump 8 4 17 4
Gauge 8 8 22 8 1
Valve 4 4 4 2
Primary pump 1
Diagnostics
BPM 1 4 12
Low frequency Signal
generator 2
ICT 1 2
Stepper motors 1 3
Camera CCD 2 3 1
RF
Klystron coil 1
Modulator 1
Amplifier 1
Canon 1
Feedback 2
Source laser
Photodiode 1
Camera CCD 1
lsquo
144
Motor 1
Mirror 2
Laser ndash FP cavity
Camera 4
CCTV Camera 10
Motor 16
Thermal control unit 2
Laser equipment 3
Scopes 3
FPGA 1
X‐ray Line characterisation
Xpad camera 1
Si PIN diode 3
Ge diode 1
Motor 13
Stepper motor 25
Others
Translation table 1 1
Slot 1
Table 81 Devices to control (TL=Transfer line OC = Optical cavity and XL = X-ray line)
842 Inputsoutputs
All devices listed above communicate with each other by digital inputsoutputs (DI DO) ie one bit
signals analog inputsoutputs (AI AO) or more complex signals as n-bit digital signals (n = 8 12 or
16) Several communication protocols mentioned in sect832 above will be used
They are detailed in Table 82
PLC InputsOutputs Bus Ethernet
connections
DAQ (ADCampDAC)
Vacuum 26 DI 13 DO 26 profibus
Diagnostics 2 gpib 30 3 x ADC 12 bits 100 kHz
Linac 2 DI 2 DO 5 AI 3 ADC 8 bits 1 GHz
Magnets 35 profibus
Feedbacks RF (x2) 10 DI 10 DO 10 AI
10 AO
USB
Ampli RF 64 DI 8 DO 32 AI 16
AO
RS232 2 4 channels ADC 12 bits
1MHz
Modulator RF 23AI 4 AO 69 DI 5 DI 5 DO
8 channels ADC 8 bits
Pulsed Elements (x3) 12 DI 3DO profibus ADC 12 bits
Laser ndash FP cavity RS232 22
Timing 2 gpib 14 15 channels DAC 16 bits
X Line 26 DI 26 DO 7
Table 82 Equipment IO to control (not complete at the time of writing)
lsquo
145
843 Front-end machine architecture PLCs and industrial PCs
The front-end architecture will comprise equipments individually dedicated to sub-systems These
machines are controlling nodes which will be located in sub-system server racks as close as possible to
the equipment to be controlled (when required)
Two kinds of front-end machines will be used
- PLCs programmable logic controllers are automation OS-based machines able to manage digital and
analog IO and to control processes carried out by devices connected to a field bus (eg Profibus) or
specialized devices They are reliable devices designed to operate in an industrial environment A
proprietary operating system runs on these machines which are powerful tools (eg automats) to
implement logic appropriate to experimentation Interconnection with the other Tango network
equipment is made through an Ethernet adapter interface
- Compact PCIs (industrial PCs) these crates can embed complex operating systems like a Linux
distribution or some Microsoft Windows Operating Systems hard drives Ethernet connections and
USB adapters Such crates are based on a PCI bus backplane to interconnect specific cards (DAC
ADC counters optoelectronics convertershellip) cPCI is a standard widely used in an industrial
environment it is compatible with numerous card suppliers These front-end machines will also allow
the running of Tango processes or specific computing tasks locally installed in a sub-system
The ThomX distributed control system will be build on these controlling nodes This structure will
allow the bandwidth optimization of the network main links and will provide cabling simplifications
The control equipment may be placed in the accelerator hall as well as in the technical rooms where
most of the electronics that must be controlled will be clustered in 19rsquorsquo standard server racks
‐ Some devices need not be controlled through these front-end machines They will be directly
connected to the control network (eg ethernet cameras or electronics for motor control) Three
ways to interconnect such devices are shown in Fig 81
‐
Figure 81 Control architecture for one sub-system
lsquo
146
These acquisition and controlling units can be Profibus PLC or a cPCI crate Each driven element will
be controlled through one of these acquisition and controlling unit
The overall control system will be composed of more than 25 front-end machines as described in Table
83
PLC cPCI Crate
Linac (wout vacuum diags modulator
magnets) 2 1
Vacuum 2
Diagnostics 2
Feedbacks 2 2
RF amplifiercavity 1 1
RF modulator 1 1
Pulsed elements 3 3
Fabry-Peacuterot cavity 1 1
X-ray line 1
Synchronization 1
Magnets 2
Total 13 14
Table 83 List of front-end machines (not complete at the time of writing)
85 IT infrastructure
- Network The ThomX local network will be organized around an IP backbone It will be based on the
Ethernet TCPIP standard in order to link user terminals front-end machines specialized servers etc
Because of the large number of devices and the requirements bearing on the bandwidth (eg for image
storage) 1 Gbitss compliant equipment will be chosen (LAN adapters switcheshellip) Depending on the
actual number of cameras an image-dedicated network could be added On the cabling side shielded
copper cables will be used as interconnection wires to limit electromagnetic interferences as much as
possible
- Data storage During the ThomX commissioning or for the operation monitoring or for the X-ray
experiments it will be crucial to store a large amount of data It is foreseen to implement the three
storage processes provided by Tango historical (long term storage at a slow rate) temporary (short
term storage at a fast rate) and snapshot (storage of parameters on operator demand or event triggered)
It will also be possible to meet the X-ray line needs for the storage of large size images during short
periods (at most for several days) This specific storage will not be necessarily on local servers it could
be convenient to move these data and store them on servers close to the final users possibly outside the
ThomX network
Two storage-dedicated servers will be installed for analyses replay of accelerator operation scenarios
and for some specific needs of the X-ray line These storage units will be specific machines which
prevent data outages or losses by redundant power supplies stored data redundancy (RAID) hard disk
hot swap etc The first of these two storage units will be a 2 To server equipped with a disk array
dedicated to the accelerator data management The second one will have a 10 To capacity it will be
available for the large size images of the X-ray line
lsquo
147
- Equipment of the control rooms Users will work in control rooms on system administration
accelerator control development and integration (device servers GUIs high level computing software
safety analysis tasks) Administration and end-user terminals should be provided in sufficient number
(see below)
The main control room will be equipped with
- 5 PCs 1 for the Tango core system 2 for operations 1 for development 1 for safety (dedicated to
visualization storage and analysis of safety related events)
- 1 server to run high-level applications and device servers
- 2 screens one displaying the ThomX status with synopses and the other one for the safety
supervision
- camera monitors for some direct controls
- 1 UPS (uninterruptible power supply)
The X-ray line control room equipment will consist of
- 2 PCs for the operation equipment and for macro development
- 1 server to run high-level applications and device servers
- 2 screens dedicated to the monitoring of the machine status to safety synopses and to the display of
safety related events
- Software needs
- Both the Linux and Windows OS must be installed in the control room PCs in order to run Tango and
many other operating processes (see the Introduction above) for development purposes and to use
configuration software Together with the Tango core and specific tools high-level software will be
installed to meet usersrsquo needs Labview and Matlab for science applications SPEC to control the
motors and for the analysis of the X-ray line data
- Data bases will run on the MySQL open-source database managing system
- An open-source http server Apache will allow the implementation of WEB interfaces for the control
system authorized users this way these users will be able to remotely visualize experiment data or
equipment status
- Finally all end-user stations will provide standard office tools
The following Figure gives an overview of the control room equipment
lsquo
148
Figure 82 Schematics of the equipment foreseen for the main control room and the X-ray line control
room
Overall the ThomX control system ndash designed to ensure harmonization and standardization and based
on open source tools ndash should be cost effective The investment cost is estimated around 200 keuro
including IT small equipment not described in this document cables a gateway to provide internet
access extra screens to display the ThomX status in the laboratory etc
lsquo
149
Chapter 9 SAFETY AND RADIOPROTECTION
91 Introduction
The ThomX facility will be sited in an existing building called lsquoIgloorsquo where a source of hard X-rays was
operated until 2003 by the Laboratoire pour lrsquoUtilisation du Rayonnement Synchrotron The ThomX linear
accelerator and storage ring will be built inside this building while the facility control room and the one of the
X-ray line will be located respectively in the D1 and D2 experimental halls The D4 hall will be devoted to
experimental tests
The ThomX operation will produce high-energy electrons and other particles such as X-rays and neutrons
which generate ionizing radiations through their interactions with matter An adequate protection against these
radiations must be ensured for people working at the facility The safety logic adopted is the so-called
lsquopositive safetyrsquo which means that any system dysfunction will immediately cause a complete stop of the
accelerator Furthermore redundant safety devices will be implemented allowing various actions that will
guarantee the staff safety such as
- Shining the beam into an area shall be submitted to prior people evacuation and to the interlocking of this
arearsquos doorways
- Access to a sensitive area shall be submitted to the prior stop of the upstream beam by using at least two
different means independent of each other
Complementary to these two procedures safety measures will be applied involving hard-wired logic safety
PLCs shielding beam dumps shutters personnel exclusion interlocks radiation monitoring of the
experimentation areas staff dosimeters and radiation hazard training of the personnel In case of dysfunction
radiological monitoring hard-wired logic or safety PLCs will act through interlocks that will break
connections to the mains voltage (Pre-Amplifier modulator or electron gun depending on the situation) in
order to stop the accelerator
92 PLC safety
The safety hard-wired logic works in association with an automaton PLC system via two completely separate
circuits A Safety Programmable Logic Controller (PLC) is a special purpose machine which is used to
provide a critical control and safety applications to automaton users This controller is an integral part of
lsquosafety instrumented systemsrsquo (SIS) which are used to detect situations involving potentially dangerous
processes When such a situation occurs the SIS is programmed to bring the process to a safe state
automatically
A safety PLC is specifically designed to accomplish two important tasks (ensured by a diversitary redundant
architrecture)
- lsquodo not fail but if this cannot be avoidedrsquo
- lsquofail only in a predictable safe wayrsquo
Hard-wired logic is a safety circuit fully independent of automatons It takes in charge in particular the
access door interlocking feedback contacts which provide a large number of checks bearing on the magnetic
field the high voltage the pre-amplifier status etc and the running control panel
The PLC safety level must be as high as the one of a positive safety hard-wired logic It manages access doors
emergency stop buttons safety rounds and reports issue authorizations to operate from a command-control
system
lsquo
150
93 Radiation monitoring
Concerning the radiation protection the ThomX facility design satisfies the following principles
- No one will receive a dose of radiation in excess of the regulatory dose limits as a result of normal
operation
- Professionnal occupation exposures will be kept As Low As Reasonably Achievable (ALARA)
- All practical steps will be taken to prevent accidents with radiological consequences and to minimize the
radiological consequences of any accident
Arrangements will be made for the monitoring of working areas such as the Igloo the bunker and the Optical
hutch For this purpose several monitoring modes will be available such as thermo-luminescent dosimeters
measurements made by the qualified person in charge of radiation protection (Competent Radiological
Protection Person) and a modular system for continuous monitoring A continuous monitoring of gamma rays
and neutrons will provide radiation dose rates and will manage alarms its measured values will be transmitted
to a central data acquisition unit Placed in the Compton room a special treatment unit will also be
implemented that will provide a local display of the dose rate signage and will manage alarms
94 Radiation protection zoning
The D1 D2 and D4 halls are not classified lsquosupervised arearsquo therefore their access is free to people who
satisfy access requirements In contrast the Igloo hall is classified lsquosupervised arearsquo
The D1 hall contains the X-ray line and technical equipment
- The X-ray line is not a classified area with the exception of the Optical hutch
- The Optical hutch is alternately classified lsquoprohibited arearsquo and lsquonot classifiedrsquo area depending on the
permission conditions issued by functional elements important to safety (shutter position permanent
radiological level)
The D2 hall contains the ThomX control room and a room which contains the laser equipment
The D4 hall is devoted to tests and mechanical assembly
The Igloo hall contains the electron accelerator located inside a bunker The Igloo is classified lsquosupervised
arearsquo therefore only workers subject to individual dosimetry are admitted However in exceptional
circumstances public access might be authorized as long as regulations and procedures are respected
The Igloo bunker on the one hand and on the other hand the basement area devoted to the facility are
alternately lsquoprohibited arearsquo during the accelerator operation and respectively lsquosupervised arearsquo and lsquonot
classified arearsquo when the facility operation has stopped
The accelerator energy is high enough to expect nuclear reactions of photons and neutrons with matter
leading to the activation of structural materials This is why it is particularly important to use appropriate low
activation structural materials or structural materials that are activated for short periods of time only
Furthermore one must ensure a nuclear waste management which will require a specific room to store and
monitor activated materials This room should be a lsquosupervised arearsquo located in the Igloo
95 Access control
951 General statement
It is a matter of absolute necessity to have appropriate procedures and devices in place to identify and screen
all persons entering the ThomX facility Consequently the LAL access control should be extended to the D1
D2 and D4 halls and to the Igloo
lsquo
151
952 Particular requirements concerning the lsquorestricted areasrsquo
The validation of door contacts as well as the validation of the position of protective shields and real-time
radiation measurement are prerequisites to access the restricted areas (ie Optical hutch and the accelerator
area) As it applies to all functional elements important to safety signage and door statements (openedclosed
lockedunlocked) from classified areas must be transmitted to the control room Local indications are
displayed alternately with the status messages The measured values of the dose rate and alarm symbols are
shown permanently
953 Access to the accelerator area and to the Igloo basement
The accelerator and the storage ring will be located in a concrete enclosure that will provide an efficient
shielding to protect people against ionizing radiations inside the igloo in line with regulations Prior to the
validation the floor load-bearing capacity must be checked by experts
The concrete containment structure will have a removable roof made of staggered concrete slabs Civil
engineering will be needed for the anchorage and supports to be used to adapt the bunker concrete blockds to
the igloo circular wall which will be part of the enclosure and to make sure that the roof of the whole
lsquoprohibited arearsquo provides a complete shielding Since doors will be made of standard steel the bunker access
will be indirect ie through (short) mazes to ensure protection against ionizing radiations
Access to the bunker and to the basement area which is part of the ThomX facility will be determined by the
equipment that guarantees peoplersquos safety in accordance with the facility operation As mentioned above
these devices ensure redundancy by using wired logic safety PLCs and also special safety door locks which
are simple but effective devices A mechanical system called lsquotrapped key lockrsquo will be implemented the key
is trapped in the lock when the door is open and it can be recovered only once the door is properly locked
Turning on the beam in lsquoprohibited areasrsquo is allowed only if the safety systems have been unlocked which
implies that the collection of access keys is complete as will be checked by the control board To meet the
safety requirements new access doors must be installed and some existing doors must be modified
954 Access to the X-ray line and to the Compton room
The D1 hall will not be a lsquoclassified arearsquo and will be open to authorized people The X-ray line sections will
be in open access during normal operation with the exception of the Optical hutch To meet the radiation
protection requirements the Optical hutch partition and its roof will be made of thick metal sheets A thick
lead shield will be installed at the end of the X-ray beam axis
The Optical hutch will be alternately classified lsquoprohibited arearsquo and lsquono classifiedrsquo area (under the worst-case
conditions it might become a lsquoregulated arearsquo) depending on permission conditions issued by safety functional
items External and internal light-signaling warning will indicate the zoning status Thus access to the Optical
hutch is firstly subordinated to the removable shutter position and secondly to the local permanent
radiological measurement values The shutter is dimensioned to protect people from high radiation rates
effectively A fail-safe closer device triggers the shutter closing automatically in case of electrical power
failure too (compressed air power supply) When a door is open the shutter command is prohibited
A local radiation detector located in the Compton room will check he radiation exposure and the dose levels
down to as low as reasonably achievable values Measured high dose rates will automatically shut down the
accelerator and activate the evacuation alarm The validation of these safety devices is mandatory to grant
access permission
lsquo
152
96 Beam dumps
The ThomX facility will comprise tow beam dumps made of concrete that will contain high energy particles
They are very important to ensure an adequate protection of people from the ionizing radiations that would be
produced if they were not used One will be aligned with the linac axis and the other one will equip the
extraction area These shields will be installed as close as possible to the radiation source to absorb ndash or at
least to moderate ndash as much as possible of the produced radiations Their size and thickness will be
sufficiently large to reduce efficiently all secondary radiation (X-rays and neutrons) produced by the stopping
of the e- beam They will be made of a lined lead shield designed to moderate back-scattered radiation placed
in a cavity made of concrete blocks or high-density polyethylene Ne peut-on pas ecirctre plus preacutecis donner des
nombres sur les dimensions et preacuteciser les mateacuteriaux utiliseacutes avec une figure
97 Interlocks
Depending on the dysfunction origin the safety system will operate and break command circuits such as the
pre-amplifier trigger the modulator high voltage the electron gun trigger or its high voltage Feedback
information from the control systems may be gradually requested according to the anomaly
98 Accelerator emergency shutdown
Accelerator Emergency Shutdown (AES) are signal and alarm devices located in specific areas A sufficient
number of AES should be installed in prohibited zones namely the accelerator area the bunker basement and
the Optical hutch For the users to spot them easily AES are devices equipped with flashing lights which are
turned on whenever the local zone is closed
If people are still present in a dangerous zone when a machine run is about to start (ie when the access is
going to be forbidden after a short while) they should signal their presence by pushing an AES This action
prevents the accelerator operation The system reactivation must follow a specific procedure
99 Control console
Placed in the control room the control console collects all access keys The accelerator may be turned on if
and only if all keys are inserted in that console
The safety circuit validation (PLC safety hard wired logic) allows one to launch the accelerator running
procedure and triggers audible warning devices for 30 seconds before operation becomes possible
910 Evacuation of dangerous areas
A specific procedure must be followed for the evacuation of the bunker and its basement Operators should be
trained in methodical evacuations with the validation of all checkpoints (roundsman function) Non
compliance with the procedure will lead to the non validation of the facility operation
A sufficient number of roundsman functions should be located in the accelerator area Also an exit path
should be marked leading to those areas that provide a possibility to regress
911 Audio system and horns
Loud-speakers and horns will be installed in the Igloo basement (in the so-called lsquorotundarsquo) and pits and
inside the bunker The sound system must be of good quality speeches and broadcast messages must be
intelligible Horns must be loud enough 80 dBA at least
lsquo
153
912 Safety notices
Local notices Posters should be put up in lsquosupervisedrsquo and lsquocontrolled areasrsquo to remind people of the
regulation requirements Such posts maintain the workerrsquos awareness whenever they are in a radiological
environment Access doors to zones with multiple classifications should be equipped with highly visible
luminous warning panels Also measured radiation levels and messages could be flashed regularly Regarding
the radiation measured level in the Optical hutch an alarm symbol must be permanently displayed that
informs of this level
Signal boards at zone entrances and within the zones will specify the real-time safety conditions display
warnings (such as presence of live voltage ongoing operation mode etc) and the access permissions (such as
open access prohibited zone etc)
913 Radiological monitoring
The control of the radiological environment will comply with the ALARA principle It will be performed by
- using bimonthly exposure dosimeters based on radio-photo-luminescence (RPL) and neutron sensitive
dosimeters
- PCR controls with live measurements
- a centralized modular system which will continuously monitor exposures feeding a data acquisition system
that can handle up to 10 sensors
This centralized system will perform dose rate and integrated dose measurements manage alarms test
measurement channels check settings and calibrations It will allow a special device placed in the Optical
hutch to display in real time local safety information
The ALARA principle is a commitment to keep radiation exposures and doses at a level as low as reasonably
achievable (social and economic factors being taken into account)
lsquo
154
Chapter 10 MANAGEMENT OF THE THOMX PROJECT
Introduction
The management of the ThomX project will have a simple structure that takes into account the three
project phases 1) The construction 2) The exploitation 3) The technology transfer Three boards will
govern the ThomX activity covering all project aspects the machine design the purchasing and the
assembly the commissioning the usersrsquo access the communication and the relationships with the
industries This management board will comprise
1) A steering committee that will supervise the project activity and define the collaboration scientific
policy
2) A scientific committee composed of the scientific coordinators of the collaboration partners It will
assess the usersrsquo projects and prioritize the beam-time allocation This committee must also define the
scientific policy of the machine exploitation and RampD of the ThomX project
3) A partnership and valorisation office This body will evaluate the cultural and the economic
potential of the ThomX innovations It will stimulate transfer initiatives and check their outcome It will
work in close contact with the steering and the scientific committees
The composition of these three bodies is given in Fig 101
Figure 101 The ThomX project committees
In parallel the communication group (see Table 101) will be in charge of all activity concerning
communication and reach-out in the ThomX project It will organise events realise the publications
relating to the project status and progressrsquo and coordinate the official announcements of the
collaboration
LAL Orsay
Achille Stocchi
CELIA Bordeaux
Philippe Balcou
UMPC
Philippe Walter
Institut Neel
Alain Schuhl
INSERM Grenoble
Franccedilois Esteve
Synchrotron SOLEIL
Amor Nadji
ESRF
Jean Susini
THALES
Yves Trinquet
Alessandro Variola
LAL Orsay
Alberto Bravin
ESRF
Geraldine Le Duc
ESRF
Jean-Louis Hazemann
Institut Neel
Philippe Walter
UMPC
Alexandre Loulergue
Synchrotron SOLEIL
Francois Esteve
INSERM Grenoble
Eric Cormier
CELIA Bordeaux
Alessandro Variola
LAL Orsay
Pierre Bordet
Institut Neel
Philippe Deblay
Synchrotron SOLEIL
Yves Trinquet
THALES
Annabelle Alves
CNRS DR4
Edward Mitchell
ESRF
Fabian Zomer
LAL Orsay
lsquo
155
The project management is organized according to a matrix structure The ThomX facility has been
divided in five sub-systems Linac Ring and transfer lines X-ray line Laser and Fabry-Peacuterot cavity
There is a coordinator responsible for the design installation and operation of each sub-system All
these sub-systems involve various technologies which have been grouped in expertise domains
Mechanics and integration RF systems Magnets Diagnostics Power supplies and Pulsed elements
Vacuum Synchronisation Controls Beam Physics Alignment Cabling Nomenclature and
Infrastructures There is a coordinator for each one of these domains These coordinators will be in
charge of the activity harmonization within each technical field and of the maintenance during the
facility exploitation (or ldquoscientific phaserdquo)
The project leader his deputy and a project engineer are responsible for the project overall
coordination Fig 102 describes the project structure
Figure 102 The project structure
Project and Quality Engineer Technical
coordinator
AVermegraves Francois Wicek
LAL Orsay
Deputy Project leader
A Loulergue
Synchrotron SOLEIL
Project Leader
A Variola
LAL Orsay
Systems
LINAC RRouxLAL Orsay
Ring and Transfer Lines A Loulergue Synchrotron SOLEIL
X line M Jacquet LAL Orsay
Laser E Cormier CELIA Bordeaux
Fabry-Perot Cavity F Zomer LAL Orsay
Groups
Mechanics and Integration MLacroix LAL Orsay
Beam Physics ALoulergue Synchrotron SOLEIL
Alignment ALestrade Synchrotron SOLEIL
RF systems P Marchand Synchrotron SOLEIL
Magnets ME Couprie Synchrotron SOLEIL
Diagnostics L Cassinari Synchrotron SOLEIL
Infrastructures BPieyre MLanglet LAL Orsay
Control PGauron LAL Orsay
Vacuum C Preacutevost LAL Orsay
Power Supplies - Pulsed Magnets P Lebasque Synchrotron SOLEIL
Synchronisation N Delerue LAL Orsay
Cabling JN Cayla LAL Orsay
Nomenclature PCornebise LAL Orsay
lsquo
156
In the following Table the support structures are presented
Support Structures
Administration Communication Quality
Administration Brigitte Renard LAL
Orsay
Procurements Anngaeumllle Mardam
Beck LAL Orsay
EDMS Support Agnes Vermes LAL
Orsay
Secretary Chloe SevestreLAL Orsay
DR04 Support Edouard Dorion
Dominique Bony LAL Orsay
Marie-Pauline Gacoin
Synchrotron SOLEIL
Claus Habfast ESRF
Sandrine Agnes Vermes
LAL Orsay
Table 101
The ThomX collaboration needs an important support from all administrative services For this an
administrative cell has been set up which is headed by the person responsible for the administration the
purchasing office the project database management the secretariat and the contact with the regional
CNRS delegation support Last but not least a quality engineer plays a major role in the project
101 Partnersrsquo complementarity
The ThomX project requires a multidisciplinary environment given the complexity of the facility and of
the set-up operation Each partner contributes its competences to reach the final goal The ThomX
collaboration was organized so as to meet the requirements of a multicultural scientific environment
where different communities are called upon to share their know-how and their knowledge to develop a
common scientific program This was already the case ndash and still is the case ndash for the MightyLaser
project In this project LAL and CELIA teams have developed a fully integrated set-up composed of a
fibre laser and a high gain optical resonator which is presently installed at ATF (KEK Japan) There the
complementarity between the two teams was crucial LAL has a solid experience in developing optical
resonators in an accelerator environment while CELIA is among the leading laboratories in fibre
amplification technology This program aims at a progressive increase of the light beam power stored in
an optical cavity in order to maximize the Compton back-scattering rate The final goal of this RampD is
to demonstrate the feasibility of a γ factory based on Compton backscattering It started as a
demonstrator for Compton-based polarized positron sources needed for the next generation linear
colliders The interest shown by other scientific communities in this technology made it clear that a low
energy Compton source would be very useful in various research fields This conclusion led to the
launch of the ThomX collaboration
The ThomX CDR has already been published and the present TDR is about to be widely circulated For
the TDR writing it was mandatory to develop close connections between the various communities
lsquo
157
participating in the ThomX project in order to merge technological developments and usersrsquo
requirements in a single document All partners have contributed to the scientific case elaboration the
study of the facility feasibility and the definition of its main components
- LAL and SOLEIL contributed the description of the Compton backscattering characteristics and
carried out the study of the accelerators and the Fabry-Peacuterot resonator
- CELIA Bordeaux Lyon provided their expertise for the laser dimensioning
- THALES provided its experience in industrial components and sub-systems
- UMPC and ESRF enriched the scientific case
This task sharing results from the multidisciplinary approach to the ThomX project where half a dozen
institutes play an important role
The writing of this TDR provided a unique opportunity of collaborating for laboratories that belong to
different fields LAL specialized in fundamental high energy physics SOLEIL and CELIA in advanced
technology in laser and accelerators LC2RMF in social sciences and compact detectors ESRF in
medical sciences In the future the diversity of this set of teams will still increase with the participation
of the Neel Institute and the GIN-INSERM in Grenoble
After a prototyping phase ThomX will operate as a mini-synchrotron whose beam line can be accessed
not only by the facility partners but also by other communities Moreover the aim of the
industrialization phase is to provide the high technology scientific and industrial with compact and
reliable sources that can be operated with little or no accelerator and laser expertise This should open
the way to a large community of CBS source users that would benefit from the complementarity
between techniques developed at synchrotron sources and Compton back-scattering facilities Imaging
techniques biological tests on cells and animals metallurgy fast chemistry lithography are examples
of future possible applications of these unique instruments
102 ThomX larger framework
ThomX is one of the EQUIPEX projects submitted for approval to the French government by the P2IO
Consortium (Physique des deux Infinis et des Origines) of the Paris-Saclay Campus This Consortium is
composed of eight laboratories or institutes (CSNSM IAS IMNC IPNO IRFU LAL LLR LPT)
some of which are among the largest French laboratories All together their staffs comprise 2000
scientists engineers and technicians working in particle physics nuclear physics accelerator science
astroparticle physics astrophysics and their interfaces P2IO represents between 20 and 90 of the
national potential in the various sub-domains Although obviously not all of the eight laboratories
participate in each EQUIPEX project each project ndash including ThomX ndash is fully supported by P2IO and
will contribute to the very ambitious common technological platform that P2IO wants to create in the
coming years in the framework of the Paris-Saclay Campus initiative Once equipped with these
EQUIPEX facilities this campus will represent a unique opportunity for French researchers and
students
lsquo
158
References
11 Geoffrey A Krafft Gerd Priebe Compton Sources of Electromagnetic RadiationlowastReviews of Accelerator
Science and Technology Vol 3 (2010) 147ndash163
12 VBesson et al NIMA 608 (2009) S75-S77
13 T Akagi et al 2012 JINST 7 P01021
14 CDR ThomX LAL RT 0928 SOLEILSOU-RA-2678 editors AVariola FZomer ALoulergue
15 PRussbueldt et al Optics Express Vol 17 Issue 15 pp12230-12245 (2009)
16 IPupeza et al OPTICS LETTERS Vol 35 No 12 June 15 2010
17 M Y Shverdin et al 2009 OSAIPNRANLOSL 2009
18 CVaccarezza et al IPAC Conference proceedings New Orleans USA2012 TUOBB01
19 AVariola IPAC Conference proceedings San Sebastian Spain 2011 WEOAA01 1903-1905
21 S Doumlbert ldquoIntegration of the PHIN RF Gun into the CLIC Test Facilityrdquo Proceedings of EPAC 2006
Edinburgh Scotland 2006 MOPLS129
22 R Roux S Cavalier M Bernard G Bienvenu MJoreacuteBLeblondBMercier BMoutonCPrevost A
Variola ldquoPHIL a Test Beamline at LALrdquo Proceedings of EPAC08 Genoa Italy 2008 WEPP078
31 J Payet Beta Code CEA SACLAY
32 JB Murphy et al Longitudinal Wakefield for an Electron Moving on a Circular Orbit Vol 57
Part Acc 1997
33 K Bane M Sand The Short-Range Resistive Wall Wakefields SLAC-PUB 95-7074
34 E Forest Phys Tech Part Photon Beam
35 J Zang et al lsquoDipole fringe field effects in the Thom-X ringrsquo Proc IPAC13
36 R Nagaoka et al Recent Studies of Geometric impedance At SOLEIL PAC2006
37 Z Huang RD Ruth Phys Rev Lett 80976 (1998)
38 S K Mtingwa African Physical Review 2 (2008) 001
39 P Chen G Horton-Smith T Ohgaki AW Weidemann K Yokoya
NuclInstrumMethA355107-1101995
310 K Ohmi Phys Rev E 55 7550 (1997)
311 S Haifets D Teytelman Phys Rev Spec Top Acc and beams 8 064402 (2005)
312 E Weihreter Compact synchrotron light sources Series On Synchrotron Radiation Techniques
And Applications - Vol 3 World Scientific
313 P Lebasque et al Synchrotron SOLEIL lsquoEddy current septum magnets for booster injection and
extraction and storage ring injection at Synchrotron SOLEILrsquo EPAC 2006 Edinburgh Scotland
314 P Lebasque et al Synchrotron SOLEIL lsquoOptimisation of the coating thickness on the ceramic vacuum
chambers of SOLEIL storage ringrsquo EPAC 2006 Edinburgh Scotland
315 P Lebasque et al Synchrotron SOLEIL lsquoFour matched kicker systems for the SOLEIL storage ring
injection a full solid-state solution of pulsed power supplies working at high currentrsquo EPAC 2006
Edinburgh Scotland
316 F Marhauser E Weihreter (BESSY) ldquoImpedance Measurements of the HOM-Damped 500 MHz
Metrology Light Source Cavityrdquo BESSY Gmbh Germany 08 January 2007
317 M Langlois P Sanchez M Cornelis F Perez (ALBA) ldquoMeasurements on the RF cavity for the ALBA
storage ringrdquo Proceedings of EPAC08 Genoa Italy
318 R A Rimmer (Lawrence Bekeley Lab) et al ldquoDevelopment of a high-power RF cavity for the PEP-II B
factoryrdquo DE-AC03-76SF00098 (LBL)
lsquo
159
319 P Fernandes R Massarino R Parodi ATarditi (INFN Italy) A Massarotti (Sincrotrone Trieste Italy)
rdquoThe Design of the RF Cavities for ELETTRArdquo IEEE 1989
320 M Svandrlik A Fabris C Pasotti (Sincrotrone Trieste) rdquoImprovements in curing coupled bunch
instabilities at ELETTRA by mode shifting after the installation of the adjustable higher order mode frequency
shifter (HOMFS)rdquo IEEE 1998
321 W Bruns web site httpwwwgdfidlde
322 P Marchand et al Development of high RF power solid state amplifiers at SOLEIL Proceedings of the
IPAC2011 San Sebastian Spain p 376
323 C Pardine et al Status of the 476 MHz 50 kW solid state amplifier Proceedings of the IPAC2010
Kyoto Japan p 3972
41 YAG(CE) data sheet Saint Gobain Crystals httpwwwdetectorssaint-gobaincomYAG28Ce29aspx
42 SPI Suppliesreg YAGYAP Comparison Table
httpwww2spicomcatalogscintillsintillator_properties_comparison_tablehtml
43 K Nakamura et al (Particle Data Group) J Phys G 37 075021 (2010)
44 Wartski et al Detection of optical transition radiation and its application to beam diagnostics IEEE
TransNuclSci20544-5481973
45]Saturation of YAG Beam Profile Monitors Measured at ATF A Murokh J Rosenzweig V Yakimenko X
J Wang
46 Radiation Detection by Cerenkov Emission in Optical Fibers at TTF E Janata M Koumlrfer Tesla-Report
2000-27
47 CERN-2009-005 p273
48 CERN-2009-005 p272
51 J Bonis et al 2012 JINST 7 P01017 doi1010881748-0221701P01017
61 J Bonis et al Non-planar four-mirror optical cavity for high intensity g ray flux production by pulsed laser
beam Compton scattering off GeV-electrons JINST SISSA Vol 7 January 2012
62 R V Pound lsquolsquoElectronic Frequency Stabilization of Microwave Oscillatorsrsquorsquo Rev Sci Instrum 17 490ndash
505 (1946)
71 httpwwwjjxraydkhome
72 RW Alkire et al Nucl Inst and Meth A 350 (1994) 13-16
73 B Lengeler et al J Synchrotron Rad 6 (1999) 1153-1167
74 RL Owen et al J Synchrotron Rad 16 (2009) 143-151
75 MS Nogueira et al Radiation Protection Dosimetry 111 (2004) 105-110
76 M Krumrey et al AIP Conf Proc 705 (2004) 861-865
M Gerlach et al Metrologia 45 (2008) 577-585
77 httpwwwcanberracompdfProductsUltra-LEGe-SS-M3843pdf
78 K Sakaue et al Rev Sci Instrum 80 (2009) 123304
79 httpsaleshamamatsucomassetspdfparts_FF9892-31_-32_TMCP1039E03pdf
710 O Chubar et al Proc DIPAC (2001) CT09
711 D Paterson et al Opt Com 195 (2001) 79-84
712 M Mahesh RadioGraphics 21 (2001) 1033-1045
713 K Medjoubi et al JINST 6 C01080 (2011)
httpwwwvariancomusxrayproductsdigital_radiographypaxscan_productshtml
714 H Elleaume et al Physics in Medicine and Biology 47 (2002) 3369-3385
lsquo
160
715 Y Prezado et al Medical Physics 38 (2011) 1709
716 I Martiacutenez-Rovira and Y Prezadoa Medical Physics 38 (2011) 4430
717 L Samain Thesis Liege university (ULG) 2012
718 P Martinetto Thesis Joseph Fourier university (UJF) 2000
719 P Bleuet et al Analytical Chemistry 29 (2010) 518-527
720 P Bleuet et al Nature Materials 7 (2008) 468-472
81 httpwwwprofibuscomncdownloadspecifications-standardsdownloadsprofibus-standard-dp-
specificationdisplay
82 httpwwwtango-controlsorgDocumentstango-kernel
lsquo
161
Table of content
INTRODUCTION 2
1 The ThomX project 2
2 Scheme and options 3
3 General layout of the machine 4
4 Machine parameters 6
5 Summary of the TDR main technical points 8
Chapter 1 THE THOMX INTERACTION REGION 9
11 Electron beam and laser pulse characteristics in the interaction region 9
12 Expected flux and its characteristics 10
13 Position stability 11
14 Timing stability 12
15 Energy stability 13
Chapter 2 INJECTOR 14
21 Injector specifications 14
22 Technical description of the linac components 16
221 Photo‐injector 16
222 Solenoids 17
223 Laser 18
23 Acceleration section 20
231 The RF source 21
232 RF network 22
24 Beam dynamics 23
241 Nominal operation 23
242 Alternative operation modes 24
Chapter 3 TRANSFER LINE and RING 28
31 Transfer line 28
32 Extraction line 31
33 Storage ring 31
331 Optics and lattice 31
332 Non‐linear dynamics 35
333 Collective effects feedbacks 37
334 CBS and IBS effects 42
335 Scattering on the residual gas 46
336 Ion effects 47
337 Electron beam injection and extraction 48
338 Orbit corrections 50
341 Dipole magnetic design 53
342 Quadrupole magnetic design 54
343 Sextupole magnetic design 56
344 Corrector magnetic design 58
345 Injection dipole 59
lsquo
162
35 Power supplies 60
351 Pulsed magnets 60
36 Storage ring RF system 65
361 HOM impedance thresholds as regard to beam instabilities 66
362 Cures to the HOM induced instabilities 66
363 RF power source 70
364 Low level RF system and longitudinal feedback 71
365 RF equipment status 73
366 RF system conclusion 73
37 Transverse feedback 74
371 Specifications 74
372 Electronics 75
373 Kicker 76
38 Vacuum 77
381 Transfer line vacuum 77
382 Ring vacuum 78
Chapter 4 INJECTOR TRANSFER LINE AND RING DIAGNOSTICS 80
41 Charge measurement 80
411 Current transformers 80
412 Faraday cups 81
42 Transverse profile measurements 81
421 Diagnostics stations 81
43 Emittance measurement 84
44 Beam position measurement 85
441 Beam Position Monitors (BPM) 86
442 Orbit feedback system 90
443 Tune measurement and feedback 90
444 Energy measurement 90
45 Bunch length measurement 90
451 Cerenkov radiation 90
452 Synchrotron radiation 92
453 Transport of the Cerenkov and synchrotron radiations to the diagnostics hutch 93
454 Streak camera for the bunch length measurement 93
46 Bunch imaging using synchrotron radiation 94
47 Beam loss measurement 94
48 Beam dumps 94
Chapter 5 SYNCHRONIZATION 95
51 Overall scheme 95
52 Specifications 96
Chapter 6 OPTICAL SYSTEM DESIGN 97
61 Feedback system and results on a Fabry‐Perot cavity locking 97
611 Electron and FPC pulse synchronization 98
612 Laser parameters to be stabilized 99
lsquo
163
613 PDH Measurement technique 101
614 Actuators 103
615 The digital feedback system 107
62 High average power laser system and its coupling to a high gain FP cavity 110
621 The MightyLaser oscillator 111
623 Stretcher and compressor 115
624 ThomX development 115
63 Mechanical design 118
631 Optical cavity vacuum 123
Chapter 7 X‐RAY BEAM LINE AND EXPERIMENTS 124
Introduction 124
71 Available extracted X‐ray beam 124
72 Global view of the X‐ray lne 124
73 Chraracterization of the X‐ray beaugrave 126
731 Optical table 1 126
732 Beam transport 128
733 Optical table 2 128
74 Demonstration experiments 130
741 Experiments using the full X‐ry beam 130
742 Experiments using the central part of the beam 134
75 Summary 139
Chapter 8 CONTROL SYSTEM 140
Introduction 140
81 General principles 140
82 Technical considerations 141
83 The TANGO control system software 142
831 Software bus 142
832 A control system for accelerators 142
84 Control system architecture 143
841 Equipment to control 143
842 Inputsoutputs 144
843 Front‐end machine architecture PLCs and industrial PCs 145
85 IT infrastructure 146
Chapter 9 SAFETY AND RADIOPROTECTION 149
91 Introduction 149
92 PLC safety 149
93 Radiation monitoring 150
94 Radiation protection zoning 150
95 Access control 150
951 General statement 150
952 Particular requirements concerning the lsquorestricted areasrsquo 151
953 Access to the accelerator area and to the Igloo basement 151
954 Access to the X‐ray line and to the Compton room 151
lsquo
164
96 Beam dumps 152
97 Interlocks 152
98 Accelerator emergency shutdown 152
99 Control console 152
910 Evacuation of dangerous areas 152
911 Audio system and horns 152
912 Safety notices 153
913 Radiological monitoring 153
Chapter 10 MANAGEMENT OF THE THOMX PROJECT 154
Introduction 154
101 Partnersrsquo complementarity 156
102 ThomX larger framework 157
lsquo
4
Fig1 Schematic diagram of the ThomX source
At the end of a storage period of 20 ms the electron bunch is extracted from the ring and dumped in a
dedicated beam dump to avoid the background noise that would be produced by uncontrolled particle
losses At the same time a new lsquofresh beamrsquo is injected in the machine
3 General layout of the machine
As previously mentioned the ThomX design takes into account a strong constraint coming from the
goal of a small size machine Nevertheless this design allows for the possible integration of two
collision regions [19] Figure 2 shows the general layout of the machine The linac includes an electron
RF photogun (1) followed by a section dedicated to diagnostics (2) the acceleration structure (3) and a
quadrupole triplet (4) to allow for a three-gradient measurement in the direct line dump (5) where a
diagnostic chamber will be located The injection line consists of four 45 degree dipoles to steer the
beam four quadrupoles and two chambers (6 7) for energy spread measurements and beam
characterization before injection The injection is performed by a septum (8) and a fast kicker (9)
Another kicker (10) is inserted in the same line to provide together with the septum a fast extraction
Once extracted the beam is transported towards a dedicated beam dump (11)
lsquo
5
Figure 2 ThomX layout
The ring design ensures a four-fold symmetry based on a double bend achromat (DBA) optics It
includes 8 dipoles 24 quadrupoles and 12 sextupoles for chromatic corrections The bunch storage
time is too short (20 ms) and the beam energy is too low for synchrotron damping to play a role in
regard to the Compton back-scattering recoil effect nor in regard to collective effects such as CSR IBS
and ion instabilities For this reason an lsquoELETTRArsquo type RF cavity (12) will be used not to restore
energy but to bunch the electrons and to perform a longitudinal feedback Transverse position
diagnostics and feedback will be provided by a strip-line and by BPMs A synchrotron radiation
chamber will be used for transverse and longitudinal optical diagnostics The required ultra-vacuum
will be maintained by ion pumps distributed along the ring and by two dedicated pumps in the Fabry-
Peacuterot resonator
In the ring lattice two of the achromatic lines have been shortened by removing all the quadrupoles (see
Fig 3) so that the Fabry-Peacuterot cavity (13) can be easily integrated between two dipoles By minimizing
the beta functions it was possible to locate the collision region (14) in such a short section
Furthermore this design provides the possibility of implementing a second interaction region in the
future
lsquo
6
Fig 3 Layout of the interaction region and of the optical cavity integration
4 Machine parameters
The ThomX machine is composed of four main systems the injector the storage ring the laser and the
Fabry-Peacuterot resonator As mentioned previously we foresee some flexibility in the machine operation
so that the X-ray energy can be varied For this all four systems are dimensioned to withstand an
operating energy up to 70 MeV Nevertheless to characterize the machine a baseline configuration has
been chosen at 50 MeV In Tables 1 2 3 and 4 the parameter values which are given below correspond
to this baseline configuration
Table 1
Injector
Charge 1 nC
Laser wavelength and pulse energy 266 nm 100 microJ
Gun Q and Rs 14400 49 MWm
Gun accelerating gradient 80 MVm 5 MW
Normalized rms emittance 44 π mm mrad
Energy spread 04
Bunch length 43
lsquo
7
Table 2
Table 3
Laser and FP cavity
Laser wavelength 1030 nm
Laser and FP cavity Frep 3568 MHz
Laser power 50 ndash 100 W
Laser pulse energy 14 ndash 28 microJ
Fabry-Perot pulse energy 28 mJ
Fabry-Perot pulse length (rms) 5 ps
FP cavity finesseGain 3000-30000 1000- 10000
FP waist 70 microm
Power circulating in the FP cavity ~ 007 ndash 07 MW
Ring
Energy 50 MeV (70 MeV possible)
Circumference 168 m
Crossing-Angle (full) 2 degrees
βxy IP 01 m
εxy just after injection 5 10-8
m
Bunch length just after injection (rms) 4 ps
Bunch length at the end of a 20 ms storage
cycle
50 ps (rms)
Beam current 1784 mA
RF frequency 500 MHz
Transverselongitudinal damping time 1 s 05 s
RF Voltage 300 kV
Revolution frequency 1784 MHz
σx IP (just after injection) 70 microm
Tune xy 317 174
Momentum compaction factor αc 00136
InitialFinal relative energy spread (with IBS
and Compton back-scattering)
0406
lsquo
8
Table 4
Source
Photon energy cut-off 46 keV (50 MeV) 90 keV ( 70 MeV)
Total Flux 1011
-1013
photons
Bandwidth (with diaphragm) 1 - 10
Divergence 10 mrad (1γ) without diaphragm 50 MeV
5 Summary of the TDR main technical points
After the publication of the CDR much work has been put in simulations design and technical choices
The goal of this TDR is to provide a reference document in view of the machine construction as well as
a more precise planning and a more precise costing
The first chapter deals with the machine performances and the tolerances to be met so that the collisions
between the electron bunch and the laser pulse do take place in the conditions specified in the CDR
Then the main systems are described and our technical choices are presented This last section is
subdivided as follows
1 Linac
2 Beam transfer and injection section ring
3 Laser and Fabry-Peacuterot cavity
4 X-ray user line
A special attention is given to the integration of each system and to the global integration of the ThomX
machine The lsquoOptical systemrsquo chapter includes a short report on recent results obtained in the laser and
Fabry-Peacuterot cavity RampD program both in the laboratory and on a beam line at KEK-ATF (Japan)
Special chapters are dedicated to diagnostics the control of the main systems and their synchronization
safety and radioprotection The last chapter presents the project management
lsquo
9
Chapter 1 THE THOMX INTERACTION REGION
11 Electron beam and laser pulse characteristics in the interaction region
The ThomX injection frequency has been fixed at 50 Hz (20 ms period) by taking into account the
Compton scattering recoil effect which continuously deteriorates the beam longitudinal emittance This
effect is all the more important as synchrotron damping is strongly suppressed since the beam energy is
low Because of this lack of damping the Compton scattering recoil effect and eventual beam
instabilities must be controlled by longitudinal and transverse feedback systems
We stress the fact that in ThomX the beam does not reach a permanent state during a storage cycle its
properties are continuously varying To describe the Compton interaction dynamics we define a set of
nominal beam characteristics which are those prevailing just after the injection of a fresh e- bunch
Simulations have been carried out to characterize the X-rays emitted during a single crossing of the
electron bunch and the photon pulse (stored in the optical cavity) as a function of the collision
geometric parameters
These nominal collision parameters are as follows
Parameter value unit
Crossing angle 2 degree
Electrons energy 50 MeV
Electrons energy spread 03
Number of electrons 625 109
Fabry Perot pulse energy 28 mJ
Repetition rate 1784 MHz
Laser pulse transverse dimension (rms) 35 microm rms
Electron bunch transverse dimension (rms) 70 microm rms
Laser pulse duration (rms) 5 ps rms
Electron bunch length at injection 4 ps rms
Table 11 Nominal collision parameters
To extend these results to other geometric configurations one has to take into account the parametric
dependence of the Compton collision rate The X-ray flux is the product of the Compton cross-section
and the luminosity It is proportional to the electron bunch intensity and to the laser pulse power The
geometry dependence is given by the luminosity formula which involves the geometric 6D size and the
crossing angle Assuming Gaussian bunches and neglecting the hourglass effect the ring luminosity is
given by the following expression
Eq 11
L = NeNγ f cosφ
2π
1
σ ye
2 +σ yγ2 σ xγ
2 +σ xe
2( ) cos2φ + σ ze
2 +σ zγ2( ) sin2φ
lsquo
10
where σeγxyz represent respectively the two transverse dimensions and the longitudinal one of the
electron bunch and of the laser pulse f is the collision frequency Ne and Nγ are the number of
electrons per bunch and photons per pulse Φ is half the crossing angle
The X-ray energy cut-off is higher for head-on collisions and for photons which are exactly back-
scattered In this case the photon energy gain is 4 γ2
This calculation provides a useful reference to assess the efficiency of other configurations but it
remains an approximation since exact results must take into account the beam evolution during a
storage cycle and should be obtained from a full revolution by revolution simulation
12 Expected flux and its characteristics
In nominal conditions the expected flux is 78 1012
X-rayss whether one uses Eq 11 or the monte-
carlo simulation code CAIN Figure 11a shows the energy spectrum of the scattered photons and its
cut-off at 45 keV As shown in Fig 12b most photons are emitted very close to the interaction region
axis within a small cone with a 2 mrad opening angle They may be selected by using a small
diaphragm The energy spectrum of the X-rays emitted in a cone of 225 mrad opening angle is shown
in Fig 12a with a FWHM energy spread of 25 keV
Figure 11 Results of simulations performed with CAIN using the parameters listed above a)
Energy spectrum b) Transverse size of the source c) Angular emission distribution d) Horizontal (x-
xrsquo) phase space of the scattered X-rays
lsquo
11
The spot size at the source is dominated by the electron bunch of 7 10-5
m as illustrated in Fig 11b
The dimansions are assumed to be the same in the horizontal and vertical plane The total divergence
of the source is around 15 mrad given by the electron beam energy (ie the 1γ parameter) with an rms
size of 4 mrad (Fig 11c) The emittance of the photon source shown in Fig 11d is 03 mm mrad The
energy-angle correlation and the collimation effect are illustrated in Fig 12 As expected the high-
energy X-ray flux is peaked with a 1γ opening angle The diaphragm which is assumed for this
simulation selects emission angles and thus photon energies resulting in a ~ 10 monochromatic
photon beam
Figure 12 Results of simulations performed with CAIN using the parameters listed above
Left panel Energy spectrum of photons emitted in a cone with an opening angle of 225 mrad
Right panel Number of photonsmradsup2s versus energy and angle (the color bar indicates the photon
number)
13 Position stability
To optimize the X-ray flux a precise transverse alignment must be acieved in the interaction region It
can be performed either by steering the electron beam or by adjusting the optical table position These
procedures will be monitored by means of a double BPM which is integrated in the Fabry-Peacuterot cavity
vacuum chamber exactly at the interaction point The resolution of this technique is a few microns A
simulation has been performed to calculate the flux reduction resulting from a possible mismatch
between the electron bunch orbit and the laser pulse trajectory Figure 13 shows the number of X-rays
produced per bunch crossing and per keV as a function of a transverse displacement of the optical
cavity axis with respect to the electron orbit from zero to 140 microns
lsquo
12
Fig13 Number of back-scattered photons and their energy as a function of a transverse alignment
error This figure clearly shows that the losses entailed by a displacement comparable to the BPM
sensitivity are negligible The color bar indicates the number of X-rays produced at each crossing
14 Timing stability
A precise synchronization between the electron bunch and the laser pulse at the interaction point is
another important requirement A timing mismatch introduces a longitudinal shift between the two
bunches at the IP This can be due to an error in the location of the Fabry-Perot cavity with respect to
the e- storage ring or to a jitter in the synchronization system Fig 14 shows the variation of the X-ray
flux as a function of a longitudinal displacement
The flux is not so much reduced as long as the jitter is less than 10 ps This number fixes the
synchronization tolerance and thus the performance of the longitudinal feedback
Fig 14 Flux and energy spectrum of back-scattered photons as a function of the relative
synchronization of the beams (longitudinal displacement)
lsquo
13
15 Energy stability
The nominal ThomX injection energy is 50 MeV Phase rotation in the RF source or phase mismatch
will generate a jitter in the injection energy Such a jitter entails synchrotron oscillations which will be
damped by the longitudinal feedback However before this feedback has stabilized the longitudinal
oscillations the collision conditions will not be nominal This situation has been simulated the results
are illustrated in Fig15 One can see that it is in the higher part of the X-ray energy spectrum that the
loss is more important
Fig 15 Flux and spectrum of the back-scattered photons as a function of the e- energy
lsquo
14
Chapter 2 INJECTOR
The most important components of the linac injector are the electron gun and the acceleration section
which brings the beam to the desired energy Other components are the laser used for the e- gun the
magnetic elements used to transport the beam the diagnostics the vacuum system the waveguides and
the radio-frequency (RF) source Each component is detailed in the following First of all the injector
specifications are given since they lead to the choice of some of the linac components
21 Injector specifications
The project goal namely to produce a high flux of X-rays of energy 50 keV leads to the following
specifications for the linac
Energy 50 MeV
Charge 1 nC
Number of bunches per RF pulse 1
Emittance (rms normalised) lt5 mm mrad
Energy spread rms lt1
Bunch length rms lt5 ps
Average current 50 nA
Repetition frequency 50 Hz
Table 21 Nominal linac parameters
First of all one notices that the low average current 50 nA does not justify the use of cold technology
(ie of a superconductor) for the acceleration structure But there is still a large choice of possibilities
for an RF structure operating at room temperature For economic reasons and because of the long
experience accumulated in the field we have chosen an RF structure operating at 3 GHz This
frequency is a European standard
Furthermore the above specifications led us to choose a photo-injector for the electron source since a
low emittance beam is required Again to minimize risks we chose to build the same gun as the one we
constructed for the CLIC lsquoTest Facility 3rsquo at CERN [21] This gun has been running successfully for 4
years To increase the current per bunch we will use a metallic magnesium photocathode which can
deliver more than 1 nC with a laser pulse energy of a few tens of a microJ at a wavelength of 260 nm The
gun is made of 2 12 copper cells magnetically coupled to the waveguide To get 1 nC with an
emittance lower than 5 mmmrad an accelerating field of 80 MVm is required which means an RF
power of 5 MW in a 3 micros pulse The electron beam energy output by the gun is around 5 MeV
We will use an industrial laser to extract the electrons from the cathode by photoelectric effect The
main specifications of this laser are the following ones Wavelength 260 nm energy per pulse 100 microJ
pulse duration around 5 ps It must be synchronized with the RF frequency and triggered at 50 Hz This
laser will be installed in an optical hutch located outside the concrete shielding which will enclose the
accelerator Therefore its beam must be transported with dielectric mirrors and lenses over a distance of
about 20 meters
Concerning the acceleration section SOLEIL a partner of the project will lend us a lsquoLILrsquo section This
section was made by LAL for the pre-injector of LEP at CERN It has been kept as a spare for the
SOLEIL linac The ThomX project will provide an other section in case SOLEIL needs one The
section length is about 46 m To reach a final energy of 50 MeV the energy gain in the section must be
lsquo
15
45 MeV The latter value implies an electrical field of 125 MVm and an RF power of about 10 MW
However the accelerator should reach higher energies in order to produce X-rays beyond 50 keV Since
the maximum targeted X-ray energy is 90 keV the linac design should allow a beam energy of 70
MeV corresponding to an RF power of 20 MW Thus taking into account the RF system efficiency the
specification for the total RF power is set at 35 MW
This specification lead us to choose a klystron producing 35 MW in a 45 micros pulse To feed the
klystron a modulator is needed that provides high voltage pulses of typically 240 kV through a high
voltage transformer soaking in an oil tank The two technologies which can be used for this component
are discussed in detail below
For the electron beam steering two coils and two magnetic correctors are required The coils are placed
around the gun Opposite sign currents circulate in these coils (-310 A 336 A) so that the magnetic field
vanishes at the cathode location Its amplitude is 023 T roughly 7 cm downstream from the cathode
Concerning the magnetic horizontalvertical correctors once a good mechanical alignment is achieved
experience has shown that beam trajectory errors are not larger than a few mm between the gun and the
acceleration section Hence the correctors must produce a beam deflection of up to 10 mrad For the
model operating at LAL [22] two bipolar power supplies of 10 A per corrector are adequate
For the diagnostics three different instruments will be installed between the RF gun and the
acceleration section An Integated Current Transformer (ICT) will measure the charge per bunch
Beyond this device a beam position monitor (BPM) will measure the transverse positions (x y) of the
electron beam We will use a BPM based on striplines which was designed for SOLEIL The third item
comprises a vacuum chamber equipped with an arm which can be translated by compressed air in order
to introduce a YAG screen to intercept the beam The YAG crystal fluoresces when ionized by the e-
beam The emitted light shows the beam transverse shape A short optical transport line and a CCD
camera allow the measurement of the transverse dimensions LAL designed and constructed the
diagnostics chamber It has already been successfully tested in the PHIL facility A drawing showing
the linac layout is shown in Fig 21
lsquo
16
Figure 21 Linac layout In the upper figure the different components of the LINAC are illustrated
starting form the gun and subsequently passing trough the stripline the ICT for current measurements
the laser introduction chamber the diagnostics station the steerers and the accelerating section
22 Technical description of the linac components
221 Photo-injector
It is a copy of a gun built by LAL for the CLIC Test Facility 3 of CERN the so-called lsquoprobe beam
gunrsquo[22] Simulations have shown that the performances of this gun fulfill the ThomX specifications
We are thus benefiting from all the technical drawings which already exist and all the fabrication
processes are mastered by LAL from the machining to the brazing This RF gun is comprised of 2 12
cells The output cell is magnetically coupled and the other end is short-circuited The main parameters
are given in Table 22
Frequency 299855 MHz
Rs 50 MΩm
Q 14500
β 1
E 80 MVm
PRF 5 MW
Table 22 RF characteristics of the photo-injector at 30degC in vacuum
The photo-cathode will be made of magnesium It has the big advantage of producing the specified
charge without requiring an in-situ photo-cathode preparation chamber which is a heavy and expensive
lsquo
17
installation Preliminary tests performed at LAL have demonstrated the possibility to obtain good
quantum efficiency with such cathodes
The main difficulty expected in operating this device is the laser cleaning of the photocathode surface
when polluted by an exposure to air To perform this cleaning a laser intensity of 160 microJmm2 is
required together with an RF electric field of at least 50 MVm The magnesium disk holder is not
designed yet it could be press fitted screwed or brazed
222 Solenoids
The gun is equipped with two solenoids one is the focusing coil while the other is the bucking coil
which cancels the magnetic field at the photo-cathode location They have been modeled by usning
POISSON A schematic is shown in Fig 22
Figure 22 Schematic of the gun with its coils
Dimensions
For both solenoids the inner diameter is 160 mm and the outer one is 500 mm The copper part is 30
mm long for the bucking coil and 56 mm for the focusing coil
The conductor used for these magnets is a standard one with a 6x6 mm2 square shape It is cooled by
water circulating in a 4 mm diameter channel One can easily calculate the current the resistance the
dissipated power and the temperature rise assuming that the cooling water is supplied at 20degC and 8
bars Values are provided for each lsquopancakersquo since the conductors are wound by columns of two The
bucking coil Bbc has two pancakes and the focusing coil Bfoc four
Bbc Bfoc units
Number of coils 84 168 MHz
Number of pancakes 2 4
Current -274 326 A
P 23 33 kW
R 306 31 mΩ
Flow 164 164 lmn
ΔT +20 +28 degC
Table 23 Solenoid electric and cooling parameters lsquoNumber of coilsrsquo refers to one pancake
Two power supplies are required one for Bbc (400A 30 V) and one for Bfoc (400A 60 V)
lsquo
18
223 Laser
The laser extracts a bunch of electrons from the photo-cathode surface by photo-electric effect Table
24 specifies its characteristics
Parameter value unit
l 260‐270 nm
Emax 100 plusmn 05 microJ
Beam transverse quality M2 M2 lt 11
Gaussian Pulse width 5 ps
Transverse profile square
Pointing stability 10 of beam diameter
Oscillator frequency 74963750 externally triggered with jitter lt 1 ps MHz
Amplifier repletion rate 50 externally triggered Hz
Table 24 Laser specifications for the RF gun photocathode
The laser architecture is shown in Figure 23
Figure 23 Architecture of the laser proposed by industry
An oscillator provides a signal at the specified frequency It can be either a fiber or a TiSaphire laser
Then the pulse is stretched amplified (with one or two amplifiers) recompressed and converted in the
UV range
The laser will be installed on an optical table in a hutch whose temperature and hygrometry will be
regulated
224 Laser injection chamber
The laser injection chamber is located at a distance of 8011 mm downstream of the photo-cathode It
contains a metallic aluminum mirror placed 30 mm away from the accelerator axis It provides an 80
reflectivity The angle of incidence of the laser beam on the photocathode is 214deg This mirror is fixed
To adjust the laser beam position on the photo-cathode and its incident angle we will use a motorized
mirror with a remote control placed on an optical table just beside the laser injection chamber
The size of this optical table will be 1x1 m2 On it there will be
Oscillator Stretcher Preamplifier Amplifier Compressor
nJ microJ
mJ
Frequency
shifter
Clock
Syncro
Line syncro
Shaper
lsquo
19
- 2 dielectric mirrors
- 2 beam splitters
- 1 UV photo-diode to monitor the laser intensity
- 1 CCD camera to observe the laser spot on a screen (in paper or lanex) facing the photo-cathode
- An optical beam expander to adjust the laser spot size on the photo-cathode
This is the minimum number of optical items that will be put on the table some changes may be made
depending on the very final geometry of the accelerator and laser transport lines The set-up design can
be finalized during the last stage of the accelerator construction since it involves standard components
only
225 Beam steering coils
The e- beam trajectory will be adjusted by sets of two steering coils which produce dipolar magnetic
fields over short distances along the X and the Y axes One such device will be located at the gun exit
and the other about 800 mm downstream Only very small position and angle errors must be corrected
Figure 24 shows the LAL design for such a coil Experience with the PHIL photo-injector at LAL [22]
showed that an integral of Bmiddotdl of 52 G m is sufficient The latter figure has been validated by
calculations made by a private company With a current of 5 A a Bmiddotdl of 58 G m was obtained that
results in a 100 mrad deviation of a 5 MeV beam Thus a 100 mm lateral displacement of the beam can
be produced at the entrance of the acceleration section
Figure 24 Schematic of the beam steering coils Dimensions are in mm
All together the steering coils need 4 bipolar DC power supplies Their maximum current is 10 A
lsquo
20
23 Acceleration section
The acceleration section is shown in Fig 25 Its characteristics are given in Table 25
Figure 25 Picture of the SOLEIL LIL acceleration section
Length 45 m
Number of cells mode 135 2π3
Frequency 299855 at 30degC MHz
Rs 63divide74 MΩ
Q 14800
Attenuation 74 dB
Energy gain 15 MW 60 MeV
τ 135 micros
Ez max 182 MVm
Table 25 Characteristics of the acceleration section
In a second stage of the ThomX project a call of tender will be issued for a new acceleration section
that should be shorter in order to make the machine more compact
lsquo
21
231 The RF source
Figure 26 shows the RF source schematic
Figure 26 Schematic of the RF source
The RF timing and its synchronization are described in chapter 5 A clock signal is sent to the linac
which uses it to trigger a 3 GHz synthesizer Its output is fed to a preamplifier which brings its power to
350 W The preamplifier is triggered at 50 Hz Then the 3 GHz signal is amplified by a klystron whose
gain can reach 53 dB to produce a 35 MW RF wave as required by ThomX This power is distributed
via a waveguide network which is described below
To drive the klystron one needs a modulator delivering a pulsed high voltage (HV) whose
specifications are given in Table 26 The klystron cathode must be heated by a power supply that
provides a current rising slowly from 0 to 25 A and a voltage rising from 0 to 30 A in 20 minutes A
precision of 1 on the current value is required The klystron is generally equipped with focusing coils
and their power supply whose standard parameters are 40 A 120 V
Repetition rate 50 Hz
Plateau length 99 gt 3 micros
Plateau ripples lt plusmn 05
Pulse duration at 50 6 micros
Rise time 10 divide90 lt 05 micros
Fall time 90 divide10 lt 1 micros
Pulse to pulse stability lt 05
Voltage range 0divide300 kV
Peak current 285 kV 300 Hz
Table 26 Modulator specifications
The classical and well-proven modulator technology relies on the high voltage (40 kV) loading of a
delay line made of inductances and capacitances The delay line is switched by a thyratron A new
technology is presently emerging based on solid-state amplifiers Each amplifier produces a 100 V
lsquo
22
pulse Thus to produce a 40 kV pulse roughly 400 amplifiers are put in a rack and their output pulses
are added The performances of both technologies are similar
232 RF network
The RF power is transported and distributed to the electron gun and the acceleration section by a
waveguide network shown in Fig 27
Figure 27 Schematic of the RF waveguide network
The WR 284 waveguide network is pressurized at two bars with SF6 between ceramic windows at the
klystron output and at the acceleration structure entrance A 5 dB bidirectional coupler splits the power
in two channels In the branch which runs toward the electron gun there is a 10 dB attenuator and a
circulator to protect the klystron from the RF power reflected by the electron gun In the branch running
toward the acceleration structure there is a 10 dB attenuator and a phase shifter to adjust the relative
phase between the gun and the acceleration section Various RF loads (five in total) ensure the system
safety by absorbing of RF power of the reflected waves
lsquo
23
The network also includes three couplers to measure the incident and the reflected RF power One is
placed at the klystron power exit one is placed at the gun entrance and the last one is placed at the
acceleration section entrance The RF gun is equipped with a loop that measures the stored RF energy
All of these loops used for measurements are weakly coupled at -70 dB The signals are measured with
crystals or C boxes
24 Beam dynamics
241 Nominal operation
The beam dynamics was simulated with the PAMELA tracking code for the ThomX nominal
parameters ie a bunch charge of 1 nC at 50 MeV The 50 MeV can be obtained with an electric field
of 80 MVm in the RF gun and 125 MVm in the acceleration section It is clear that a different set of
field values can lead to the total of 50 MeV In particular one may wish to operate the gun at a higher
field in order to improve the beam quality We will operate the linac with this possible improvement in
view
The beam will be accelerated on crest Thus the coil magnetic fields are the only parameters savailable
to minimize the transveres and the longitudinal emittances Simulations have shown that for precise
magnetic field values (see Table 27) the e- beam emittances are optimized as illustrated in Fig 28
20
15
10
05
00
σxy
(m
m)
6005004003002001000
z (cm)
20
15
10
5
0
em
it (
mm
mra
d)
6005004003002001000
z (cm)
20
15
10
05
00
ΔE
E (
)
6005004003002001000
z (cm)
46
44
42
40
38
36
34
32
30
σz (p
s)
6005004003002001000z (cm)
lsquo
24
Figure 28 Beam characteristics (rms values) as a function of the distance along the accelerator (z
coordinate) Upper left panel Transverse size upper right panel normalized emittance lower left
panel Energy spread lower right panel Bunch length
The beam parameter values at the linac end are given in Table 27
Beam parameters value unit
Energy 50 MeV
Beam size 17 mm
Normalised Emittance 44 π mm mrad
Energy spread 04
Bunch length 43 ps
Table 27 Beam parameter rms values at the linac end
The accelerator parameters which should ensure the above beam characteristics are given in Table 28
RF structure parameters value unit
Ez in the gun 80 MVm
PRF in the gun 5 MW
Ez in the section 142 MVm
PRF in the section 14 MW
Solenoid parameters value unit
Bz max 2500 G
Ibc -274 A
Ifoc 326 A
Laser parameters value unit
Energy impinging on a Mg photocathode 10 microJ
Square transverse profile diameter 2 mm
Variation of the intensity from axis to border 14
Table 28 Summary of the linac parameters
242 Alternative operation modes
Concerning the injector other operation modes can be envisaged One at least will occur namely an
operation at 70 MeV in order to extend the X-ray energy range up to 90 keV Such an option has
attractive applications In this mode the electron beam performances should remain exactly the same as
the ones prevailing at 50 MeV (cf Table 27) For the accelerator parameters the only change is the RF
power required for the acceleration section it must reach 195 MW instead of 14 MW at 50 MeV
The operator can also be tempted to change the RF phase The one used in the simulations gives the
minimum emittance A phase shift of -10deg of the RF gun with respect to the laser could lead to an
energy spread reduction by a factor 2 at the expense of an emittance degradation Whatever the goals
the linac operator must be aware of the high sensitivity of the beam parameters to the RF phase This is
illustrated in Fig 29 An error of 5deg in the acceleration section phase results in almost a doubling of the
energy spread
lsquo
25
Figure 29 Energy spread at the linac end as a function of the RF phase in the acceleration section all
other parameters keeping their nominal values
The RF electrical field in the gun also plays an important role in the beam performances The results of
simulations of the emittance variation as a function of the gun electric field are presented in Fig 210 If
this field reaches 120 MVm the emittance is reduced by 20 with respect to the 80 MVm case
However the possibility to operate the gun at such a high gradient can only be assessed experimentally
Figure 210 Injector beam emittance as a function of the electric field in the electron gun (Gaussian
laser beam 1 nC e- bunches)
The injector performances as a function of the bunch charge are summarized in Table 29 Three modes
will be available in order to reduce the effect of collective instabilities at injection
Parameters Mode 1 Mode 2 Mode 3 Unit
Q 1 05 01 nC
σxy 165 15 08 mm
εxy 47 33 11 π mmmrad
σz 44 4 33 ps
σEE 04 03 02
Table 29 Linac beam performances (rms values) as a function of the bunch electric charge
As one can see in the above Table reducing the bunch charge by a factor 10 leads to an emittance
reduction by a factor 4 Although the X-ray flux is reduced the source brightness ie the ratio of the
flux to the divergence is improved by a factor 16 As a consequence a higher number of X-rays can be
20
18
16
14
12
10
08
06
04
02
δΕ
Ε (
)
151050-5-10
φ (deg)
10
9
8
7
6
5
ε (πmmmrad)
1401201008060
E0 (MVm)
lsquo
26
transported to a sample (number per unit area) Whatever the eventual benefits to the X-ray production
the advantage for the accelerator to operate at a lower charge per bunch is clear the reliability of the
accelerator is much enhanced the laser energy requirement is reduced and it becomes also possible to
use a copper photo-cathode instead of a magnesium one easier to handle But the main advantage lies
in the ring operation Indeed a 1 nC bunch whose length is only 4 ps induce a strong wakefield
stemming from the interaction between the beam and the vacuum pipe Such a wakefield perturbs the
beam and may cause oscillations of the bunch centroid possibly instabilities and thus a reduction of the
X-ray production rate
25 Photo-injector and acceleration section vacuum
Simulations of the photo-injector vacuum have been performed assuming a pressure of 3 10-8
Pa at the
cathode location and requiring a vacuum time constant (VS) that ensures an easy gun conditioning This
constraint must be respected to maintain a 10-7
Pa pressure at the acceleration section entrance where the
system will be baked to obtain an outgassing rate τ = 5x10-10 Pa m s-1
(N2)
Pressure in the line of injection
The use of a semi-conductor cathode (Cs2Te) puts severe constraints on the vacuum and requires a preparation
and a transfer chamber To avoid these complications some tests have been made on a metallic cathode (Mg)
The first attempt ran into difficulties (it took 12 h to reach a gradient of ~ 60 MV m-1
) we observed pressure
rises and sparks In presence of only the RF power we observed hydrogen CO and CH4 outgassing (as the
main components) A slight reduction in the quantity of water and oxygen was noticed due to chemical re-
composition When the laser beam was introduced to extract electrons a pressure increase by a factor of ten
was observed The 24th
25th
and the 26th
peaks also significantly increased for the HxCy compounds but no
Mg was measured
lsquo
27
Gas analyzes during the conditioning of cathode Mg
On the basis of simulations and experimental results we identified the need for the following items
‐ 2 ion pumps 40 l s-1
(upstream of the cathode amp injector) with their baking systems and power
supplies
‐ 2 ion pumps 55 l s-1
(for the RF network) with their baking systems and power supplies
‐ 1 ion pump 150 l s-1
(for the laser introduction chamber) with its baking system and power supply
‐ 1 ion pump 300 l s-1
(for the acceleration section) with its baking systems and power supply
‐ 1 gate valve manually actuated DN 100 (injector- upstream) and a baking valve
‐ 1 gate valve electro-pneumatically actuated DN 63 (injector - downstream) and a baking valve
‐ 1 gate valve electro-pneumatically actuated DN 40 (injector- downstream) and a baking valve
‐ 3 Pirani gauges (before the injector injector laser introduction acceleration section)
‐ 3 cold cathode gauges (after the injector injector laser introduction acceleration section)
‐ 2 power supply gauges
‐ 1 residual gas analyzer (RGA)
‐ 1 turbo pump (DN 63 ~ 80 l s-1
)
‐ 1 rotary vane pump (~ 12 m3 h
-1)
‐ 4 angle valves manually actuated for pre-pumping DN 63
‐ 2 T with RF grids
lsquo
28
Chapter 3 TRANSFER LINE and RING
31 Transfer line
The transfer line (TL) has to transport the beam from the linac to the ring and to match the optics of
these two devices In addition a set of beam diagnostics needed for an efficient injection in the ring are
localized along the line as sketched in Fig 31 The transfer line main characteristics are summarized in
Table 31
Figure 31 Layout of the injection and extraction transfer lines
A particular attention is paid to ensure a versatile optics that allows the matching of different linac
beams to different ring optics Simulations have been carried out using the BETA code [31] The
transfer line optics is optimized in order to accommodate some emittance degradation which may arise
from collective effects
Screen station + dump
Linac emit + pulse length diags
Screen + Scraper
Linac energy diags
Screen station
TL emit diags
Extraction line dump
lsquo
29
Figure 32 Transfer line optics including the ring The vertical line indicates the ring entrance point
The nominal transfer line optics is shown in Fig 32 from the linac to the ring together with the ring The
nominal input bunch Twiss parameters are x = z = 34 x = z = 42 for Q = 1 nC bunch length = 4
ps (rms) and an optimized normalized emittance of about 5 πmmmrad The strong bend produced by the four
dipoles of the transfer line increases the risk of emittance degradation under collective effects but simulations
show that the emittance increase is only of about 20 including coherent synchrotron radiation (CSR [2]) and
3D space charge effects (see Fig 33)
Figure 33 Emittance increase due to chromatic (cont lines) CSR (dashed) and space charge (dot dashed)
effects for 1 nC and 4 ps (rms) bunches at 50 MeV along the TL and the first revolution in the ring
The magnetic elements are identical to those of the ring
lsquo
30
Parameter value unit
Length 14 m
Nominal energy 50 MeV
Beta max x z 70 40 m
Maximum dispersion 1 m
Dipole numberfamily 4 1
Quadrupole number 7
Biplane corrector number 5
Table 31 Transfer line parameters and components
Besides ensuring the matching between the linac and the ring the transfer line must provide the measurement
of the beam emittances the beam energy and the energy spread Care has been taken to locate the diagnostic
stations in a way that ensures accurate measurements of these important parameters
The emittance measurements will be done by means of the lsquothree-gradientrsquo scheme (The pepper-pots
alternative is also foreseen) For instance with an adequate refocusing that produces a double waist (vertical
and horizontal) at the screen it is possible to scan both planes at the same time by varying only one
quadrupole gradient (see Fig 34) Other scan variations are also possible The following figures show the
optics retuning as well as the beam spot size variation at the screens of both stations the one at the linac end
and the one downstream of the U-turn
Figure 34 Optics retuning and beam spot size scan as observed on the screen (simulation)
The electron energy and its spread are measured in between the dipoles by a dedicated optics Note that it is
also possible to retrieve the bunch length by measuring the energy spread as a function of the linac phase
(three-phase scheme)
lsquo
31
32 Extraction line
To dump the residual beam at the end of the storage time (typically from 20 to 50 ms) an extraction line
composed of one small dipole and two quadrupoles is also foreseen This line will allow the measurement of
the beam properties after it has circulated in the ring The injectionextraction devices are symmetrically
implemented in the ring injection section
33 Storage ring
331 Optics and lattice
Thom-X must produce a high X-ray flux of 1011
divide1013
phs in a stable way The flux depends on the
characteristics of the electron bunch and of the laser pulse Assuming a 1 nC electron bunch charge at
50 MeV and a 20 to 30 mJ laser pulse ( 124 microm) the optics constraints at the interaction point (IP)
are as follows
- an rms size of the electron beam less than or equal to 70 microm in the transverse plane requiring
low beta functions of 01 m with an rms geometric emittance of 50 nmrad injected from the
linac (5 mmmrad normalized)
- a vanishing dispersion at the interaction point in order to avoid a beam size increase resulting
from an e- energy spread increase as well as to avoid potential synchro-betatron coupling
resonances
- a bunch length less than 20-30 ps to ensure a good overlap with the laser pulse (the crossing
angle will be 2deg) and long enough to reduce collective effects stemming from the pipe
impedance CSR bunch internal scattering etc
Besides these specific IP constraints there are two general layout constraints namely
- a ring circumference as short as possible with enough straight sections to accommodate the
interaction region the RF the injection and possible other equipment as RF and transverse
feedback systems
- the need for some flexibility as far as the optics and the working point are concerned
The ring layout and its optics are based on a two-fold symmetric lattice with four achromatic arcs and
four straight sections two short ones (02 m) and two long ones (16 m) This layout preserves the ring
compactness and provides an adequate interaction region in one of the two straight sections between
two adjacent dipoles (cf Fig 35) The main ring characteristics are summarized in Tables 32 and 33
lsquo
32
Figure 35 Ring layout
The laser pulse frequency is the harmonic 2 of the e- bunch revolution frequency in the ring ie 357
MHz This leaves open the possibility of storing two bunches in the ring in order to increase the
Compton flux but at present this operation mode is not taken into account because of injection
constraints The optical cavity mirrors are located outside the ring vacuum chamber thus enabling a
mirror substitution without affecting the ring In addition the extraction of the X-rays can be performed
closer to their source than in the case where the optical cavity is inserted in a long straight section
The two long free straight sections accommodate the injectionextraction devices for one and the RF
and feedbacks for the other
The ring is composed of 8 dipoles 24 quadrupoles and 12 sextupoles Its circumference is 168 m long
the revolution frequency is 1785 MHz The RF frequency is set at 500 MHz ie at the 28th
harmonic
of the ring revolution frequency The nominal optical functions have been optimized with the BETA
code [31] They are plotted in Fig 36 The dispersion vanishes in the long and in the short straight
sections (RF injection and interaction regions) The beta functions decrease down to 01 m at the IP to
provide a 70 microm focus assuming that the injected beam has an emittance of 50 nmrad (5 mmmrad
normalized) in both the vertical and the horizontal planes
RF and feedback section
Injection extraction section
IP 1
Interaction Point (IP)
lsquo
33
Ring parameters value unit
Circumference 1680 m
Nominal energy 50 MeV
Betatron tunes nx nz 317 174
Beta max xz 11 11 m
Dispersion max 08 m
Beta xz dispersion IP 01 01 0 m
Momentum Compaction Factor 0014
RF frequency 500 MHz
RF harmonic 28
RF voltage 300 kV
Synchrotron frequency 360 kHz
Period Revolution frequency 56 1784 nsMHz
Natural chromaticities -32 -82
Dipole nb family 8 1
Quadrupole nb families 24 6
Sextupole nb families 12 3
Corrector nb families 12 12
Table 32 Ring parameters
Max field 70 MeV Bore length Gap Diameter
Dipole 07 T 2765 mm
(R=352 mm)
40 mm
Quadrupole 5 Tm 150 mm 41 mm
Sextupole 15 Tm2 60 mm 42 mm
Correctors 510-4
Tm
Table 33 Main magnet characteristics
Taking into account the constraints imposed by the optical functions the chromaticity and the dynamic
aperture the operating point was chosen with tunes νx=317 and νz=174 There is still enough
flexibility to vary these tunes by plusmn 02 without spoiling the main ring performances The momentum
compaction factor can also be changed over a large range in order to scan the best working point
regarding the beam dynamics and the X-ray flux
Figure 36 Nominal beta functions and dispersion over one super-period
lsquo
34
The RF acceptance for the nominal momentum compaction factor (MCF) and a larger one including the
non-linear MCF and the vacuum pipe limit are plotted in Fig 37
Figure 37 RF acceptance Left panel nominal lattice Right panel for a MCF twice as large as the
nominal one VRF= 300 kV
RF acceptance nominal r56 = - 02 m r56 = - 04 m
Linear (RF bucket) 10 10
Non linear -32 +18 - 68 +4
Pipe limit 25 25
Table 34 RF acceptance for two different values of the momentum compaction factor
The two following plots (Fig 38) show (i) the vacuum chamber occupation by the beam at injection
when the emittance is 50 nmrad and the energy dispersion is 03 and (ii) at the end of the storage
cycle when the emittance reaches 300 nmrad and the energy dispersion reaches 06 (see section
438) The orbit components (misalignment errors) are not included since they are negligible once
corrected The choice of the pipe aperture (40 mm in H by 28 mm in V full aperture) takes into account
several constraints the beam pipe occupation the magnet bore radius the vacuum and the coherent
synchrotron shielding [32]
b a
lsquo
35
Figure 38 Transverse occupation (3 rms) of the vacuum chamber at injection (left) and at the end of
the storage cycle (right)
332 Non-linear dynamics
The natural horizontal and vertical chromaticities are respectively -32 and -68 Chromaticity
corrections are achieved by means of 12 sextupoles (3 families) located in the dispersive sections
Without performing a specific optimization the dynamic aperture is large enough about 50 times the
rms transverse size of the injected beam (see Fig 39) the quadrupole fringe fields being taken into
account Furthermore this dynamic aperture remains about the same for off-momentum particles (up to
plusmn3) compared to particles having the nominal energy The fringe field of the dipole magnets
introduces high order non-linear components Since the Thom-X dipoles are short with a large gap
their magnetic design (see section 34) has been carefully worked out in order to maintain the dynamic
aperture
Pole number 6 8 10 12 18 20 28 30
Inner dipoles +10 0 +3
Outer dipoles -16 -1 -6
Quadrupoles +10 +5 -5
Sextupoles +10 -10
Table 34 List of multipole tolerances for each kind of magnets expressed in 10-4
relative field variation
at 18 mm radial position
Figures 39 and 310 show the change of the dynamic aperture with and without considering the
multipole (the fringe field of the quadrupoles is also taken into account in these plots) These
Dynamical Apertures (DA) have been calculated with two different codes BETA and TRACYIII They
lsquo
36
both give very similar results In addition the TRACYIII code dedicated to long symplectic tracking
performs Frequency Map Analysis (FMA) giving more detailed information concerning the particle
diffusion process
Figure 39 On-momentum DA and FMA for the nominal ThomX lattice Beta code left and TracyIII
code (right) sextupoles only (top) and including magnet multipole tolerances (bottom)
lsquo
37
Figure 310 Off-momentum DA and FMA for the nominal ThomX parameters Beta code left and
TracyIII code (right) sextupoles only (top) and including magnet multipole tolerances (bottom)
333 Collective effects feedbacks
At low energy and thus low beam rigidity the electron bunch is sensitive to electromagnetic self-
perturbations that act revolution after revolution in the ring Several instability sources can be
identified
‐ Interaction with surrounding metallic items such as the resistive walls (RW) the RF cavity and
various discontinuities such as bellows BPMs and flanges HOMs etc
‐ Space charge forces
‐ Coherent synchrotron radiation (CSR)
These interactions affect the bunch energy and the bunch longitudinal time-profile leading to
distortions emittance growth bunch oscillations or even above some threshold instabilities and beam
losses Similar interactions may also affect the transverse beam dynamics but to a lower extent The
two relevant parameters for the instability threshold are the bunch charge and its length The strength of
the interaction is proportional to the bunch charge and varies inversely with the bunch length
The two plots in Fig 311 show the longitudinal wakefield over one revolution at 50 MeV generated by
a 1 nC Gaussian bunch with an rms length of 12 and 6 mm (4 and 20 ps) Five different contributions
are compared in this figure
‐ the shielded CSR following the Murphy and Skrinsky [32] formulation with a beam pipe full height
of 40 mm a dipole field of 05 T and a curvature of 0352 m
‐ the longitudinal space charge (LSC)
‐ the resistive wall (RW) following the K Banes and M Sands formulation [33] with a full beam
pipe height of 40 mm and an electrical conductivity of 14 106
‐ the vacuum vessel broadband component including 12 BPM 12 flanges and 12 bellows based on
the synchrotron SOLEIL type This potential has been calculated by means of the HFSS finite
elements code [34]
‐ the 500 MHz ELETTRA type cavity broadband component This potential has been calculated by
using the HFSS finite element code
They exhibit a very strong CSR wake field (up to few per mil) with the shorter bunch case
Figu
re
311
Lon
gitu
dina
l
wak
e
field
over
one
lsquo
38
turn generated by a 1 nC Gaussian bunch at 50 MeV for a bunch length of 4 ps (left panel) and 20 ps
(right panel)
The ThomX low energy operation (50 MeV) practically suppresses the natural damping effect of the
synchrotron radiation and complicates the bunch stabilization Each kind of perturbation grows
irreversibly implying that feedback systems in the three planes are essential to cope with the induced
oscillations of the bunch center-of-mass In addition to these collective effects the unavoidable
injection jitters have also to be taken into account To calculate these effects 6D tracking simulations
that include the longitudinal impedance as well as the large mismatch between the linac and the ring
optics in the longitudinal phase space have been performed (see Fig 312)
Large longitudinal oscillations
without feedback at injection
induced by collective effects
Longitudinal oscillations damped by the
RF phase feedback system operating at
F~360 kHz
Damping lt 50 micros
Figure 312 Tracking results (50 MeV
1 nC bunch) over the first 3000
revolutions just after injection without
and with the RF phase feedback system (VRF = 300 kV)
Large oscillations start very quickly after injection when the bunch is still short (4 ps) and are of the
same order of magnitude as the ones induced by the energy-phase jitter that affect the linac injection
This effect is the one to address in the design of the longitudinal feedback As far as the transverse plane
is concerned the growth times characterizing various instability sources are listed in Table 35 The
revolution-by-revolution feedback strength indicated for each instability source is the one needed to
suppress the oscillations
Source Type Growth time Revolution-by-
revolution kicker
strength
Beam pipe
geometry
TMCI
Head-Tail
-
160 micros
gt10 nrad
Resistive Wall
600 micros gt 2 nrad
Ions lt 100 micros gt20 nrad
Table 35 Transverse instability growth time and feedback kicker strength for a 1 nC bunch at 50 MeV
lsquo
39
In contrast with what prevails in the longitudinal phase space the main transverse oscillation source is
the injection position jitter Since the non-linear sextupole components are large this jitter may produce
a large emittance increase as a consequence of the phase space painting process (see Fig 313)
Figure 313 Sketch of the transverse phase space painting process
Simulations show that a very fast increase of the transverse emittance can result from steering errors at
injection in about 500 revolutions or 25 micros To overcome this effect a fast transverse feedback is
needed it has to damp the oscillation in about 100 revolutions or ~5 micros The strength of the revolution-
by-revolution feedback has to be on the order of 2 microrad This strength is far larger than the ones
estimated to cope with the transverse instabilities For this transverse feedback a dedicated strip-line
acting in both planes is foreseen in the RF section (see Fig 314)
Figure 314 Simulated transverse emittance increase induced by a 05 mm injection jitter with and
without feedback
After N
turns
lsquo
40
Phase space Specifications Device
Horizontal amp vertical Fr Frequency = 178 MHz (F0)
Strength = 2 microrad
Dedicated strip-line for
both planes
Longitudinal Frequency = 360 kHz (Fs)
Strength = 105 rads
Main RF phase loop
Table 36 Summary of the feedback specifications
Other collective effects such as the intra-beam scattering (IBS) which is important at low energy or the
Compton recoil effect play an important role during the 20 ms storage time Their dynamics differ from
an instability dynamics but they also increase the electron energy spread and the bunch emittance mdash
and thus reduce the X-ray flux
To summarize three different contributions to the e- bunch dynamics must be dealt with
‐ at injection jitters which may affect either the injected bunch energy or its trajectory or the precise
timing of its injection in the ring generate oscillations which may be large and which must be
quickly damped by feedbacks in all three dimensions (x y z)
‐ during a transition period corresponding to the first ~1000 revolutions strong collective effects
enhanced by a possible synchronization error between the linac and the ring may lead to a complex
turbulent regime in the longitudinal plane prior to ring matching (elliptical phase space density
profil)
‐ during the 20 ms storage period IBS and CBS produce a progressive increase of the bunch size in
all three directions This evolution cannot be stopped by a feedback system In contrast RF HOMs
and other perturbations may generate instabilities that should be controlled by feedbacks
To take into account collective effects a dedicated code has been developed including a collection of
wake field as well as non-linear dynamics It is based on a 6D multi-particle symplectic tracking
restricted to perfect hard edge drift quadrupoles and dipoles of finite length Other higher order
elements as sextupoles are treated in the thin length approximation The RF cavity is simply treated as
a longitudinal cosine kick The mapping through elements is given by the exact Hamiltonian solution
[34] for the drift sections and the dipoles (including their small curvature and dipole edge effects) The
quadrupole mapping is restricted to small radial angles It has been compared to various codes
(TRACYIII ELEGANT BETA and MADX) by performing simulations of the SOLEIL and the
ThomX rings with special care given to the dipole edge effects [35]
The transverse beam size at IP and the bunch length are plotted in Fig 315 over the first 10 000
revolutions or 056 ms with a 1 nC bunch charge This study shows that a small positive chromaticity in
the horizontal plane is necessary to stabilize the horizontal emittance During the very first revolutions
the tune dependent emittance behavior has not been fully investigated nor understood It appears that
this behavior follows from the bunch length mismatch between the linac and the ring The fact that the
linac produces very short bunches of 4-5 ps (rms) leads to a complex turbulent dynamics (see Fig
316) The effect of this mismatch disappears progressively over the first 1000 revolutions then the
bunch length reaches 20-25 ps (rms)
lsquo
41
Figure 315 Red RMS beam size and bunch length at IP Magenta bunch centroid position over the
first 10 000 revolutions (056 ms) for a 1 nC bunch The beam energy is 50 MeV
Figure 316 Turbulent dynamics in the longitudinal phase space that us taking place just at injection
These plots show the phase space density after 100 and 3000 turns
lsquo
42
334 CBS and IBS effects
The electron beam dynamics that prevails in a ring where Compton back-scattering is taking place is
similar to the one resulting from synchrotron radiation (SR) in a synchrotron light source CBS gives
rise to random excitation in the transverse and the longitudinal planes In the transverse plane the
emittance turns out to remain close to the one at injection [37] In contrast the CBS excitation strongly
affects the dynamics in the longitudinal phase space It leads to a substantial energy spread increase
over 50 ms storage time (see Fig 317a) resulting in a significant bunch lengthening
Based on the lattice parameters the calculation of the Touschek lifetime leads to a value on the order of
10 s this effect can thus be neglected
Figure 317 a) Energy spread increase and b) Radial (x xrsquo) relative emittance increase during a 100 ms
storage time Blue curves CBS effect only red curves IBS effect only black curves (almost on top of
the blue ones) CBS and IBS combined effect The IBS calculation is based on the Mtingwa formalism
[38]
In contrast the multiple Touschek effect (lsquoIBSrsquo) plays a significant role in the radial phase space (x xrsquo)
as shown in Fig 317b After 20 ms this horizontal emittance is increased by a factor ~ 4 from 50 to
200 nmrad and the relative energy spread by ~20 from 03 to 036 as can be seen in Fig 317a
Figure 317 shows that the Compton back-scattering process plays a role by far more important than the
IBS does It also shows that because of the large emittance increase produced by the CBS the bunch
density decreases by a large amount making the IBS much less effective (see Fig 317b where the
curve showing the combined effects lies below the curve illustrating the IBS effect by itself)
When the electron beam energy is increased up to 70 MeV the IBS effect on the horizontal emittance
becomes less important During a 20 ms storage cycle this effect is reduced by a factor 2 On the other
hand the energy spread degradation increases by more than a factor two compared to the 50 MeV case
This change is due to the higher energy of the back-scattered photons In contrast if the electron beam
energy is decreased down to 25 MeV the energy spread is reduced by a factor two while the horizontal
emittance increases by a factor seven compared to the 50 MeV case
Assuming an optimal overlap between the e- bunch and the laser pulse in the interaction region and
neglecting the e- bunch size variation across this region (the lsquohourglassrsquo effect) as well as the variation
of the laser beam envelope near its waist (characterized by the Rayleigh length) the X-ray flux is given
by the following formula (see Eq 11)
The nominal storage time is 20 ms only
lsquo
43
where σth is the Thomson cross section Nx is the number of photons per laser pulse f is the e- bunch
revolution frequency ϕ is half the collision angle σxyze σxyzγ are respectively the three dimensional
bunch rms sizes for the electron (e subscript) and for the laser pulse (γ subscript) In fact because of the
increase of the e- bunch size during the storage time the flux is reduced by roughly a factor two at the
end of a cycle as shown in Fig 318
Figure 318 X-ray flux as a function of the storage time These results take into account the energy
spread increase and the emittance degradation due to IBS only (blue curve) and to the combined CBS
and IBS effects (black curve)
To calculate more precisely the X-ray flux produced by Thom-X the code CAIN [39] (cf sect 12 above)
has been included in the previous 6D tracking code including the non linear effects of the lattice
magnets as well as collective effects The X-rays characteristics are computed at each ring revolution
The intra-beam scattering effect is calculated also at each e- bunch revolution based on the Bjorken and
Mtingwa formalism [38] To reduce the computing time the code extrapolates the IBS effect whenever
there is only a little change from one revolution to the next (this method reduces the computing time by
a factor 4) This code runs on linux OS Using the computer farm of the IN2P3 computing center in
Lyon it takes about 15 days to simulate a full Thom-X cycle (20 ms ie 400 000 revolutions) when the
e- bunch is modeled by 500 000 macro-particles
The way the total X-ray flux decreases during the first 10 000 revolutions is shown in Fig 319 This
decrease is due to the e-
beam quality degradation mainly due to the CBS effect Also shown in this
figure is the X-ray flux evolution as a function of the storage time for three different lower cut-offs in
the X-ray energy spectrum Figure 320 shows a typical angular distribution of the X-ray emission
Figure 321 plots the bunch longitudinal phase space for the first revolutions The macro-particles in
green are those which have been hit by photons at the last crossing while those in blue have been hit
before the others are in red
( ) ( ) φσ+σ+φσ+σσ+σ
φfNNσ=N
2
γzez
2
γxex
2
γyey
γeth
x22222
sincos
2πcos
lsquo
44
Cross comparison and benchmarking of this code against other codes developed for ThomX (6D
tracking without Compton effect or Compton + IBS without tracking) are in progress and indicate a
good agreement between the codes
Figure 319 Left panel Number of X-rays produced as a function of the X-ray energy at the end of the first
10 000 first revolutions (in red) and during the last 10 000 revolutions before extraction (in blue) Right
panel Number of X-rays produced summed over 2500 revolutions as a function of the revolution number
Figure 320 X-ray intensity as a function of the emission angle in mrad (horizontal axis) and their
energy in eV (vertical axis) The color bar indicates the number of X-rays
lsquo
45
Figure 321 Upper nine panels Evolution of the e- beam longitudinal phase-space after injection The macro-
particle positions are plotted along the horizontal axis while their energy deviations are plotted along the
vertical one Green electrons which have collided with photons at the last crossing blue electrons which
have collided before red electrons which have not yet undergone any collision These plots reveal a turbulent
dynamics Lower nine panels Time evolution of the beam longitudinal phase space as a result of a turbulent
regime from the initial lsquobanana shapersquo at injection to a Gaussian distribution after a few ms (colors indicate
the electron number-density red corresponds to the most populated region)
lsquo
46
Figure 322 Evolution of the X-ray energy spectrum when the e- beam energy is increased from 475 to 505
MeV Left panel The X-ray energy spectrum is plotted vertically for seven e- beam energies Colors
characterize the X-ray intensity Right panel Each curve shows the total number of X-rays above some lower
energy cut-off Green cut-off set at 20 KeV blue cut-off at 30 KeV red cut-off at 40 KeV
335 Scattering on the residual gas
Coulomb scattering of the electrons on the residual gas molecules induces an increase of the circulating
beam emittance The amplitude of this effect is proportional to the average of the beta functions and to
the gas partial pressure Moreover it varies as the square of the atomic number of the residual gas
atoms and is inversely proportional to the electron energy With a low beam energy of 50 MeV it is
therefore essential to keep a low pressure to minimize the time-averaged emittance Fig 323 shows the
growth of the transverse emittance during the 20 ms storage time in the nitrogen case and in the
hydrogen one Assuming that one deals with nitrogen at a pressure of 8 10-7
Pa the emittance increases
by a factor of two (see Fig 323) while the degradation is negligible in the dihydrogen case We
conclude that a pressure lower than 3 10-7
Pa must be maintained to keep the emittance growth at an
acceptable level
Figure 323 Emittance increase due to Coulomb scattering assuming that the residual gas is composed
of a) 100 nitrogen b) 100 dihydrogen
lsquo
47
336 Ion effects
The beam-ion instability [310 39 312] originates in the ionization of the residual gas molecules Ions
can subsequently be trapped by the electron beam electrostatic potential We recall that the on-axis
potential is inversely proportional to the beam size
When the trapped ion number density reaches some threshold beam instability is triggered Besides this
effect trapped ions damage the beam quality by shifting the tunes increassing the emittance and
lowering the beam lifetime since the local pressure is increased
For the Thom-X ring the critical mass above which ions are trapped is on the order of one in units of
the proton mass Thus all ions whatever their mass can be trapped in our case
Once trapped ions oscillate transversally in the potential well Their oscillation frequency depends on
the electron beam size and on the ion species (see Fig 324 1c) Because of the e- beam bunching
when the ion oscillation frequency becomes larger than the beam betatron frequency a lsquofast ion
instabilityrsquo may develop the e- bunch is submitted to the electric field of the oscillating ions eventually
resulting in an exponential growth of the oscillations of all particles (electrons and ions) This instability
arises above some value of the neutralization factor which depends on the lattice
The neutralization factor and the growth time of either the vertical or the horizontal oscillations are
shown in Fig 324 as a function of the horizontal beam size for several ion species For a horizontal
beam size averaged over the ring of 05 to 1 mm (which is about what is expected) the growth time is
very short (less than one micros) but for this would happen for a neutralization factor much larger than one
obviously this is excluded To keep under control the other ion harmful effects mentioned above one
may prevent ion accumulation over many storage cycles by introducing gaps between short series of
injections or by clearing the ions by an electrostatic potential that cancels out the potential well
produced by the e- beam A simple solution consists in polarizing the beam position monitors (BPMs)
thus avoiding the insertion of electrodes along the ring This scheme should be effective since ions will
drift longitudinally as the kicks they receive at each e- bunch revolution has a longitudinal component
The amplitude of these kicks along the z direction is shown in Fig 325 where the BPMs are
represented by circles They are placed near accumulation points where the longitudinal kick vanishes
ie where ions will converge A difficulty may arise at s =125 m This point is opposite to the IP in the
lattice at this point the e- beam has small transverse dimensions and a vacuum pump is placed Indeed
with a small beam size the neutralization factor threshold is very low (10-3
) and the growth time
constant is on the order of one micros This potential difficulty can be overcome by adding an electrode in
the short section between the two dipoles to clear the ions at this accumulation point
Ion clearing with polarized BPMs or gaps introduced in the injection sequence should keep other ion
effects at an acceptable level (more work is needed to quantify this statement)
Defined as the total number of ions divided by the total number of electrons
lsquo
48
Figure 324 a) Threshold neutralization factor b) growth time and c) ion oscillation frequency for the
following fractional becirctatron tunes 1) 064 and 2) 017 for different ion species The dashed lines show
the e- bunch betatron frequency σx is the mean beam size
Figure 325 Longitudinal kick given to on-axis ions at each e- bunch revolution
337 Electron beam injection and extraction
Originally the linac beam was injected in the ring by using only a septum followed by a fast kicker
Given the high repetition rate (50 Hz) radiation safety issues and a possible noise in the X-ray
experimentation area lead to set up an e- beam extraction line The symmetric septum magnet located in
lsquo
49
the middle of one of the long straight sections is used in a double pass mode for injection and
extraction It is flanked by two identical kickers one downstream for the injection and one upstream for
the extraction (Fig 326)
Figure 326 Sketch (top panel) implementation (middle panel) and simulation (lower panel) of the
injectionextraction process
The main characteristics of the septum and the kicker magnets are listed in Table 37 Since in both
processes the beam runs through quadrupoles some margins with respect to the nominal deflection
values must be provided to cope with the fact that quadrupole fields are not uniform
Kicker Septum
Deflection 15 mrad max (11 nominal) 170 mrad (40 mTm 70 MeV)
Pulse duration lt 56 ns half sine 130 micros full sine
Effective length 200 mm
250 mm
Overall length 450 mm 500 mm
Table 37 Main characteristics of the pulsed devices
Septum center
x =14 mm
xp=9 mrad + 170 mrad
Kicker center
x =0 mm
xp=0+11 mrad
2 fast kickers
Inj Ext
1 slow septum
lsquo
50
338 Orbit corrections
A set of 12 beam position monitors (BPMs) and 12 correctors in both planes are implemented along the
lattice in order to control and correct the orbit An additional BPM located at the IP is fixed on the
table that supports the laser cavity The main error sources are listed below Uncorrected and corrected
orbits are illustrated in Fig 327
rms value dBB dL dX dZ dS tilt
Dipoles 10-4 01 mm 02 mm 01 mm 02 mrad
Quadrupoles - - 02 mm - -
Dipoles 10-4 01 mm 02 mm 01 mm 02 mrad
Quadrupoles - - 02 mm - -
Table 38 Main magnet misalignment errors
Uncorrected orbit with typical magnet alignment errors
rms value xmax (mm) zmax (mm)
Max 7 3
IP 05 03
rms value xʹmax (mrad) zʹmax (mrad)
Max 12 4
IP 6 3
lsquo
51
Corrected orbit
rms value xmax (microm) zmax (microm)
Max 200 140
IP 200 30
rms value xʹmax (microrad) zʹmax (microrad)
Max 600 300
IP 350 60
Figure 327 Uncorrected and corrected closed orbits Red lines horizontal projection blue lines
vertical projection
Quadrupoles and sextupoles are fixed on girders with a maximum error of 50 microm The magnetic field
center error relative to the magnet yoke being of the same order the expected total alignment errors are
~100 microm in each plane The girders supporting all the elements will be aligned as solid blocks Since
lsquo
52
dipole misalignments have a large effect on the orbit dedicated additional support tools will allow the
readjusting of the dipole positions relative to the girder
One needs a maximum corrector strength of 05 mrad when this strength is computed by assuming just
one sigma alignment errors to cope with alignment errors which would be twice as large corrector
strengths of 1 mrad are thus necessary
Figure 328 Orbit bumps (red horizontal blue vertical) that can be produced at the IP
In addition to this standard orbit correction procedure a dedicated orbit scan at the IP will be very
useful to ensure an optimal overlap between the e- bunch and the laser pulse in the interaction region
(see Fig 328) To provide a scan range of up to 1 mm or 1 mrad the corrector strength must be further
increased by 1 mrad Thus the corrector maximum strength will be 2 mrad
The orbit correction will be performed by a slow feedback operating around 01 The correctors are in
the sextupoles magnets (combined magets) The orbit feedback correction will also control the RF
frequency in order to keep the orbit length at the ring design value A specific synchronization scheme
that ensures the proper injection timing and that maximizes the electron-photon collision rate is
described in chapter 5
Position amplitude scan at IP location up to 1 mm
Angle amplitude scan at the IP
location up to 1 mrad
lsquo
53
34 Magnets
Table 39 describes the magnets that will equip the transfer line the ring and the extraction line
Magnet type Dipoles Dipoles for inj Quad Sext Corr
Number 4+8+2 2 7+24+2 12 4+12+1
PS number 3 2 33 3 34
Strength 0180 T m 5 Tm 40 Tmsup2 05 mTm
Length 2765 mm 150 mm 60 mm 30 mm
Table 39 ThomX magnet parameters
341 Dipole magnetic design
The dipole bending angle is 45deg A magnetic field of 05 T is needed at 50 MeV Since their radius of
curvature is very small (0352 m) special attention has been paid to the field profile between the magnet poles
and to the pole edge shape where large multipole components are present Lateral shims and pole edge
specific profiles have been optimized with the TOSCA3D code to minimize the impact of the multipoles on
the beam dynamics
Characteristics unit value
Gap mm 40
Field at centre T 063
Overall dimension mm3 350 x 400 x 400
Current A 263
Voltage V 177
Pole width mm 100
Pole height mm 100
Yoke length mm 2765
NI Aturns 13000
Coil area mmsup2 67x40
Current density Ammsup2 116
Number of turns turns 48
Transverse size of the conductor mmsup2 6x6
Cooling channel mm 4
Water pressure drop bar 7
Temperature increase degC 47
Flow rate (per dipole) lmn 86
Electric power (per dipole) W 3100
Table 310 ThomX dipole parameters
lsquo
54
Figure 329 Drawing of a ThomX dipole
342 Quadrupole magnetic design
The quadrupole design (see Fig 330) was also performed with the TOSCA software The magnetic field
is produced by four coils carrying a DC current The conductor is wrapped around four poles which
concentrate and define the quadrupole magnetic field map
Figure 330 Quadrupole magnetic design
A quadrupole can be manufactured in four identical parts each one comprising one pole and one quarter
of the yoke or the yoke and the four poles can be manufactured separately The quadrupole will be
equipped with special shims which will be adjusted during the magnetic measurement to ensure a good
alignment of the magnetic axis and low harmonic components The ThomX quadrupoles are air-cooled
to reduce the infrastructure cost This is possible since the maximum current density is 15 Ammsup2
Figure 331 shows the magnetic field intensity versus the radial distance from the quadrupole axis The
maximum gradient is 5 Tm Figure 332 shows the magnetic field intensity at a distance of -20 mm
from the magnet axis versus the longitudinal coordinate z The maximum magnetic field is 01 T
lsquo
55
Figure 331 Magnetic field (in T) versus the distance (in mm to the magnet axis
Figure 332 Magnetic field intensity (in T) versus z (the longitudinal coordinate in mm)
lsquo
56
The main parameters of the ThomX quadrupoles are given in Table 311 Each quadrupole is powered
by a 10 A 32 V DC power supply thus the released heat power remains below 32 W A dedicated
magnetic bench equipped with a rotating coil will be used for the quadrupole magnetic measurements
This bench has been developed for the magnetic measurements of the SOLEIL storage ring
quadrupoles
Parameters unit value
Bore mm 41
Gradient Tm 5
Current density Ammsup2 15
Overall dimensions cm3 19x19x18
Current A 10
Voltage V 35
NI Aturns 810
Pole width mm 20
Pole height mm 50
Coil dimensions mmsup2 15x36
Number of turns turns 81
Wire dimensions mmsup2 5x125
Yoke length mm 150
Yoke thickness mm 20
Chamfer mm 5 (not optimised yet)
Table 311 Main parameters of the ThomX quadrupoles
343 Sextupole magnetic design
The ThomX sextupole design (see Fig 333) was also performed with the TOSCA software The magnetic
field is produced by six coils that carry a DC current The coils are wrapped around six iron poles which
concentrate the field and define the sextupole field map These magnets are combined function and are also
equipped with additional coils to produce the corrector dipole fields
Figure 333 Sextupole magnetic design
lsquo
57
Each sextupole yoke is manufactured as a single piece and the six poles are screwed on this yoke Then a coil
is wound around each pole During the design phase a chamfer was calculated with TOSCA to guaranty a low
harmonic content The sextupoles will be air-cooled to reduce the infrastructure cost The maximum current
density is 15 Ammsup2
The magnetic field intensity versus the distance to the magnet axis is shown in Fig 334 The maximum
sextupolar strength is 40 Tmsup2
Figure 334 The ThomX sextupole magnetic field versus the distance to the magnet axis
Fig 335 shows the variation of the magnetic field at a distance of -20 mm from the sextupole axis versus the
longitudinal coordinate z The maximum magnetic field is 0016 T
Figure 335 ThomX sextupole magnetic field versus z (the longitudinal coordinate)
Table 312 gives the main parameters of the ThomX sextupoles They will be powered by a 10 A 05 V DC
power supply
lsquo
58
Parameters unit value
Bore diameter mm 42
Sextupolar Strenght Tm2 40
Field 20 mm T 0016
Current density Amm2 25
Overall dimension cm3 19x19x8
Current A 10
Tension V 05
AmpereTurn At 100
Pole width mm 20
Pole height mm 40
Coil dimension mm2 6x40
Number of turns Tours 10
Conductor dimension mm2 2x2
Yoke length mm 60
Yoke thickness mm 20
Chamfer mm 15
Table 312 Main characteristics of the ThomX sextupoles
A dedicated bench equipped with a rotating coil will be used for the magnetic measurements of the sextupoles
This bench has been developed for the magnetic measurements of the SOLEIL storage ring sextupoles
344 Corrector magnetic design
Simple window frame correctors based on an already existing SOLEIL design will be used for the orbit
corrections of the two transfer line
Figure 336 Magnetic design of the window frame correctors
lsquo
59
345 Injection dipole
The two injectionextraction dipoles compensate the septum deflection with a 170 mrad deflection (cf sect 337
see Fig 326) They are also air-cooled Their design and their technical characteristics are illustrated in Fig
336 and Table 313
Figure 336 Magnetic design of the injection dipole
Parameters unit value
Gap mm 40
Field at the magnet centre T 0224
Current A
10
Voltage V 6
Pole width mm 60
Pole heigth mm 70
Yoke length mm 150
AI (per coil) At 3600
Coil cross section area mmsup2 60x60
Current density Ammsup2 09
Number of turns tours 360
Conductor size mmsup2 10 x 1
Pressure drop of the cooling water bar NC
Temperature increase degC NC
Power (per dipole) W 120
Table 313 Parameters of the injection and extraction dipoles
lsquo
60
35 Power supplies
In order to provide a high operation flexibility ThomX will be equipped with the following power supplies
ndash One power supply for the ring dipoles (the latter will be connected in series) one for the TL dipoles
and one for the DL
ndash Individual power supplies for all the other magnets (quadrupoles sextupoles and correctors)
All the power supplies will have a 16 bit-resolution and a plusmn20 ppm reproducibilitystability over 24 hours The
technology is a switched-mode one in order to achieve good efficiencies (gt 85)
Magnet Nb PS Nb mag current
(A)
voltage
(V)
type load
R
(mOhm)
L
(mH)
Dip ring 1 8 300 170 Unipol 565 48
Dip TL 1 4 300 85 Unipol 280 24
Dip DL 1 2 300 45 Unipol 150 12
Dip inj 2 1 12 18 Unipol 1330 180
Quad 33 1 12 6 Unipol 500 10
Sext 12 1 12 3 Unipol 250 08
Corr ring 24 1 plusmn12 plusmn3 Bipol 250 08
Corr TL DL 10 1 plusmn12 plusmn3 Bipol 250 08
Table 314 Overview of the ThomX power supply equipment
351 Pulsed magnets
3511 Layout
As discussed above the e- beam injection in the storage ring and its extraction will take place in the
same straight section (see Fig 337)
Figure 337 Injectionextraction section layout
Eddy current septum magnet
(under vacuum) Fast Kicker Fast Kicker
lsquo
61
The first injection step involves a thin septum magnet It bends the incoming bunch trajectory to bring it
inside the ring vacuum chamber close to the stored beam orbit Then a fast kicker compensates for the
residual trajectory angle and brings the injected bunch on-axis
Extraction is realized by another fast kicker followed by the same septum magnet as the one used for
injection This kicker deflects the bunch trajectory so that it enters into the septum channel in turn the
thin septum bends the bunch trajectory which enters then into the extraction channel that brings the
bunch into a beam dump
In this configuration the same thin septum magnet is used for both the bunch injection and the bunch
extraction This requires a symmetrical design with two flanges one dedicated to the injection and the
other to the extraction The magnetic gap is wide enough to accommodate the two bent trajectories
When operating at a 50 Hz injection rate the septum current pulse consists of a slow full sine whose
same positive half sine will be used successively for the lsquoold bunchrsquo extraction and the for the lsquonew
bunchrsquo injection with a suitable delay between the two steps
The two fast kicker magnets are identical their length has been reduced as much as possible because of
the very limited space available The current pulse in the kickers will have a half-sine shape One must
achieve a rise time (from zero current to peak current) and a decay time (from peak current to zero)
each less than a ring revolution period Thus the current pulse can have a maximum duration of two ring
revolution periods Consequently their charging voltage reaches high values even though most of the
deflection is performed by the septum magnet
The design of these three devices ensures that they can operate with a beam energy of 50 MeV
(nominal) or 70 MeV (maximum) and at a 50 Hz repetition rate
3512 Slow septum magnet and its pulsed power supply
The main constraints on the septum design are as follows This septum must produce a large trajectory
deflection (160 mrad nominal peak value 170 mrad maximum peak value) to bring the injected beam
very close to the ring straight section axis with no significant disturbance of the stored beam The
magnet mechanics should preserve the ring vacuum chamber geometry to keep its impedance as low as
possible Furthermore its stray field must remain very low within the vacuum chamber where the stored
beam circulates as the septum excitation is done by a full sine pulse whose width (130 micros) is very long
compared to the ring revolution time (56 ns)
An eddy current septum magnet installed in vacuum will fulfil these constraints The effective septum
blade thickness can be as small as 3 mm thus ensuring the closeness of the stored beam orbit and the
desired bunch trajectory at the output of the septum yoke An internal chamber connected to the magnet
yoke extends the ring vacuum chamber and preserves the impedance budget The injected bunch
deflected by the septum magnet field penetrates through a small hole in this internal chamber Such a
septum magnet is very asymmetric in the transverse plane it is driven by a copper loop that wraps the
steel yoke
In order to contain the magnetic flux the whole yoke is enclosed in a copper box which carries the eddy
current On the side facing the stored beam orbit the copper box thickness is reduced so that it
constitutes the thin septum blade As a result of the electric shielding provided by the eddy current
circulating in the copper septum such a structure is very effective to avoid a large magnetic stray field
outside the magnet gap The fact that the excitation pulse is a full sine reduces the duration and the
lsquo
62
amplitude of the stray field (the slow delayed stray field tends to be the sum of the two opposite half
current arcs this feature brings about a reduction of the stray field)
An additional magnetic shielding is required to strongly reduce the residual strayfield This shield is
provided by a 05 mm thick sheet of high permeability mumetal that will completely surround and
enclose the internal chamber (thus shielding the stored beam orbit from the strayfield)
Such a design that combines an eddy current shield a full sine pulse excitation and the use of a high
permeability magnetic material reduces the stray field to a very low level (a few 10 ppm of the main
pulsed field) [313]
Assuming a septum opening of H x V = 30 x 12 mm2 and an active length of 250 mm (the length was
chosen as short as possible because of constraints coming from the septum implementation in the
straight section) the septum magnet inductance is estimated to be 16 microH Connexions and cables
increase this inductance up to 23 microH
This design leads to the following parameters for the septum magnet and its power supply (values for a
70 MeV ring operation)
Septum deflection field integral field peak current charging voltage
170 mrad 40 mTm 160 mT 1500 A 226 V
Table 315 Septum main parameters
The pulsed power supply will be based on the lsquoclassicalrsquo resonant discharge circuit scheme composed
of a storage energy capacitor (190 microF) a fast high-current thyristor switch and a transmission line to
the magnet comprising a few low impedance coaxial cables
A 300V-20A power supply is foreseen It will be connected to the storage energy capacitor through a
current-limiting power resistor
3513 Fast kicker magnet and its pulsed power supply
The two fast kicker magnets must be identical Because of the very limited space available their length
has been reduced as much as possible The final numbers are 200 mm and 450 mm for the active length
and for the overall length respectively They will be located outside the vacuum vessel with their
lsquowindow framersquo rectangular ferrite yokes surrounding the ceramic vacuum chamber (see Fig 338)
Figure 338 Front view of the kickers
The two ceramic sections of the vacuum chamber will have a flange-to-flange length of 450 mm Their
aperture will be the same as the one in the ring standard sections but with a racetrack transverse shape
Both extremities of each ceramic chamber include a smooth shape transition and a short bellow that
lsquo
63
allows for RF finger contacts These bellows can accommodate thermal dilatations especially during
bake-out The ceramic chambers will be coated with a metallic deposit that will carry the bunch mirror
current and thus avoid an increase of the chamber impedance This coating needs to be very thin It will
consists of a titanium layer with a thickness le0050 microm The thickness has been calculated so as to
avoid the deterioration of the fast pulse shape and to avoid the temperature rise due to the mirror current
[314]
Figure 339 Time profile of the kicker pulse versus the titanium layer thickness
The kicker dimensions follow from
- the aperture needed by the stored beam H x V = 40 x 28 mm2
- the required deflection kick nominal 11 mrad maximum 15 mrad
- the beam energy nominal 50MeV maximum 70MeV
- the short revolution period of the stored e- bunch Trev = 56 ns
- the very short length of the injected bunch from 4 to 25 ps (rms)
With a ceramic thickness of 6 mm the effective magnet aperture must be H x V = 55 x 55 mm2 These
parameters determine the current and voltage pulse characteristics
The kicker magnets will be powered by a circuit delivering half-sine pulses [315] In fact because of
the e- bunch short revolution period (56 ns) the field pulse duration has to be less than twice the ring
revolution period (less than 112 ns) and thus must a have very fast rise time and decay time (both less
than a ring revolution period) On the other hand with such a very short bunch length (4 or 25 ps (rms)
whether one deals with the incoming bunch or the extracted bunch one may assume that the pulses do
not have to present a marked flat top
These considerations lead to the following kicker parameters
00000E
-2
17000E
-10
34000E
-10
51000E
-10
68000E
-10
85000E
-10
10200E
-9
11900E
-9
0000E+00
2000E-01
4000E-01
6000E-01
8000E-01
1000E+00
1200E+00
Thom-X injection kicker pulse field attenuation relating to Titanium thickness
Bext(t)
Bint(t) 002microm
Bint(t) 004microm
Bint(t) 005microm
Bint(t) 006microm
Bint(t) 008microm
Bint(t) 01microm
Bint(t) 02microm
Time
Fie
ld a
mp
litu
de
lsquo
64
Kicker beam energy deflection field integral
Operation at Enominal 50 MeV 15 mrad 25 mTm
Operation at Emax 70 MeV 15 mrad 35 mTm
field peak current estimated voltage
92 mT 406A 10 kV
175 mT 771 A 18 kV
A specific RampD program is necessary to guarantee that these constraints will be fulfilled by the power
supply and by the magnet Preliminary studies were based on the standard circuit used for such a pulsed
power supply (see Fig 340) involving a high voltage (HV) capacitor to store the energy in series with
a HV switch and a load whose inductance is close to the one of the kicker magnet In this circuit the
HV switch is a very fast MOSFET switch followed by a HV fast recovery diode module
Figure 340 Power supply circuit for the ThomX kickers
These first studies lead to unsatisfactory preliminary results
- The BEHLKE Ultra-Fast 17 kV 480 A MOSFET module can switch ON in a very short time less
than 8 ns Then the pulse shape is normally determined by the passive component parameters in the
circuit provided that the negative current is quickly interrupted either by HV diodes in series or by
a turn Off control of the HV switch
- But the different types of HV diodes used are not able to switch OFF the pulsed current quickly
enough Even the lsquofast recovery diodesrsquo have a reverse recovery time too much long to stop cleanly
the pulse at its zero crossing Thus it was necessary to work on other topologies and to find better
suited components
Further simulations were performed to see how the circuit could be improved It turned out that
controlling the switch both at the closing and at the opening can improves the shape of the current
pulse A prototype using the same Behlke MOSFET modules and including a coaxial transmission line
was built and tested A plot of the current pulse in the kicker coil that was obtained is shown below
lsquo
65
This plot was obtained with the following prototype circuit
As one can see more work is still needed to improve the shape of the current in the coil (namely one
should reduce the level of reflections improve the current pulse shape and work on the peak current to
meet the 70 MeV requirements) We are also looking for an other supplier who could provide controlled
HV switches ensuringr both ON and OFF fast operation
Mechanical and magnetic studies of the kicker magnets are ongoing
36 Storage ring RF system
The choice of 500 MHz for the RF frequency is a compromise between several constraints the cavity
fundamental and high order mode (HOM) impedances space requirements the availability of RF power
sources and other components The required accelerating voltage of 500 kV can be provided by only
one 500 MHz single-cell cavity with about 40 kW of RF power As shown in the following the choice
of the cavity design is mainly dictated by the need for a strong damping of the HOM
lsquo
66
361 HOM impedance thresholds as regard to beam instabilities
When a cavity mode is excited in resonance it generates beam oscillations with a rise time
approximately given by one of the two following expressions
(1τ)l = Ib α fm Rl (2 Qs Ee) in the longitudinal case and
(1τ)t = Ib βt fo Rt (2 Ee) in the transverse case
where fm is the HOM resonant frequency Rl [Ω] and Rt [Ωm] are respectively the longitudinal and the
transverse HOM impedances Qs = fs fo is the synchrotron tune (fs and fo are the synchrotron and the
revolution frequencies) βt is the beta function amplitude at the cavity location α is the momentum
compaction factor Ib is the average beam current and Ee is the beam energy in eV
Assuming typical values for the cavity HOM impedances (Rl fm = 01 to 1 MΩ GHz Rt = 1 to
10 MΩm) and the machine parameters listed in Table 315 the instability growth time is around 10 micros
in both the longitudinal and the transverse cases
Orbit length C 168 m
Electron energy E 50 MeV
RF frequency fRF 500 MHz
Revolution frequency fo 1786 MHz
Harmonic number h 28
Momentum compaction factor α 0015
β-function amplitude at the cavity location βxz 3 m 4 m
Relative energy spread σE E 40 10-3
Peak acceleration voltage VRF 300 (500) kV
RMS bunch length σs 27 (21) ps
Synchrotron frequency fs 358 (462) kHz
Bunch charge Qb 1 nC
Number of stored bunches Nb 1
Maximum current Ib 1786 mA
Table 315 Ring parameters used to estimate the rise time of the HOM induced instabilities
In a low energy ring like ThomX radiation damping is so weak (the damping time is ~ 1 s) that a
stationary state is not reached during the beam storage time (20 ms) On the other hand to ensure a
good ring operation it is sufficient to make all instability rise times much longer than this storage
period This can be achieved by attenuating the HOM impedances very strongly typically by a factor
104
362 Cures to the HOM induced instabilities
There are essentially two ways of coping with such HOM induced instabilities either by performing a
strong de-Qing of the HOM resonances or by tuning their frequencies away from the beam spectral
lines in order to prevent resonant excitation
In practice HOM de-Qing is achieved by extracting the cavity HOM power and transferring it towards
external dissipative loads But in this approach it is difficult to reach attenuation factors higher than
102 - 10
3 over a large frequency range and this technique requires heavy and cumbersome equipment
close to the cavity (coaxial lines with coupling antennas or ferrite loaded waveguides) The DAMPY
lsquo
67
cavity designed at BESSY [316] and which is used in ALBA [317] and the PEP II cavity developed
at LBNL [318] are two variants of this technique
The other method which consists in controlling the HOM frequencies is better suited to a small
circumference machine like ThomX where the beam spectral linesrsquo spacing (~ 18 MHz) is very large
compared to the HOM bandwidth (this bandwidth is of a few tens of kHz for Qo equal to a few 104) As
long as the HOM density is not too high and the modes can easily be tuned far enough from the beam
spectral lines (δf gtgt fm Qo) it should be possible to reduce their effective impedances (the ones lsquoseenrsquo
by the beam) down to tolerable levels ie Reff asymp Rs (2 Qo δf fm) 2 ltlt Rs
We are investigating the potential performance of this technique by applying it to an ELETTRA-type
cavity (see Fig 344) [319] which combines three frequency tuning means The HOM frequencies can
be precisely controlled by an appropriate choice of the temperature of the water cooling the cavity while
the fundamental frequency is maintained constant by a mechanism which deforms the cavity
longitudinally In addition a movable plunger tuner also called HOM frequency shifter (HFS) located
in the cavity equatorial plane provides an additional degree of freedom [320]
Tables 316 a and b list the characteristics of such a cavity fundamental and HOM resonances They
have been calculated with the 3D GdfidL code [321] One finds 18 significant HOM (9 monopoles and
9 dipoles) with resonant frequencies lower than the cavity cut-off frequency Figures 345 a and b
represent the corresponding HOM spectra fm Rl(f) and Rt(f) folded over a single base band of
18 MHz The horizontal dashed line in the graph represents the impedance threshold corresponding to
an instability rise time of 20 ms the beam storage time Although they remain somewhat critical these
results tend to confirm that it should be possible to find acceptable operating conditions as long as the
HOM frequencies can be shifted by a few MHz by combining temperature and HFS tuning
lsquo
68
Figure 344 Drawing and photo of the ELETTRA cavity
lsquo
69
Figure 345 (a) Longitudinal and (b) transverse HOM spectra folded over a single base band of
18 MHz for the ELETTRA-type cavity
τL = 20 ms
lb
os
I
eEQ
τα
2
ttob
o
fI
eE
τβ
2
τt = 20 ms
lsquo
70
a) Longitudinal modes fm [MHz] Q0 Rs Q0
[Ω]
Δfm (T)
[MHz]
Δfm (HFS)
[MHz]
L0 (fundamental) 499132 44164 765 000 000
L1 958618 48158 336 037 -100
L2 102781 57861 036 080 369
L3 14087 52562 220 174 021
L4 151528 69088 783 112 086
L5 155129 66375 850 240 721
L6 186912 55785 097 142 -092
L7 19495 81752 008 183 143
L8 203985 62729 086 400
L9 206896 87300 530 445 667
Table 316 Main characteristics of the monopole (a) and dipole (b) modes computed with GdfidL for
an ELETTRA-type cavity Δfm (T) and Δfm (HFS) are the effective temperature and HFS tuning ranges
assuming a constant fundamental frequency
It is worthwhile mentioning that in any event fast longitudinal and transverse feedback systems are
required to damp the beam oscillations resulting from injection transients (see section 333) They will
contribute to the HOM damping as well
363 RF power source
An RF power of about 40 kW is required to generate the desired voltage of 500 kV in a single 500 MHz
ELETTRA-type cavity For this frequency there exists a wide choice of power sources based either on
vacuum tubes (klystron IOT diacrode) or on solid-state technology The low operating voltage and
high modularity of the solid-state version offer significant advantages compared to vacuum tubes
Experience acquired at SOLEIL after few years of operation has definitely demonstrated all the benefits
of this technology [322] Figure 346 shows the 35 kW - 352 MHz solid-state amplifier used for the
SOLEIL booster synchrotron It consists of a combination of 147 modules Each one has a power of 330
W a gain of 11 dB and an efficiency of 62 This amplifier was commissioned about 7 years ago
During a running period of more than 35 000 hours it tripped only once and this trip was caused by a
human mistake
b) Transverse modes fm
MHz]
Qo Rsperp Qo
[Ω]
Rsperp[MΩm] Δfm(T)[MHz] Δfm(HFS)
[MHz]
T1 73701 50267 335 2599 04 075
T2 74606 52566 92 7557 155 285
T3 115202 47196 264 3006 049 -312
T4 125565 55386 51 7428 068 725
T5 130249 71674 36 7039 097
T6 169981 69325 103 2542 188
T7 173650 91039 165 5463 gt 2
T8 177337 59409 32 7061 gt 2
T9 183617 77697 55 1643 gt 2
T10 206142 88484 59 2254 gt 2
T11 222475 99092 11 5079 gt 2
T12 225933 56641 13 3484 gt 2
lsquo
71
This technology has already been extended to 476 MHz in the framework of a SOLEIL-LNLS
collaboration aimed at realizing two 50 kW amplifiers for the LNLS storage ring [323] More recently
the SOLEIL team has developed modules which can deliver up to 650 W at 500 MHz with a gain of
17 dB and an efficiency of 62 while still reducing the thermal stress [322] About a hundred of such
modules will be combined to obtain the 50 kW required by ThomX
Figure 346 35 kW - 352 MHz solid-state amplifier of the SOLEIL Booster (147 amplifier modules
and DCDC converters bolded on 8 water-cooled dissipaters)
364 Low level RF system and longitudinal feedback
The proposed low level RF (LLRF) system is conventional As shown by the synoptic diagram of Fig
347 it consists of three relatively slow loops (frequency amplitude and phase) aimed at controlling the
cavity voltage with an accuracy of about plusmn 02 in amplitude plusmn 02deg in phase and a response time
around 1 ms This scheme is suitable for compensating the pressure and temperature drifts as well as
the power supply ripples
Such a system is not fast enough to deal with the beam phase oscillations at the synchrotron frequency
fs on the order of 400 kHz that originate in injection transients These oscillations can produce an
undue emittance blow up and a phase jitter at the interaction point Beam dynamics simulations show
that preserving the beam quality requires a damping of these oscillations within a few tens of a micros (see
section 333) As the radiation damping is negligible during the storage time a longitudinal feedback
600 W 280 Vdc 28 Vdc converter
Power combination
330 W amplifier module
lsquo
72
(LFB) must be implemented It can be performed by a dedicated broad-band longitudinal kicker that
acts on the beam Since ThomX is operating with a single bunch we propose to use the main cavity as a
longitudinal kicker despite its narrow bandwidth of 25 kHz ltlt fs This difficulty can be overcome by
making the cavity effective bandwidth much larger than this value in fact larger than fs We will use a
high gain direct RF feedback (RFFB) that allows the operation of a fast phase feedback (ϕFB)
Assuming a cavity-to-amplifier distance of about 10 m the loop delay will be less than 150 ns Then
one expects an RFFB gain Go larger than 50 which leads to an effective bandwidth larger than 1 MHz
Simulations show that a LFB that combines such a RFFB with a ϕFB (see Fig 348) can damp injection
phase errors of up to 10 degrees within a few micros ie faster than the above specification
Figure 347 Basic LLRF system scheme with lsquoslowrsquo frequency amplitude and phase loops
lsquo
73
Figure 348 Schematics of the longitudinal feedback system combining a phase feedback with a direct
RF feedback
365 RF equipment status
For the ThomX commissioning we will use an ELETTRA cavity The design of all the components of
the 50 kW solid-state amplifier (600 W modules combiners power supplieshellip) was validated in 2011
The next two milestones are first the validation of a 9 kW unit (combining 16 modules) and secondly
the commissioning of the complete amplifier on a dummy load
Several versions of the LLRF system including LFB are under investigation either fully analogue (see
Fig 349) or mixed analogue for the fast feedback loops and digital for the slower ones (see Fig 350)
or even fully digital based on the new generation of fast ADC DAC and FPGA
366 RF system conclusion
For the RF system of the ThomX storage ring we plan to use a 500 MHz single-cell cavity of the
ELETTRA-type Maintaining at a tolerable level the impact of the RF cavity HOMs on the beam is a
challenge However this goal should be achievable with a proper control of the HOM frequencies It
can be done by combining three different means of tuning the cavity temperature control longitudinal
elastic deformation of the cavity and motion of a plunger The fast longitudinal and transverse feedback
systems - which in any event are required to damp the beam oscillations resulting from injection
transients - will contribute to the damping of the HOM excitations as well
In order to provide the required accelerating voltage of 500 kV the cavity will be powered by a solid-
state amplifier of the lsquoSOLEILrsquo type able to deliver a power of up to 50 kW The cavity voltage will be
controlled with an accuracy of plusmn 02 and plusmn 02 degree by means of conventional amplitude and phase
loops (either analogue or digital) with a few kHz bandwidths In addition a fast phase feedback system
combined with a high gain direct RF feedback will provide the longitudinal damping which is required
to preserve the beam quality
lsquo
74
Figure 349 Complete LLRF system longitudinal feedback included (analogue version with phase and
amplitude modulation ndash the option with IQ modulation is also considered)
Figure 350 Complete LLRF system longitudinal feedback included (mixed analogdigital version)
37 Transverse feedback
371 Specifications
Table 317 summarizes the instability growth time constants and the corresponding kicker strength
requirements for the main types of instability It shows that for Thom-X the betatron oscillations due to
the injection jitter in position and angle are the most critical ones They require a feedback damping
time constant of 5 micros or less (cf chapter 3)
lsquo
75
Source type growth time constant kicker strength
Beam pipe
geometry
TMCI
Head-Tail
-
160 micros
gt10 nrad
Resistive Wall 600 micros gt2 nrad
Ions lt 100 micros gt20 nrad
Injection Jitter 5 micros 2 microrad
Table 317 Overview of expected instabilities (1 nC 25 ps rms at 50 MeV)
372 Electronics
To cope with these instabilities we will use a SPring-8 based high performance digital transverse
feedback system which consists of a wideband detector (a BPM) an RF front-end an FPGA based
processor a power amplifier and a stripline kicker (see Fig 351) This system is capable of detecting a
coherent transverse motion and applying a counter kick to damp it bunch-by-bunch and turn-by-turn
with one or two bunches stored in the ring
Figure 351 Spring-8 based digital feedback system
Two BPMs (one for each plane) will provide the beam position information The RF front-end will
extract the betatron oscillation information which will be sampled by ADCs From the sampled data the
FPGA will compute a suitable kick to damp the oscillation The signal processor output will be
amplified by a 10 W wideband amplifiers before being fed to the kicker
In the RF front-end (see Fig 352) a baseband down-converter will be implemented in order to amplify
the beam signal and to minimize the jitter effect for the same reason the digital kick will also be
extended over four RF periods
lsquo
76
Figure 352 Front-end of the Thom-X RF electronics
For the commissioning phase a digital system (rather than an analog one) will be used to benefit from
the tuning flexibility provided by the digital parameters of the FIR filter Once the machine tune will be
fixed a more suitable low cost analog system will be installed
373 Kicker
The kicker is under development It consists of a stripline composed of four electrodes connected to
vacuum feedthroughs on each side (see Fig 353) The amplifiers will be connected to the downstream
ports whereas upstream ports will be used for signal detection and they will be equipped with 50 Ω
loads The same kicker will be used to act on the beam in both planes
Electrode line impedances must be adapted (50 Ω) Electromagnetic 3-D simulations are being
performed to optimize the bandwidth and shunt impedance (ie to maximize the stripline efficiency)
and also to minimize the kicker impedance to wakefields
Figure 353 Schematics of the Thom-X stripline kicker
lsquo
77
38 Vacuum
381 Transfer line vacuum
Concerning the design of the transfer line vacuum system three main constraints have been taken into
account
- Pressure of 10-7
Pa at the acceleration section output
- Ion pump operating at 10-6
Pa
- Ring pressure of 10-8
Pa
Simulations have been performed assuming that two 03 m3 s
-1 triode pumps will be installed one at the end of
the acceleration section and the other at the septum The vacuum chamber is a DN40 The septum output
conductance is 018 m3 s
-1 Without baking the outgassing rate would be σ τ = 2x10
-8 Pa m s
-1 and the
pressure at the ring entrance would be about 10-7
Pa Thus the septum and its environment must be baked to
bring τ down to 5x10-10
Pa m s-1
and obtain a pressure of 10-8
Pa at the ring entrance
Pressure in the line of transfer
The vacuum equipment envisaged for the transfer line is the following one
‐ 3 ion pumps 150 l s-1
with their baking systems and power supplies
‐ 1 ion pump 500 l s-1
with their baking system and power supply
‐ 4 Pirani gauges
‐ 4 cold cathode gauges
‐ 2 power supplies gauges
‐ 2 gate valves electro pneumatically actuated DN 40 (acceleration section output ring entrance beam
dump)
‐ 1 turbo pump (DN 63 ~ 80 l s-1
)
‐ 1 rotary vane pump (~ 12 m3 h
-1)
‐ 4 angle valves manually actuated for pre-pumping DN 63
lsquo
78
382 Ring vacuum
Concerning the ultra-vacuum to be maintained in the ThomX ring one must take into account first the
pipe relatively low specific conductance c = 452x10-3
m3 m s
-1 and secondly the pipe specific
outgassing rate θ τ = 108x10-10
Pa m2 s
-1 Also the pumpsrsquo effective pumping speed is reduced by the
T port with the grid Seff = 19x10-2
m3 s
-1 In a first stage the ionic pumps chosen for the simulations
are 0040 m3 s
-1 diode pumps This corresponds to S = 0030 m
3 s
-1 at 10
-8 Pa and with a grid pumping
port whose conductance is 00362 m3 s
-1 to an effective pumping speed Seff = 0019 m
3s
-1 The pressure
has been estimated by modeling a quarter of the ring including half of the non-baked optical cavity ie
in the most unfavorable case
Figure 354 Pressure distribution in the vacuum chamber
The calculated average pressure is 306x10-8
Pa It can be improved by increasing the grid pumping port
conductance by replacing the DN35 with a DN63 Monte-Carlo simulations show an improvement of ~
16 in efficiency
The entire ring must be baked Geometrical and temperature constraints (respectively a distance of only
07 mm between the magnet coils and the vacuum chamber a baking temperature of 150 degC for the
chamber while keeping the coilsrsquo temperature below 80deg C) are strict An ultra-thin baking system has
been constructed tested and validated
Figure 355 The vacuum chamber ultrathin baking set-up
lsquo
79
The following is the vacuum equipment needed for one quarter of the ring
- 3 ion pumps (40 l s-1
) with their baking equipment and their power supplies
- 1 residual gas analyzer (RGA)
- 2 turbo-pumps (DN 63 ~ 80 l s-1
)
- 2 rotary vane pumps (~ 12 m3 h
-1)
- 2 Pirani gauges
- 2 cold cathode gauges
- 1 gauge power supply
- 1 DN 63 gate valve electro-pneumatically actuated with an HF feed-through (one on each side of
the RF cavity and similarly for the septum) and the baking equipment
- the vacuum chamber baking equipment
- 3 angle valves manually actuated for pre-pumping DN63
- 3 T with the grid
lsquo
80
Chapter 4 INJECTOR TRANSFER LINE AND RING DIAGNOSTICS
Several diagnostic devices will be implemented in the ring to measure the electron beam parameters
listed in Table 41
Parameter unit value
Energy MeV 50
Average current mA 1784
Bunch charge nC 1
Energy dispersion 03 at injection to 06 at storage end
Emittance x mrad 5 e-8 at injection to 30 e-8 at storage end
Normalized emittance x mrad 5 e-6 at injection to 30 e-6 at storage end
Emittance y mrad 5 e-8
Normalized emittance y mrad 5 e-6
Bunch length ps-rms revolutions 1 ~ 4
revolutions 100-200 ~ 25
revolutions 200-300000 ~ 50
RF frequency MHz 500
Revolution frequency MHz 1784
Period ns 56
Injection repetition rate Hz 50
Ring circumference m 1680
(a) Ring parameters
Magnetic field T 05
Radius of curvature m 0352
R51 m 0 to -025
R52 m 0 to -045
R56 m 07 to -07
Beta x m 06
Beta y m 6
Eta x m 02 - 03
Transverse dimension x mm (rms) 06
Transverse divergence x mrad 2
Transverse dimension y mm (rms) 054
Transverse divergence y mrad 2
(b) Parameter values at the location of the bending magnets
Table 41 ThomX beam parameters
41 Charge measurement
411 Current transformers
The beam charge will be measured non-destructively with commercial in-flange high resolution
current transformers (ICT) Three devices will be installed
- one in the linac (LIDGICT010) located before the accelerating sections
- one at the transfer line entrance (TLDGICT010)
lsquo
81
- one at the transfer line exit (TLDGICT020)
In-flange ICTs are directly mounted in the beam line They are UHV compatible can be baked up to
150deg C (185deg C on request) and they are available for many pipe diameters (also with an elliptical
aperture or any other aperture shape) We will use the CF6rdquo model its internal diameter is 604 mm and
it is 60 mm long This device includes a ceramic gap a shield and a wall current bypass Bellows are
not required
The signal from each integrating current transformer will be processed by a BCM (beam charge
monitor) A built-in calibration generator provides an on-line check of the measurement reliability An
Integrate-Hold-Reset (BCM-IHR) device is suited for pulse repetition rates ranging from 10 kHz down
to single pulses and for bunch charges up to 2 nC The acquisition will be performed by using a 100
kHz 12 bit ADC
412 Faraday cups
Interceptive absolute beam charge measurements will be performed by Faraday cups There will be two
of them placed inside the two beam dumps (see Fig 2 of the Introduction) They should provide a
better resolution for small electric charges
42 Transverse profile measurements
421 Diagnostics stations
Five diagnostics stations will be installed to measure the transverse beam profile as shown in Fig 41
Diagnostics station 1 (DS1 ndash LIDGSST010) will be located after the RF gun at the beginning of the
linac Diagnostics station 2 (DS2 ndash TLDGSST010) will be located between the end of the linac and
the linac beam dump Diagnostics station 3 (DS3 ndash TLDGSST020) will be placed in the middle of the
transfer line Diagnostics station 4 (DS4 ndash TLDGSST030) will be located at the end of the transfer
line and Diagnostics station 5 (DS5 ndash ELDGSST010) will be located before the ring dump
lsquo
82
Figure 41 Location of the diagnostics stations (see text for the nomenclature)
The ThomX station design draws from the station built by LAL for PITZ (itself drawn from a SOLEIL
design) It is illustrated in Fig 42
Figure 42 3D view of a ThomX diagnostic station
Each station will have a motorized arm on which several screens will be mounted A stepper motor will
move this arm and insert the screens laterally The following screens will be available
- 1 YAG(Ce) screen in all stations
- 1 optical transition radiation (OTR) screen in all stations
lsquo
83
- 1 target to allow an online calibration of the camera in all stations
- one Cerenkov screen for bunch length measurements in station DS2 (described in more detail
below)
The YAG(Ce) and OTR screens and the calibration target will be tilted at 45deg with respect to the beam
axis so that the emitted or reflected light will travel downward and exit the vacuum through a DN60 CF
fused silica window A CCD camera will be located below the station to image the screens The
Cerenkov screen will be titled so that its light exits at the top of the diagnostic station Shutters will be
used to close either the Cerenkov window or the YAG(Ce)OTR window when not in use to prevent
measurement noise due to background photons Several LEDs located just after the bottom window will
be used to illuminate the inside of the chamber when needed
At the beginning of the Linac at station DS1 the beam size will be about 05-07mm (rms) in both
planes at the end of the Linac at DS2 the beam size will be 15 mm (rms) in both planes In the middle
of the transfer line at DS3 the beam size is expected to reach 25 mm (rms) in x and slightly less than
1mm in y At the end of the transfer line at DS4 it will be about 1 mm in both planes After the
extraction at DS5 the beam size will be about 05-07mm (rms) in both planes Given these beam sizes
we chose a screen diameter of 25 mm for all stations
Screen photon yield
The YAG(Ce) screen will be 100 microm thick Such screens have a light yield of 8 photons per deposited
keV [41] The YAG radiation length is 35 mm [42] assuming the usual model for energy deposition in
matter [43] each electron will deposit about 143 keV in the screen this corresponds to the emission of
~1100 photons Thus the total number of photons produced by an e- bunch crossing the screen will be
71 1012
We recall that YAG(Ce) emission occurs in all directions
The second screen will be of the OTR type The screen support design allows either the use of a 100 microm
aluminized silicon wafer or of an ultra-thin aluminized mylar film The silicon wafer has the advantage
of giving a better image quality while the mylar film allows a beam injection through the screen without
significantly affecting its quality The OTR yield within a cone of half-aperture θ for photons whose
wavelengths range between λ1 and λ2 is given by [44]
Thus for a standard CCD camera whose sensitivity ranges between 400 nm and 800 nm and assuming
an acceptance cone of 003 radian this expression gives ~0002 photon per electron at 50 MeV (station
DS2-5) A similar yield is reached at 5 MeV with an acceptance of 03 radians (station DS1)
To image the screens a lens will be located 500 mm away from them A small telescope made of two
lenses will be used to focus the light on the CCD camera At least one of the telescope lenses will be
movable to provide the needed dynamic range A Gigabit Ethernet camera with at least 600 x 400 pixels
will record the images The camerarsquos square pixels are about 74 x 74 microm2
With such a configuration one expects that a 50 MeV bunch passing through the YAG(Ce) screen will
emit 71x1012
photons and assuming that the CCD chip has an average efficiency of 50 1010
counts
will be recorded by the camera taking into account the solid angle covered by the lens
lsquo
84
The same bunch passing through the OTR screen will emit 82 107 photons within the lens acceptance
Thus 3 106 counts will be recorded by the camera
For a 5 MeV bunch (station DS1) the YAG(Ce) screen will emit 2 1012
photons and 109 counts will be
recorded by the camera The OTR screen will emit 82 107 photons within the lens acceptance and 3 10
6
counts will be recorded by the camera
Assuming a transverse beam size of 15 mm one expects 8 106 counts per pixels with a YAG(Ce)
screen and 25 105 counts per pixel with an OTR
If the 50 MeV beam is defocused (for example during a quadrupole scan) to produce a 5 mm spot size
these numbers become 8 105 for the YAG(Ce) screen and 2 10
4 for the OTR
Number of OTR
photons emitted
Number of
OTR
photons
recorded by
the camera
Number of
YAG
photon
emitted
Number of
YAG
photons
recorded
DS1 82 107 3 10
6 2 10
12 10
9
DS2-5 82 107 3 10
6 71x10
12 10
10
Assuming a YAG Cerium dopant density of 018 we get a Cerium density of 25 1019
cm-3
[45] and
a 15 scintillation efficiency It follows that the scintillating site density is 37 1018
cm-3
Therefore the
100 microm YAG(Ce) screen will saturate when the photon density reaches 36 1014
mm-2
This last figure
translates into an electron density of 32 1011
mm-2
for an e- beam energy of 50 MeV In these
conditions the YAG(Ce) screen will saturate if the beam is focussed on a spot smaller than 20 microm This
is five times less than the smallest beam foreseen at ThomX
Additional targets will be used to calibrate the cameras when necessary They will be made of stainless
steel plates with etched grids Each square of the grid will be 2 by 2 mm2 In some of these squares
smaller lines of known thickness will be etched Images of these targets will allow one to check the
whole optics
To minimize a possible radiation deterioration of the cameras lead-glass windows will be inserted
between the vacuum windows and the focusing lenses
43 Emittance measurement
The beam emittance will be measured with the quadrupole scan technique In this technique a triplet
(set of three quadrupoles) is used to find the minimum transverse beam size in each plane A fit of the
beam size variation versus the quadrupole strength enables one to retrieve the projected transverse
emittance
The sets of triplets and the diagnostic stations implemented along the ring allow an emittance
measurement at three different locations at the exit of the linac with DS2 in the transfer line with DS4
and in the ring with DS5 by using the extraction kicker The beam emittance measurement with DS5
lsquo
85
will provide a check of the turn-by-turn emittance growth thus bringing precious information on the
transverse beam dynamics
Pepper-pot measurements are also foreseen The advantage of this technique with respect to a
quadrupole scan is that it consists in a single shot measurement independent of machine operation
fluctuations
A pepper-pot set-up comprises a plate with evenly distributed holes Such a plate is placed in the beam
path together with a scintillating screen located downstream and an imaging system A pepper-pot
mounted on a movable arm could be located at the end of the linac between the last dipole and the
diagnostic station DS2 and another one could be located just before station DS5
At station DS2 the beam size will be about 15 mm (rms) and the geometric emittance about 5 10-8
m
rad Therefore the maximum beam divergence will be about 50 microrad By adjusting the quadrupoles one
may focus the beam down to a 20 microm spot and increase its divergence up to 250 microrad To measure the
transverse emittance one would have to position this focal point two meters before DS2 (or DS5) and
to position a pepper-pot mask with 100 microm holes or slits one meter upstream of DS2 (or DS5)
44 Beam position measurement
The beam position will be measured by a set of BPMs placed all along the ring Their locations are
shown in Fig 43
lsquo
86
Figure 43 Location of the BPMs along the ring (note the BPM located at the IP has been suppressed
since this drawing was made)
441 Beam Position Monitors (BPM)
Two types of BPMs will be used short circuited striplines in the linac and the transfer line and buttons
in the storage ring
Compared to button BPMs striplines generate a higher signal but need more space and contribute more
to the wake-field generating impedance For these reasons striplines are preferred for the linac and the
transfer line where we need single pass measurements and where wakefield is not an issue In the
storage ring button BPMs will be chosen for their small size and small contribution to the ring
impedance The lower signal produced by the small electrodes will be compensated by the high
repetition rate of the stored beam (averaging is possible)
BPM electronics detection frequency on the storage ring is usually equal to the RF frequency as it is the
only common frequency component present in the BPM data whatever the filling pattern is Even
though only one bunch will be stored in ThomX the same approach is chosen Therefore the storage
ring BPM electronics detection frequency is 500 MHz
Since it is more convenient to adopt the same electronics for all the BPMs (linac transfer line and
storage ring) stripline BPMs on the Linac and transfer line are optimized to give a signal with the
highest possible component at 500 MHz Their electrode length is thus adapted to this frequency which
means that it has to be equal to λ4 (150 mm 500 MHz) where λ is the RF wavelength
4411 BPM for the linac
The linac BPM equipment will be as follows
- Number of BPM 1
- Type short circuited stripline
- Electrode length 100 mm
- Required resolution 100 microm for a single pass measurement and a 1 nC bunch charge
Figure 44 Linac BPM
lsquo
87
Due to lack of space in the linac section where a stripline must be installed the electrode length cannot
be 150 mm but must be reduced to 100 mm Transfer impedance calculations show that at 500 MHz
the signal amplitude will be reduced by 13 compared to the optimal length of 150 mm (see Fig 45)
This amplitude reduction is acceptable
Feedthroughs will be soldered on CF16 flanges
Figure 45 Stripline transfer impedance versus frequency for various electrode lengths With an
electrode length of 100 mm the signal is reduced by 13 compared to its maximum value for a
frequency of 500 MHz
4412 BPMs for the transfer line
The transfer line will be equipped as follows
- Number of BPM 4
- Type short circuited stripline
- Electrode length 150 mm
- Required resolution 100 microm for a single pass measurement and a 1 nC bunch charge
lsquo
88
Figure 46 Drawing of the BPMs that will equip the electron transfer line
Feedthroughs will be soldered to CF16 flanges
4413 BPMs for the storage ring
The specifications relevant to the ring BPMs are as follows
- Number of BPM 12
- Type button
- Button diameter 107mm
- Required resolution ~1 microm 50 Hz and for a bunch charge of 1 nC
- Absolute precision plusmn 250 microm
In order to provide additional BPM signals for other applications (see below) 4 BPM blocks will be
equipped with a double set of electrodes In that case only 4 electrodes (among 8) will be used for
position monitoring The button size and the beam pipe size remain unchanged
Figure 47 Simple and double BPM blocks for the ThomX storage ring
The button positions and dimensions have been determined by simulations with Poisson Superfish (an
electrostatic simulation software) An evaluation of the impedance due to the button discontinuities has
shown that their effect will be negligible
Feedthroughs will be soldered directly to the BPM block They are bakable Connectors will be SMA
males to prevent any deformation during baking
lsquo
89
Alignment
BPM blocks will have an external mechanical reference This reference will be used for BPM alignment
during the installation phase and to position precisely the block in a calibration bench
Calibration
Storage ring BPMs will be calibrated by using a dedicated bench An antenna inside the block will be
fed with a 500 MHz wave of constant amplitude Calibration is done by measuring the signal output by
the 4 buttons A precise position of the antenna is not needed as position errors can be eliminated by
repeating the measurement with the BPM block turned by 180deg and averaging the results
The second halves of the blocks equipped with two sets of buttons will be dedicated to
‐ longitudinal (beam phase) feedback (one set of BPMs)
‐ transverse feedback (one set of BPMs)
‐ ion clearing To suppress ions which may generate beam instabilities BPM buttons will be
polarized with a high voltage (max plusmn200V) This electric field will deflect the ion trajectories
toward the vacuum chamber
Electronics
Electronics will be the same for all BPMs (whether equipping the linac the transfer line or the storage
ring)
Data rate
Revolution-by-revolution beam position measurements would be best But because of the high
revolution frequency (178 MHz) it might be difficult to get electronic devices that handle the
corresponding data rate Then one may work with averages over two turns This data will be provided
on demand and the acquisition will be triggered The buffer size should accommodate data collected
during at least 20 ms to cover full storage cycles Revolution-by-revolution data will be used for
machine physics studies and tune measurement The position measurement resolution must be better
than 100 microm
Slow acquisition
Slow data acquisition will be used for slow orbit correction and slow orbit feedback It will be obtained
by averaging the fast data flow with a sampling rate of ~10 Hz A data rate of 50 Hz would provide just
one measurement per storage cycle The resolution aimed at for this measurement is one micron or
better
Acquisition must be synchronous with the machine revolution frequency Moreover modules must be
synchronized together
Connections and cables
The four cables that carry the signals of each BPM should be put together in a strand Their electrical
lengths will be matched so that phase differences are kept below 13deg 500 MHz The cable attenuation
should not differ much from one BPM to another If the cable lengths cannot be the same for all BPMs
cable sections will be chosen in order to ensure the same attenuation for all cables To minimize both
lsquo
90
attenuation and cost cable lengths below will be kept below 30 m The LMR240 cable is an example
of what could be used its attenuation is ~6 dBm 500 MHz over a distance of 33 m
As radiation close to the machine can damage the cable insulator short (~1 m) pigtails (that can be
easily replaced if needed) will be inserted between the BPMs and the long cables
442 Orbit feedback system
A feedback system will keep the beam orbit stable within plusmn1 microm It will use the position measurements
provided by the 12 ring BPMs The correction rate will be comprised between 01 Hz and 10 Hz
Correction signals will be applied to the 12 correctors located along the ring The orbit feedback system
will be a matlab based application
443 Tune measurement and feedback
The tune will be obtained from the revolution-by-revolution data provided by the BPMs There is no
need for any kicker to excite betatron oscillations as the beam transverse position will naturally oscillate
after each injection (ie every 20 ms) If necessary a tune feedback will be implemented to keep the
tunes constant
444 Energy measurement
In the middle of the transfer line ie downstream of the dipole located at the end of the linac the
dispersive function is non-zero Therefore the beam energy and its spread can be deduced from the
electron horizontal distribution shown by the YAG screen in DS3 The same measurement will be
performed with DS4 using the dispersion induced by the extraction magnet
45 Bunch length measurement
The bunch length will be measured at two different places at the end of the linac and in the ring
Depending on the observation point two different physical processes are used to produce visible light
pulses which are then transported by dedicated optical lines through the ThomX shielding wall to a
double-sweep streak camera located in the experimental hall
451 Cerenkov radiation
Cerenkov radiation (CR) produced in a target placed at the end of the linac will be used to measure the
e- bunch length Cerenkov radiation is emitted when a charged particle passes through a medium at
velocity greater than the speed of light in that medium It is emitted at an angle θcr ndash with respect to the
particle momentum ndash determined by n(λ) the medium index of refraction and β the particle velocity as
follows
Above 10 MeV the emission angle is almost independent of the particle velocity Ncr the number of
radiated photons whose wavelengths range between λ1 and λ2 summed over a path of length d along the
charged particle trajectory is
( )
minus
minus
λnβλλπαd=N
cr 22
21
11
112
( )
minus
minus
λnβλλπαd=N
cr 22
21
11
112
( )( )λβn
=θcr
1cos
lsquo
91
where α=73 10-3
is the fine structure constant
Two types of radiator are usually used to produce Cerenkov radiation an aerogel or a solid target
Aerogels dramatically spoil the vacuum and therefore have to be mounted in a separate vacuum vessel
to maintain ultra-high vacuum conditions Solid targets are UHV compatible This is why they are
preferred
At the end of the linac when the bending magnet is off the beam goes straight into a beam dump
Before the dump several types of targets can be inserted in the beam trajectory for diagnostic purposes
For the production of Cerenkov radiation a sapphire target is adequate This materialrsquos index of
refraction is 177 With a 50 MeV beam the Cerenkov angle is θcr=5563deg Assuming that the window
thickness is 025 mm 16 photonse- are created with a wavelength ranging between λ1 = 300 nm and λ2
= 800 nm Their spectrum is given in Fig 48 For a 1 nC electron bunch 1011
photons are created The
energy deposited in such a target is negligible
Figure 48 Spectrum of the Cerenkov radiation emitted by a 50 MeV e- beam passing through of
sapphire window 025 mm thick
For simplicity the sapphire target is oriented so that the Cerenkov radiation exits in the vertical
direction For a Cerenkov angle θcr = 5563deg the window angle with respect to the beam axis must be θP
= 11475deg (see Fig 49)
8x10-8
6
4
2
0
In
ten
sity
[au
]
800700600500400300200
Wavelength [nm]
lsquo
92
Figure 49 Orientation of the sapphire target used to observe the Cerenkov light emitted by the e- beam
at the linac end
At the target location the beam dimensions will be σxy= 14 mm (rms) It will produce a light spot
whose dimensions will be σx = 14 mm (rms) and σy = 154 mm (rms) in the sapphire To intercept the
whole light beam and to leave some margin for the alignment we will use a target whose diameter will
be equal to 2 x 6 x σy ~ 20 mm
The Cerenkov radiation will exit the vacuum chamber via a UHV viewport made of fused silica (FuSi)
with a 90 transmission over the 300-800 nm wavelength range The viewport is mounted on a CF63
flange that provides a 68 mm clear aperture At the exit of the viewport a FuSi lens is used to focus the
light This collimated beam is then transported via a set of planar mirrors across the shielded area and
then through the ThomX shield to a dedicated hutch in the experimental hall (see sect 453)
452 Synchrotron radiation
Most of the synchrotron radiation (SR) emitted when the electrons circulate in a bending magnet (BM)
will be in the far infrared but visible light will also be present The cut-off wavelength of this radiation
corresponds to ~077 eV Thus the X-ray content is negligible This last feature makes the extraction of
the visible part of the spectrum easy The spectral flux per unit bandwidth per unit surface ΦBM
[Photons01bwmmsup2] is shown in Figure 410
Figure 410 On-axis spectral flux 1 m downstream of a bending magnet Photon energy in eV The
visible spectral range corresponds to [400 nm - 800 nm] or [155 eV - 31 eV]
The radiation is extracted at the exit of the BM ndeg5 through a porthole which provides a 63 mrad x 32
mrad aperture at 15deg with respect to the 0deg incident angle in the dipole The maximum power density is
lsquo
93
4 microWmmsup2 in the horizontal plane of this extraction porthole thus no dedicated absorber is required
(But for the porthole region the radiated power will be dissipated in the vacuum chamber wall) The
radiation will exit the vacuum chamber through a UHV viewport made of MgF2 (this material offers
the highest transmission coefficient over the 200-800 nm range) The synchrotron radiation distribution
at 5 eV one meter downstream of the bending magnet is around 6 mm (FWHM) So the spectral flux
integrated over a collecting area of 6 mm x 63 mm is about (around 5 eV)
Φe [Phs] = ΦBM [Phs01bwmmsup2] x 6 mm x 63 mm x 1000
Since photons of lower energy thus with a higher divergence will also contribute to the total spectral
flux this value gives a low limit estimate of the real total spectral flux Figure 411 shows the Φe
spectral flux
As was the case for the Cerenkov radiation a FuSi lens will be used at the exit of the viewport to focus
the beam The collimated radiation will then be transported via a set of planar mirrors across the
shielded area and then through the ThomX shield to a dedicated hutch in the experimental hall (see next
section)
Figure 411 Spectral flux Φe in a 63 x 32 mrad2 aperture 1 m downstream of a bending magnet
453 Transport of the Cerenkov and synchrotron radiations to the diagnostics hutch
Both the Cerenkov light and the synchrotron radiation have to be transported to the diagnostics hutch
located in the experimental hall Starting from different locations both radiations will be transported via
a set of planar mirrors to a lsquomeeting pointrsquo inside the shielded area A flip mirror (ie a double-
orientation mirror) will enable the selection of one of the radiations The one selected will be further
transported again with a set of planar mirrors through the shielding wall to the diagnostics hutch
Inside the diagnostics hutch the last lens will focus the radiation on the streak camera entrance The
lens aperture and its focal length will be adapted to the selected incoming radiation All the line
components and their parameters are chosen to minimize spatial aberrations and to maximize the
number of photons entering the streak camera
454 Streak camera for the bunch length measurement
We plan to use a double-sweep streak camera to measure the bunch length and to visualize its evolution
over short time scales Since the bunch length may vary from 4 to 20 ps (rms) a 2 ps (FWHM)
resolution streak camera is required
lsquo
94
The time resolution of the bunch length measurement depends on the radiation type (CR or SR) which
is used and involves several contributions the electron transverse beam size at the light emission point
chromatic effects etc All these contributions will be minimized as much as possible to maintain the
effective resolution as close as possible to the one of the streak camera
46 Bunch imaging using synchrotron radiation
The synchrotron radiation transported to perform the bunch length measurement (see section 45) can
also be used to image the bunch while it circulates For this purpose a beam splitter could be installed
at the radiation exit port the main part of the radiation would be sent to the streak camera while a small
fraction would be directed to a CCD camera mounted nearby A focusing lens placed between the
viewport and the CCD would enable the imaging of the stored beam
In the orbit section where the bunch emits the synchrotron radiation used for the above measurements
the e- beam transverse size is about 05 mm (rms) Since the size of a CCD chip is typically 5 mm the
lens gain should be about 10 Such a set-up would enable one to follow the evolution of the beam size
in real time (we will use a CCD whose acquisition rate is about 10 Hz) It would be a non-destructive
and cheap diagnostic Similar setups are currently used on storage rings to measure the electron beam
emittance assuming that the beta function is known
47 Beam loss measurement
Particle losses will be measured by using optical fibers [46] When a charged particle crosses a glass
fiber Cerenkov radiation is emitted within the fiber and propagates along the fiber It can be detected
by a photomultiplier that collects the light at the fiber end The measurement eg with an oscilloscope
of the pulse production (in the fiber) time enables one to recover the particle loss location
Two fibers will be installed One will be used to monitor e- losses along the linac and the transfer line
The second fiber will be used to monitor particle losses along the ring Each one of these fibers will be
connected to a fast photomultiplier (PMT) The PMT time resolution will determine the accuracy with
which the losses can be localized
A fused silica fiber has a yield of 227 photons per cm of a relativistic electron track [47 48] Therefore
a 5 mm thick fiber will have a yield of ~100 photons per relativistic electron which crosses it Such a
signal should be easily detected by a PMT
As it may be difficult to place the fiber close to the beam pipe in some parts of the ring the fiber may
have to be located at a distance d away from the beam axis Then the number of lost particles which will
cross the fiber will be reduced by a factor dsup2
48 Beam dumps
The ThomX set-up will include two particle dumps to stop the e- beam in such conditions that the
radioactivity then generated is minimal The first dump will be located at the linac exit The second
dump will be located downstream of the injectionextraction section The latter dump can be used to
stop the beam either at the exit of the transfer line or after any number of bunch revolutions in the ring
Both dumps are based on the SOLEIL design and can handle the 35 W power deposition (1 nC 70
MeV 50 Hz repetition rate)
lsquo
95
Chapter 5 SYNCHRONIZATION
51 Overall scheme
An ultra-low noise 500 MHz synthesizer will be used as the main clock for ThomX The 10 MHz
output of this synthesizer will be used to drive a second synthesizer (the so-called lsquolinac synthesizerrsquo)
operating at 3 GHz The RF output of the latter synthesizer will be used for the linac operation It will
be split in two lines The first one will be used to synchronize the gun laser oscillator (after appropriate
down conversion of the frequency) The second line will be used to drive the gun and the linac RF
The RF output of the main synthesizer (500 MHz) will be used for the ring RF it will be split in three
lines The first line will drive the ring RF (after appropriate phase shifting) The frequency of the second
line will be divided by 14 this line will synchronize the Compton cavity As in the MightyLaser
experiment [51] the laser used to inject power in the Compton cavity will be synchronized with that
cavity by using the PDH method The frequency of the third line will be divided by 28 and will
therefore be synchronous with the electron revolution frequency This line will be used to synchronize
programmable delays delivering turn-by-turn triggers and injectionextraction triggers To ensure that
injection is always taking place at a zero crossing of the mains the programmable delays will also be
synchronized with the mains A counter will reduce the mains frequency to the operations frequency
(when lower than 50Hz) The gun trigger will be generated when the mains will be at a zero crossing
and the ring RF at a preset phase that takes into account the time it will take for the electrons to travel
from the linac gun to the ring accelerating cavity
To ensure that the e- bunches will always be injected at a triple zero (50 Hz 500 MHz and 3 GHz) we
need to specify the tolerance on the 50 Hz zero crossing We may take 1 (ie 02 ms) for this
tolerance this means that the coincidence between the ring RF (~500 MHz) and the linac RF (~3 GHz)
must occur at a frequency greater than 5 kHz Therefore by choosing a heterodyne frequency (ie the
frequency difference between the linac RF divided by 6 and the ring RF) greater than 5 kHz we ensure
that there will always be a common zero crossing between the ring RF and the linac RF within 1 of
the mains zero crossing This sets a constraint on the ring radius of 10-5
ie 168 micrometres on a
168 m long orbit (see Table 2)
The RF bucket must be positioned with an accuracy of 3 ps in the ring RF Thus the phase noise
between the linac RF and the ring RF must be lower than 3 ps when integrated between 50 Hz and
500 MHz Most good-quality synthesizers achieve such a performance
The information collected by the ring diagnostics will be used to adjust the exact value of the frequency
(around 500 MHz) generated by the main synthesizer so that it matches the exact ring length This
information will also be used to adjust the phase between the injection trigger and the ring
To reach the correct signal level each RF line may have to be amplified An isolator will be added to
avoid reflections that may degrade the overall performance of the system
To ensure long-term low frequency synchronization a Network Time Protocol (NTP) server will be
available to allow all computers working at ThomX to synchronize their internal clock
lsquo
96
52 Specifications
Synthesizer
The main synthesizer will be used to generate a 500 MHz adjustable signal Its phase noise jitter
integrated between 50 Hz and 20 MHz must be less than a few picoseconds It must support a frequency
modulation of at least 100 kHz Typical mid-range synthesizers meet such specifications
Measurements done at LAL in collaboration with SOLEIL have shown that the addition of an external
frequency reference does not necessarily improve the phase noise of these synthesizers in the frequency
range of interest
The linac synthesizer specifications are the same ones except that it must be able to generate a
frequency of 3 GHz
Programmable delay
Two low jitters units with 8 channels each will be used
Phase shifter
The phase shifters must be voltage controlled and cover 360deg (two 180deg units)
Figure 51 Synchronization layout
lsquo
97
Chapter 6 OPTICAL SYSTEM DESIGN
Since the writing of the ThomX CDR three directions have been pursued improvement of the feedback
system improvement of the laser amplification and mechanical design of the optical resonator The
results are described in the following three sections
In the first section the ThomX feedback system is described During the past year a strong RampD
program has been developed on the locking of a TiSapph picosecond laser beam on a two-mirror high
finesse cavity at OrsayLAL Very good results in operating for the first time a 30 000 finesse cavity
in the picosecond regime were obtained The feedback techniques used for this task are directly usable
for ThomX and are reported in this section A 3000 finesse four-mirror cavity at KEK was also installed
and operated and we succeeded in producing γ rays off the ATF electron bunches (Mightylaser project
[61]) Hardware damages were however caused by the Japanese 2011 earthquake At the time of
writing this document we are recovering all our equipment to reach our final goal locking a 30000
finesse four-mirror cavity to produce a high γ ray flux at ATF Thanks to a contact with the SYRTE
laboratory we have acquired a new laser oscillator with a 1030 nm wavelength that we are testing and
that we plan to use for ThomX With this oscillator by embedding an intra-cavity EOM we are able to
increase our feedback bandwidth by more than one order of magnitude
The second section illustrates the laser amplification developments done at CELIA First a non-all-
connected fiber amplifier for the Mightylaser project was set up that allows us to extract an average
power of 50 W This amplifier was used to produce γ rays at ATF A new setup has been designed and
built to reach both the reliability and the power level required by ThomX An important activity is
devoted to the construction of a fully all-connected amplification chain As mentioned above the new
laser oscillator and the new laser amplification chain are being integrated in order to fill a 30000 finesse
high finesse cavity to conclude the ANR Mightylaser project
In the third section the details of the implementation of the cavity in the electron ring and the cavity
vacuum system are described with the opto-mechanical system that will be used for a precise alignment
of the cavity mirrors
61 Feedback system and results on a Fabry-Perot cavity locking
To produce a high X-ray flux the ThomX facility produces high intensity laser pulses and collides them
with electrons stored in a ring Hence the locking system (see figure below) has a double goal
- Stabilize the synchronization of the Fabry-Perot cavity (FPC) photon pulse with the electron bunch
- Stabilize the synchronization of the laser pulses with the FPC to obtain amplification
For this last point as will be seen in section 62 firstly the laser beam will be amplified with a CPA
technique using optically pumped fibers and then it will be amplified again by accumulation during
many round trips into a high finesse FPC (the target is a gain of 10000 which leads to a finesse of
30000)
This section describes the following topics related to the stabilization of the laser pulses relative to the
FPC
- Parameters to be stabilized repetition rate and carrier-envelop phase (CEP)
- Error signals the Pound-Drever-Hall (PDH) measurement technique
lsquo
98
- Actuators dynamic range and bandwidth
- Behavior of the digital feedback system (DFS)
General scheme for the locking system Detection and actuation elements are shown in red
The results on the locking of a TiSapph laser oscillator on a 30000 finesse cavity will also be
illustrated
Figure 61 Locking scheme for the synchronization of the electron bunch and the FPC pulse for the
ATF experiment at KEK
611 Electron and FPC pulse synchronization
This kind of stabilization has already been achieved for the MightyLaser experiment at KEK (ATF) a
1785MHz repetition rate A digital feedback system was used (i) to acquire two clocks one from the
laser and one from the master oscillator of the electron ring (ii) to perform a digital phase detection
(iii) to build a digital integrator filter to remove phase drifts and (iv) to react on a PZT inside the FPC
to change its length according to the measured phase drift (see Fig 61)
lsquo
99
612 Laser parameters to be stabilized
We will choose a laser with a repetition rate corresponding to twice the e- bunch revolution frequency
in the ring (two laser pulses per electron bunch revolution) around 500MHz14 = 357 MHz A phase
noise measurement on a similar product operating 1785 MHz was already carried out to characterize
the repetition rate phase noise and the drift (see Fig 62 and Fig 63)
Figure 62 Repetition rate phase noise spectrum Figure 63 Repetition rate drift measured
X-axis offset frequency from the carrier in Hz during about one hour X-axis repetition
Y-axis single side band phase noise power rate drift in Hz Y-axis time duration in
density in dBcHz seconds
The global shape of the phase noise in 1f will permit to use three feedback actuarurs a high bandwidth
low dynamic one (EOM for repetition rate and AOM or pump current for CEP) a middle
bandwidthmiddle dynamic one (PZT for repetition rate) and a high dynamiclow bandwidth one
(mirror motor and isochronic wedge motor)
For this kind of laser the harmonic content (index m) is well known and can be represented either in the
time domain or in the spectral domain (see Fig 63 and Fig 64)
( )2
CEL REP CE REPf m m f f m f
φ
π
Δ = + = +
lsquo
100
Figure 64 Time domain and spectral representation of the laser pulses
Since one is dealing with two parameters (the repetition rate and the CEP) one needs two different
sources of information to measure them The standard method to measure these two parameters consists
in using the Pound-Drever-Hall (PDH) technique [62] which gives the position of the laser harmonics
relative to the FPC resonances
( )FPC CAVf m m f=
One of the parameters is deduced from the whole beam signal using a central photodiode the second
one is derived from a part of the spectrum selected by a grating and a slit and measured by the grating
photodiode
If the repetition rate and the FPC round trip frequency are close enough the error signal coming from
the central photodiode (index m0) is proportional to the difference between the laser harmonics and the
corresponding resonances of the FPC
1 0 0
2
CEREP CAVm f m f
φε
π
Δ prop + minus
Frequency
domain
lsquo
101
As the locking system cancels the error the central laser harmonic m0 is perfectly aligned on the FPC
resonance but all other laser harmonics (m=m0+Δm) are slightly detuned from the corresponding FPC
resonances This introduces a correlation between the CEP and the repetition rate
( )( )
1 0 0
2 0
0 0
02
1 12
CEREP CAV
CEREP CAV REP CE
m f m f
m mm m f f f f
m m
φε
π
φε
π
Δ = rArr + =
Δprop minus minus = minus = minus
As a result of these equations the signal received by the PDH system located after the grating (ε2) is
directly proportional to the offset frequency (fCE) and then to the CEP Thus if we change the CEP of
the laser we should clearly see on the grating photodiode PDH signal ndash which measures the laser-FPC
detuning on the border of the spectrum (for an m index far from m0) ndash a linear relation between the error
signal and the CEP
This method well known for femtosecond lasers has been tested in our laboratory in the picosecond
regime Figure 65 shows our results (submitted to Applied Optics) on the locking of a 76MHz
TiSapph picosecond laser oscillator on a 30000 finesse cavity
Figure 65 Top experimental measurement of the CEP value of a TiSapph laser oscillator Bottom
measurement of the corresponding grating photodiode signal
In Fig 65 one sees that when the CEP is measured close enough to zero the grating photodiode
exhibits a linear relation with the CEP otherwise the photodiode is saturated (due to the PDH signal
saturation) In our experiment the ratio between the cavity reflected power and the incident beam
power was ~20 which leads to a value of 80 for the coupling of the incident beam power to the
cavity
These results demonstrate for the first time that a picosecond laser oscillator can be locked to a 30000
finesse cavity (implying a power enhancement factor of 10000) with a power coupling of 80
613 PDH Measurement technique
Having shown that we are able to measure independently both parameters of interest (the repetition rate
and the CEP) we will now describe the apparatus to transform these measurements into error signals by
lsquo
102
the PDH technique (see Fig 66) Side-bands to the laser optical harmonics are produced by an electro-
optic modulator (EOM) modulated by a generator coming from the digital system Side-bands and laser
harmonics are reflected by the FPC the first ones without acquiring any phase the second ones shifted
in relation with the discrepancy between the laser and the FPC frequencies All the harmonics and the
side-bands of the reflected laser are combined into the photodiodes and the front-end electronics giving
an error signal at the modulation frequency
Figure 66 Pound-Drever-Hall scheme with digital phase detection
The photodiode front-end (see Fig 67) amplifies these signals at a level compatible with the digital
feedback system (DFS)
Figure 67 Electronic scheme and picture of the PDH front-end
Inside the DFS the PDH signals are filtered by a band-pass filter to remove unwanted frequencies and
multiplied by the same modulating signal with a phase shift to allow for the phase detection A low-pass
filter at the end of the chain removes the image signal at twice the modulation frequency Figure 68
shows an example of the input and the output signals
lsquo
103
Figure 68 Modulated input (blue) error signal (yellow) FPC transmitted signal (pink)
614 Actuators
In the previous sections we described how the two parameters (the repetition rate and the CEP) of a
pulsed laser have to be controlled in order that all the laser harmonics match the FPC resonances and
we also detailed how to measure the variations of these two parameters We will now describe the
devices inside or around the laser on which we need to react to correct the variations of the laser
harmonics relative to the FPC resonant frequencies
Figure 69 Typical scheme of a laser with all its actuators
The actuators will produce an action on the two parameters to be controlled with different dynamic
ranges and different bandwidths (see chart in Fig 610)
Figure 610 Dynamic ranges and bandwidths of the laser actuators
lsquo
104
All of the actuators are located inside the laser except the AOM (see Fig 610) They are the key items
to achieve a good stabilization
All the actuators measurements have been made on the 1785MHz laser version used at KEK
- Mirror motor dynamic range precision and bandwidth
The motor is a step motor with a precision of 140 nm on the round trip variation (equivalent to 15 Hz on
Frep) Figure 611 shows the linearity and the maximum dynamic range available with the motor stage
while Fig 612 shows a step response which indicates the maximum bandwidth we can obtain
Figure 611 Mirror motor range Figure 612 Time response of Frep for
1k motor steps X-axis repetition rate
offset (Hz) Y-axis repetition rate
The repetition rate range is 178033 - 179244 MHz (gt 1MHz)
- Piezoelectric sensitivity dynamic range and bandwidth
The PZT sensitivity is approximately 3Hz V 28 nmV (length variation to be compared to the
length of a round trip of the FPC pulse)
The PZT can be operated up to 150V providing a total dynamic range of 42 microm (round trip length
change) Its first peak resonance has been measured and found above 40 kHz
lsquo
105
Figure 613 Repetition rate change produced by a Figure 614 PZT frequency response X-
PZT voltage variation from 0 to 28 V frequency range 01-5 kHz Y-axis
Attenuation (dB) ndash arbitrary offset
Figures 613 and 614 show the PZT sensitivity and its transfer function
- EOM sensitivity measurement
Measuring the sensitivity of the EOM is not direct thus we chose to apply a 10 V amplitude ndash 10 Hz
frequency modulation ndash to the EOM and to measure the power of the harmonics (see Fig 616)
Assuming a pure frequency modulation the power of the harmonics fis a Bessel series that depends on
the sensitivity Thus taking the ratio of two consecutive harmonics gives directly the sensitivity
Figure 615 EOM modulation spectrum used for sensitivity measurement X-axis frequency centered
on the repetition rate Y-axis relative optical power around the repetition rate due to the modulation
The table on the left gives the frequency and the power of each harmonic
0 5 10 15 20 25 300
10
20
30
40
50
60
70
80
90
100
PZT voltage (V)
Fre
p v
ariation (
Hz)
Frep variation vs PZT voltage
AJ0(z)
AJ1(z)
AJ2(z)
lsquo
106
We now check that the sensitivity calculated from the ratio of the first harmonic to the fundamental and
the one obtained from the ratio of the second harmonic to the fundamental are the same
∆
∆
~1
2
∆
10~ minus 2909 rarr ∆~07
∆
∆
~1
8
∆
10~ minus 6398 rarr ∆~07
Since the ratio is 07 Hz for an applied sinusoidal signal of 10 V the sensitivity is around 007 HzV
- Isochronic wedge
It is made of two glass prisms with different refraction indexes chosen to maintain a constant group
delay in order to avoid repetition rate variation
We measured that the maximum variation of the repetition rate for a full CEP range (several times 2π)
is about 300 Hz for a repetition rate of 1785 MHz (see Fig 616 which shows the measured repetition
rate shift plotted along the X-axis versus the date of the measurement plotted along the Y-axis) Thus
the influence of this actuator on the repetition rate is about 210-6
Figure 616 Measurement of the influence of wedge displacement on Frep
- Acousto-Optic Modulator (AOM)
Such an actuator has already been used with a laser which is not the same as the one used for the other
actuators namely a 76 MHz laser locked on a two-mirror FPC
In Fig 617 we present the closed loop measurement The lowest frequency part (lt10 kHz) is
attenuated by the loop gain If the loop were open this part of the transfer function would be quite flat
To estimate the AOM bandwidth limits we can use the frequencies where the gain starts to drop
significantly this bandwidth is about 300 kHz
300 Hz
lsquo
107
Figure 67 Frequency response of the AOM in closed loop
615 The digital feedback system
The feedback system will deal with several coupled inputs and outputs To ensure flexibility we have
chosen a digital system Since we also want to work with a large bandwidth in closed loop (gt 1MHz) a
low latency system (lt 100 ns) is needed A digital system featuring such performance does not exist on
the shelf Thus we decided to design and build such a system at LAL (see Fig 618)
Figure 618 Scheme of the DFS and picture of the DAC daughterboard
This system will perform several functions (see Fig 619)
103
104
105
106
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
Frequency (Hz)
Magnitude (
dB
)
Bode plot
lsquo
108
Figure 619 Organization of the digital feedback functions
- The first important function is to produce the error signal from the modulated signal coming from the
photodiodes It is a phase detection block (Digital PDH DPDH) the main problem being to reduce the
latency as much as possible This will be achieved by optimizing the low-pass filter and the modulation
frequency (see Fig 620 for a digital implementation)
Figure 620 Digital phase detection (DPDH) to produce the PDH error signals
- The second important function is to provide Infinite Impulse Response (IIR) filters to build
Proportional-Integrator (PI) filters in order to cancel the system drifts
lsquo
109
Figure 621 Digital IIR filter implementation with adders and multipliers
These filters can be programmed by the user to change the filter coefficients and it can be cascaded to
realize complex filters They have been cut in Second Order Sections (SOS) to minimize the
propagation of fixed precision errors due to quantized data and coefficients (see Fig 621)
- The last important function integrated in the digital system is performed by an identification block
which works like a network analyzer by (i) sending a small harmonic signal (ii) looking at the system
response in closed loop at this frequency and (iii) sweeping this small harmonic signal frequency to
cover all the range to be measured (see Fog 622)
Figure 622 Identification system
lsquo
110
This kind of identification system can provide information on (i) the feedback system itself (ii) the
system to be stabilized or (iii) the open and closed loop transfer functions by producing Bode or
BlackNichols diagrams (see Fig 623) These informations are very useful for system analysis and for
tuning or designing the feedback filters
Figure 623 Typical Bode (top-left magnitude bottom-left phase) and Black-Nichols (right) diagrams
produced by the identification system
62 High average power laser system and its coupling to a high gain FP cavity
The all-fibered laser system will draw from the system developed in the MightyLaser project It will
consist in an oscillator associated with three amplifying stages involving single and double clad Yb-
doped fibers (see Fig 625) At present the amplifiers are based on commercial components to facilitate
a potential replacement of damaged parts The architecture is designed to provide the mandatory
diffraction-limited beam This relatively simple and compact architecture will implement the lsquoChirp
Pulse Amplificationrsquo (CPA) scheme In order to limit non-linearity effects in the fiber the signal is
initially stretched in time thus reducing its intensity in the waveguide then amplified in the fiber and
finally recompressed to its minimal duration The difference between the MightyLaser and the ThomX
repetition frequencies (respectively 1785 MHz and ~357 MHz imposed by the different repetition rates
of the two accelerators) requires an adjustment of the laser design
Figure 625 Sketch of the all-fiber laser system
lsquo
111
621 The MightyLaser oscillator
This oscillator is an all-normal dispersion mode-locked fiber laser based on the nonlinear polarization
evolution technique equipped with a fibered output It uses an ytterbium doped fiber which allows an
emission centered around 1030 nm with a full width at half maximum of 10 nm (see Fig 626) It
delivers short pulses at a repetition rate of 1785 MHz with a low average output power (~10 mW) in
order to minimize the noise level (see Fig 628) The main advantage of the all-normal dispersion
regime resides in the significant peak power reduction provided by the pulse stretching during the
propagation along the cavity Non-linear effects are thus reduced allowing a high-energy extraction
The output pulses are naturally chirped with an estimated duration of 2 ps
Figure 626 Emission spectrum of the oscillator
The oscillator is passively mode-locked by taking advantage of the non-linear evolution of the
polarization This technique is based on the optical Kerr effect The polarization of a pulse propagating
in a fiber evolves non-linearly as a function of intensity Thus by combining polarization controllers
and a polarizer one may generate losses that depend on the intensity (see Fig 627) Beyond the
polarizer all the pulse components are linearly polarized The wave plates transform the linear
polarization into an elliptic one The latter polarization state then evolves in a non-linear way while it
propagates along the fiber This evolution results from the combined effect of the self-phase and the
cross-phase modulations induced on the two polarization components The polarization rotation of the
more intense central part of the pulse is larger than the polarization rotation of the pulse wings Since
the pulse center crosses the analyzer with a reduced loss compared to the wingsrsquo loss the pulse duration
is shortened
Figure 627 Principle of mode locking based on the non-linear polarization evolution
lsquo
112
Figure 628 Spectrum of the oscillator phase noise noise level (dBcHz) versus offset frequency (Hz)
622 Amplification stages
The amplification chain is divided in three stages in order to distribute the gain and to saturate the
second and the third amplifying stages The first and the second stages are based on the same
architecture It is an all-fibered amplifier whose central component is a polarization maintaining single
mode using an Ytterbium doped fiber with a core diameter of 6 microm and a clad diameter of 125 microm (see
Fig 629) A wavelength division multiplexer (WDM) is spliced on the doped fiber to transfer the pump
power into the gain medium and to couple the signal of the picosecond oscillator to the amplifier The
first stage is pumped by a single mode diode centered at 976 nm and delivering 200 mW The second
one is pumped at a higher average power namely600 mW The two amplifying stages are separated by
an optical isolator to avoid any parasitic feedback from one amplifier to the other
Figure 629 Schematic of the first and the second amplification stages
The average power level at the input of the first amplifying stage is about 1 mW A limitation is
imposed on the output of this stage which should remain below the damage threshold of the isolator
fixed at 100 mW Consequently we adjust the pump level in order to obtain 80 mW at the output of the
ytterbium-doped fiber The 3 dB losses of the isolator allow one to inject 40 mW into the second
amplifying stage The evolution of the average output power versus the pump level is shown in Fig
630 For the maximum pump level of 600 mW the second amplifying stage delivers 330 mW
lsquo
113
Figure 630 Second stage output power plotted versus the pump power
Beyond the second isolator the third amplifying stage (see Fig 632) is built by using as its central
component a polarization maintaining double-clad Ytterbium-doped fiber with a core diameter of 25
microm and a clad diameter of 250 microm The amplifier is clad-pumped through a special combiner 6+11
(see Fig 631) In practice this component has an input signal fiber 6125 microm directly spliced to the
previous amplification stages an output signal fiber 25250 microm spliced to the double-clad Ytterbium
doped fiber and six multimode fibers of 105 microm to couple the pump diodes Here only three pump
ports are spliced to multimode diodes emitting up to 55 W at 976 nm corresponding to a pump power of
at most 165 W The development of a mode field adapter was necessary to splice the pump combiner to
the double-clad Ytterbium-doped fiber because their numerical apertures are different Losses entailed
by this component are compensated by the last amplification stage
Figure 631 Pump combiner Figure 632 Schematic of the third amplification stage
Figure 633 is a plot of the average output power versus the pump level For a pump power level of
76W the last amplification stage delivers 52W This was the first objective The fiber laser power
stability was measured during 1 12 hour at this power level (see Fig 634) The residual pump power
the pump and signal feedbacks into the pump diodes are also plotted in this figure
lsquo
114
Figure 633 Third stage output power Figure 634 Laser stability at 52 W
versus the pump power
New measurements were made after optimizing the central wavelength of the multi-mode pump diodes
The efficiency was increased to reach 787 for an output power of 24 W (see Fig 635) The fiber
laser power has been tested at this level during two hours (see Fig 636) The observed power variation
is less than 01
Figure 635 Output power versus pump power Figure 636 Laser stability at 24 W
after wavelength optimization
The all-fibered amplifier setup has been integrated in a 19rsquorsquo rack as shown in Fig 637 An FCPC
connector placed in the front panel allows the connection of the fibered oscillator to the master
amplifier module Several 955 optical fiber couplers have been inserted in our setup at strategic
locations to monitor the power level in real time by using photodiodes they may also signal potential
problems The output fiber comes out of the module through its front panel The back-side is dedicated
to all the electronic controls (diode pump photodiodes and modulators for noise management described
elsewhere)
lsquo
115
Figure 637 Pictures of the 52 W all-fibered setup integrated in a 19rsquorsquo rack
623 Stretcher and compressor
A chirp volume Bragg grating (CVBG) is used in our setup to stretch and compress the pulse with a
very good efficiency of 94 The CVBG transmission bandwidth is around 2 nm (see Fig 638) leading
to a pulse duration ranging from 200 to 250 ps depending on the grating dispersion It is possible to
recompress the pulse after amplification by using a second CVBG Then the pulse length ranges
between 20 and 87 ps depending on the second CVBG dispersion (see Fig 639)
Figure 638 Transmission bandwidth of the CVBG
1‐2 couple 3‐2 couple 3‐1 couple
Figure 639 Time profile of the recompressed pulses for different grating pairs
624 ThomX development
Some changes must be made to the all-fibered lsquoMightylaserrsquo design in order to use it in the ThomX
project The oscillator repetition rate needs to be scaled down to match the frequency imposed by the
accelerator As far as the laser design is concerned the stretchercompressor couple must be modified to
limit non-linear effects The increase of the output power from 50 to 150 W imposes an upgrade of the
second amplification stage and eventually of the third one
lsquo
116
6241 Oscillator
The oscillator for ThomX will be based on the same principle as the one used for Mightylaser The only
difference is the repetition frequency imposed by the accelerator (357 MHz for ThomX) The low
average output power around 5 mW permits to limit the phase noise A particular attention will be paid
to the noise level of such a long cavity as no commercial products have been tested at this repetition
rate
6242 Amplification stages
For the ThomX project the lower repetition rate and the higher average output power require a change
of the second amplification stage architecture in order to limit non-linear effects and to saturate the last
amplification stage Consequently the second pre-amplifier will be based on a polarization-maintaining
double-clad Ytterbium doped fiber with a core diameter of 15 microm and a clad diameter of 130 microm (see
Fig 640) The amplifier will be clad-pumped through a special combiner 2+11 with an input signal
fiber 6125 microm directly spliced to the previous amplification stages an output signal fiber of 15130
microm spliced to the double-clad Ytterbium doped fiber and two multimode fibers of 105 microm to couple
the 9 W pump diodes
Figure 640 Schematic of the second amplification stage of the ThomX laser
Concerning the final amplification stage various fibers are available (see below) Micro-structured
fibers can be engineered and tailored but suffer from the lack of splicing techniques Hence their use in
an all-fibered setup will require additional developments dedicated to splicing (see Fig 641) and
integration If splicing is not possible we still have the possibility of proceeding with air propagation
and injection However this approach requires precise mechanics control and advanced engineering
The lsquocoherent beam combining techniquersquo will also be considered at this point In such a configuration
amplification would be supported by two parallel amplification stages dividing by two the energy level
in the gain medium Yet a first version will be developed with commercially spliceable flexible fibers
and preliminary tests of cavity locking with high power from these fibers will be performed to validate
or invalidate operating with a high level of non-linear effects
lsquo
117
Fig641 A micro-structured fiber
6243 Stretcher and compressor
Because of the high average output power required by ThomX we must modify the stretcher size Two
techniques can be considered First we may continue to use a CVBG which can stretch the pulse up to
450 ps with a maximum size of 25 cm (see Fig 642) It would be possible to stretch the pulse up to
900 ps with an efficiency of 88 through a double pass in a CVBG or a single pass in cascaded CVBG
The second way involves a chirp fiber brag grating (CFBG) as depicted in Fig 643 When associated
with an optical circulator it allows a pulse stretching with up to 800 ps in an all-fibered setup The
efficiency is around 70 (92 for the CFBG and 75 for the circulator) in this configuration This
solution is less efficient than the CVBG but the all-fibered configuration ensures the set-up robustness
and stability and the losses can be easily recovered by the amplifier chain However dispersion issues
have been reported for these devices and a detailed investigation is needed prior to a definitive
integration
Fig 642 Examples of CVBG Fig643 Example of CFBG
The high output average power forces us to use a CVBG to compress the pulse one or two units
depending on the stretcher configuration The final pulse duration can be varied between 1 ps and 50 ps
thus leaving the possibility to optimize the X-ray production by adjusting the pulse duration
We also performed simulations to evaluate the influence of the couple stretchingfiber core diameter on
the non-linearity with 3 different fiber cores 25 microm 30 microm and 40 microm Our results are summarized in
Table 61
lsquo
118
Fiber
dimensions
mode field
diameter stretching B-Integral
25250 20 microm 250 ps 47 rad
25250 20 microm 500 ps 263 rad
25250 20 microm 1 ns 118 rad
30150 22 microm 1 ns 09 rad
40200 30 microm 1 ns 0525 rad
Table 61 Pulse stretching as a function of the fiber core diameter (simulations)
A first configuration will be developed with a stretching of 500 ps If the non-linearity effects remain
too high to allow for an efficient locking on the Fabry-Perot cavity we will modify the setup to induce
a larger stretching factor or insert a fiber with a larger core diameter in the last amplification stage or
both
63 Mechanical design
Collisions between the laser pulse and the stored electrons will take place between two dipoles (see Fig
643) The constraints on the optical cavity mechanical design are as follows
- High stability level (vibrations)
- Good accessibility to the cavity mirrors
- Weak impact on the electron beam dynamics
- Ultra high vacuum (UHV) system with baking capability
- Motorized granite table for micrometric scan oof the electron beam
lsquo
119
Figure 643 Implementation of the cavity in the electron ring
The cavity mirror system is mounted on a massive granite table which can be driven to perform a
micrometric electron beam scan (see Fig 643)
lsquo
120
Figure 644 Scheme of the Fabry-Perot cavity mounted on the optical table
The Fabry-Perot cavity is composed of two cylindrical UHV vessels linked by a dipole chamber
Bellows are placed between each element to absorb the dilatation due to the backing (see Fig 644)
e‐
e‐
X‐Ray
1rsquorsquoMirrors
IP
Bellows
1rsquorsquoMirrors
Vessel
Vessel
Dipole
chamber
lsquo
121
Figure 645 On the left detail of the standard mirror assembly and on the right detail of the piezo
electric mirror assembly
The Fabry-Perot cavity is composed of two spherical and one flat mirror mounted in a standard
assembly and one flat mirror mounted on a special assembly with a piezo electric actuator for very
small longitudinal displacements (see Fig 645)
Figure 646 Detail of a truck system
Mirrors
Piezo
electric
actuator
Mirror
Gimbal
Actuators
lsquo
122
Figure 647 Cut of an UHV actuator
All the mirrors are mounted on a truck system that porvides three degrees of freedom (two rotations and
one translation) driven by three micrometric actuators These actuators are specially designed to be
compatible with UHV and backing constraints (see Fig 646 and Fig 647)
Figure 648 Cut of a vacuum vessel
Trucks (two trucks per vacuum vessel) and plugs with special cables composed of ceramic connectors
oxygen- free copper cables and ceramic pearls and spacers for electrical isolation are placed in a UHV
vacuum vessel One of the vacuum vessels has a beryllium window to extract the X-ray beam (see Fig 648)
Micrometric Actuator
Capsule
Trucks
Beryllium
window
lsquo
123
631 Optical cavity vacuum
A simulation was carried out in the most unfavorable case when the whole optical vacuum chamber is not
baked The area of one half of the optical cavity is about 2 m2 the conductance of the rectangular chamber
connecting the two cavity halves is C =70810-1
m3s
-1 (value for one half of that rectangular chamber) The
conductances of the slits connecting the optical cavity to the ring are respectively C = 45210-2
m3s
-1 (9 x 48)
and C = 16010-2
m3s
-1 (6 x 30) The outgassing rate of one optical cavity half without any baking (σ = 210
-8
Pams-1
) is Q = 410-8
Pam3s
-1 The outgassing rate of one rectangular chamber half (σ=10
-9 Pams
-1) is Q =
7610-10
Pam3s
-1 The outgassing rates of the two slits (σ=10
-9 Pams
-1) are respectively Q = 22810
-13
Pam3s
-1 (9x48) and Q=14410
-13 Pam
3s
-1 (6x30) The diode ion pump used for the optical cavity has a
pumping speed of 05 m3s
-1 DN 150 S = 0320 m
3s
-1 at ~10
-10 mbar The conductance of the pumping port
being C = 136 m3s
-1 the effective pumping speed is Seff= 0278 m
3s
-1 The diode ion pump used for one half
of the rectangular chamber has a pumping speed of 015 m3s
-1 DN 100 S = 0090 m
3s
-1 at ~10
-10 mbar The
conductance of the pumping port being C = 0046 m3s
-1 the effective pumping speed is Seff= 0075 m
3s
-1
The outgassing rate of the optical cavity without baking is very high 2 x 410-8
Pam3s
-1 (which is
approximately 40 times the outgassing rate of the whole ring) With such a rate one would need a pumping
speed of 8 m3s
-1 to reach the required pressure level of 10
-8 Pa (the present pumping speed is 07 m
3s
-1) The
conductance of the rectangular chamber that connects the two halves of the optical cavity (710-1
m3s
-1) and
the conductance of the two slits between the optical cavity and the ring (610-2
m3s
-1) are high These figures
do not allow a differential pumping between the cavity and the ring Simulations of the ring vacuum level
show that it is mandatory to bake the whole optical cavity
Figure 649 Location of the cavity pumps
The vacuum equipment needed for the optical cavity is as follows
‐ 2 ion pumps 150 l s-1
with their baking equipment and their power supplies
‐ 2 ion pumps 500 l s-1
with their baking equipment and their power supplies
‐ 2 gate valves manually actuated (pipe diameter DN100) and their baking equipment
‐ 4 Pirani gauges
‐ 4 cold cathode gauges
‐ 2 power supplies for the gauges
‐ 4 residual gas analyzers (RGA)
‐ 1 baking equipment for the vacuum chamber
‐ 4 angle valves manually actuated for pre-pumping (pipe diameter DN63)
lsquo
124
Chapter 7 X-RAY BEAM LINE AND EXPERIMENTS
Introduction
The equipment of the X-ray beam line should firstly ensure a continuous detection and monitoring of
the beam (mainly in the first optical hutch) and secondly allow a complete characterization of the
beam (in the experimental hutch) This characterization consists in measuring the beam absolute
intensity its total energy range its spectral and angular distributions and its degree of transverse
coherence Time analyses of the bunched X-ray beam will also be performed This line also aims at
confirming the ThomX assets in such fields as cultural heritage biomedical and materials sciences by
performing demonstrative experiments
71 Available extracted X-ray beam
The X-ray beam is extracted through a 50 mm diameter beryllium window as schematized in Fig 71
Because of limitations due to the dimensions and the location of the Fabry-Peacuterot cavity mirrors the X-
ray beam available for characterization and experimentation is a cone with a half-aperture of 16 mrad
Figure 71 The X-ray beam extracted from ThomX through a beryllium window The beam fills a cone
whose total aperture is 2 x16 mrad
In the case of a lsquoperfectrsquo beam (ie without taking into account the electron beam emittance) the X-ray
spectral and energy distributions are shown in Fig 72 as a function of the scattering angle for two
electron beam energies 50 and 70 MeV and a laser wavelength of 106 microm In Fig 73 the energy
bandwidth and the flux per unit solid angle are shown for on-axis Compton photons produced by either
a 50 MeV or a 70 MeV electron beam
72 Global view of the X-ray lne
As can be seen in Fig 74 the positioning of the machine in the Igloo imposes the splitting of the X-ray
line into two sections a first optical table (3x2 m2) located just after the beryllium exit window and a
second table (4x2 m2) located outside the Igloo in hall D1 Along its path and until the optical table 2
the beam is transported in the primary vacuum inside stainless steel pipes with the exception of a short
upstream section of a few tens of cm between the beryllium window and the optical table this
insulating section will prevent the transmission of harmful vibrations to the Fabry-Peacuterot cavity This
primary zone is kept oxygen free by a nitrogen flow that prevents ozone production
lsquo
125
The beam diameter at the entry of the first optical table is 45 cm After this first optical table the beam
reaches the second table via a 13 cm diameter beam pipe inserted in a hole which was drilled long ago
in the Igloo wall About 8 mrad beam angular divergence are preserved at the entry of the second
optical table where the beam diameter is 13 cm
Figure 72 X-ray spectral distributions (left panel) and X-ray energy (right panel) versus the scattering
angle for an e- beam of 50 MeV (black) and 70 MeV (red) The laser wavelength is 106 microm Grey
areas correspond to emission angles larger than 16 mrad
Figure 73 X-ray flux (left panel) and X-ray energy bandwidth (right panel) of on-axis photons per unit
solid angle versus the emission angle for an e- beam of 50 MeV (black) and 70 MeV (red) The laser
wavelength is 106 microm
lsquo
126
Figure 74 Global view of the X-ray line in ThomX
73 Chraracterization of the X-ray beam
731 Optical table 1
A global view of the optical table 1 design is presented in Fig 75 Just upstream of this optical table a
60 mm diameter beam shutter activated when one needs to take background data will allow stopping the
beam A few millimeter thick tungsten is sufficient to stop the full beam (tungsten attenuation mass
factor microen ~ 4043 cm-1
for 100 KeV photons) This shutter does not have to be cooled since the ThomX
radiated power will not exceed 100 mW A drawing of the shutter mechanical design is shown in Figure
76 (a)
Figure 75 Global view of the optical table 1 showing the location of the main devices placed on it
lsquo
127
Beam monitoring
The optical table 1 equipment (see Fig 75) is mainly devoted to the detection and the continuous
monitoring of the X-ray beam The first device monitoring the beam presence is a fluorescent screen
mounted on a two-position motor (up or down) as shown in Fig 76 (b) It is inserted in the beam path
and viewed by a CCD camera Its screen absorbs almost the totality of the beam and thus has to be put
in the up position when not in use Downstream of the fluorescent screen a device composed of two Si
PIN diodes measures the beam scattered by a thin Kapton foil inserted in the beam path This setup
shown in Fig 76 (c) will monitor the beam flux variations and beam position displacements The
Kapton foil is fixed at an inclination angle of about 60o with respect to the beam path The sum and the
difference of the two diode currents will provide a relative measurement of the beam flux and of the
beam position respectively This set-up which is nearly transparent to 20-90 keV radiations is a non-
intrusive monitor and can be used continuously Then a high precision double slit [71] intercepts the
X-ray beam and allows one to let through only the desired part of the Compton cone (see Fig 76 (d))
The minimum dimensions of the slit aperture are 10 microm in the horizontal and the vertical directions
Beyond the double slit a wire monitor [72] shown in Fig 76 (e) is inserted in the beam This device
consists in two tungsten wires stretched horizontally and spaced by a few tens of a millimeter This
monitor will be driven vertically through the beam The difference of the two wire signals allows the
measurement of the absolute beam position
Figure 76 Drawings of the X-ray line devices located on the optical table 1 (a) Beam shutter
(b) Motorized two-position fluorescent screen (c) Kapton foil and Si diodes (d) Double slit (e) Wire
profiler
lsquo
128
Focusing device
Downstream of the beam monitoring instruments a focalizing device will be installed (see Fig 75) to
make the X-ray beam parallel andor to focus it on a sample It can be either a device based on
compound refractive lenses [73] (a so-called lsquotransfocatorrsquo) or two bendable mirrors in the Kirkpatrick-
Baez configuration (KB Optics) or a capillary optical device These three techniques will be
investigated to evaluate the advantages and disadvantages (aperture efficiency focusing strength etc)
of their eventual implementation in the ThomX facility Such a focusing apparatus has to be located as
close as possible to the X-ray source because of the rapid growth of the beam transverse dimension
Finally at the end of the optical table 1 some space is kept free for any specific study which might
concern optics detectors or samples
732 Beam transport
As shown schematically in Fig 77 beyond the optical table 1 the X-ray beam crosses the Igloo wall
inside a beam pipe that runs through a 15 cm diameter hole and reaches the D1 hall where the second
optical table is installed in the lsquoexperimental hutchrsquo At the entrance of the wall hole a second beam
shutter in lead around 15 cm in diameter and 25cm thickness is installed This shutter is connected to
an interlock system that ensures radioprotection It absorbs radiations coming from the electron
machine which propagate through the Igloo cylindrical hole and allows X-ray users to work in the
experimental hutch without stopping the electron machine
Figure 77 Schematic of the X-ray beam transport across the Igloo wall from the optical table 1 located
in the Igloo until the optical table 2 located in the D1 hall
733 Optical table 2
Several kinds of instruments will be placed on the optical table 2 The goal is to study in detail the X-
ray beam produced by ThomX firstly to determine how the beam characteristics depend upon the
machine operation parameters and secondly to prepare the demonstration experiments A global view
of the optical and detection instruments placed on that table is provided by Fig 78
lsquo
129
Figure 78 Global view of the optical table 2 indicating the main instruments that will be used to
characterize the beam
Absolute and spectral flux measurement
The measurement of the X-ray beam absolute flux and of its energy spectrum will be performed by
using two detectors a calibrated Si diode [74] and a cooled high-purity Ge solid-state detector (SSD)
[75] The Si diode will be bought already calibrated [76] A lsquogoodrsquo Ge SSD has an energy resolution
(ΔEE) around 1 at 20 keV 06 at 45 keV and 045 at 90 keV [77] The energy calibration of the
Ge SSD will be carried out in the X-ray beam line by using the Kα fluorescence lines of reference
materials (see Table 71)
Element Ge Mo Sn Cs Ce
EKα1 (keV) 9886 17479 25271 30973 34719
EKα2 (keV) 9855 17374 25044 30625 34279
Element Eu Er W Pt Pb
EKα1 (keV) 41542 49128 59318 66832 74969
EKα2 (keV) 40902 48221 75981 65112 72804
Table 71 Kα fluorescence lines of chemical elements used to calibrate the Ge SSD in the 10-90 keV
energy range
The two detectors will be mounted alternately on a (X Y) translation stand to measure a large number
of points of the emission spectrum
- the mean emission energy E (Ge SSD)
- the energy spread ΔE (Ge SSD)
- the absolute flux (photon numbers) (Si diode)
These beam characteristics will be measured as a function of the size of the selected X-ray sample the
selection being carried out with a double slit device (see Fig 76 (d)) Our goal is to check
lsquo
130
experimentally the theoretical angularspectral distributions shown in Fig 12 of the Introduction by
measuring the number of Compton photons per second per mrad2 and per 01 of energy bandwidth
Time analysis of the bunch structure
The X-ray source has a time structure which involves two main scales a 50 ns period which is the
interval between two successive collisions between the laser pulse and the electron bunch and the 20
ms e- storage cycle To perform a pulse by pulse analysis we will use a high resolution time detector
(~1-5 ns) with a high repetition frequency (~10-100 MHz) eg a micro-channel plate (MPC) detector
[78] or an avalanche photo-detector (APD) Such time measurements have already been performed at a
similar facility by Sakaue et al [79] MCP Hamamatsu devices can reach a 07 ns (FWHM) time
resolution which is good enough not only to resolve each one of the collisions between the laser pulse
and the electron bunch but also to measure their relative intensity accurately from the beginning of the
e- storage cycle to the end These measurements will be performed on the X-ray beam axis They will
have to be synchronized with the ThomX clock
Transverse coherence of the beam
Some simple diffraction and interference measurements will be performed in order to analyze the
source spatial coherence The diffraction of the X-ray beam by a slit of a known aperture or the
interference effect produced by the insertion of a phase-shifting object in the beam allows one to
quantify the transverse coherence of the beam [710] In these experiments the on-axis photon intensity
distribution will be recorded with a 2D high resolution detector (such as for instance an XPAD CdTe
camera with 130 microm pixels) This measurement will be repeated at several positions along the beam
The pattern analysis will inform us about the degree of coherence of the source A monochromator will
be inserted upstream of the diffractive slit or the phase-shifting object to select a well defined energy
Later on Young double-slit experiments could also be carried out [711] to measure the beam
coherence length beam
74 Demonstration experiments
In parallel with the complete characterization of the ThomX beam described in the previous
sections some simple experiments will be performed in order to demonstrate the potentialities
of the ThomX source In this phase we will focus on medical and materials science
applications An important aim of the ThomX project is to perform medical and materials
imaging exploiting the high and tunable energy of its beam A second aim of the ThomX
project is to show its capability in the structural investigation of materials using X-ray
diffractionscatteringspectroscopy techniques
741 Experiments using the full X-ry beam
Three experiments issued from the expertise of the ESRF-INSERM teams in medical applications of
synchrotron radiation have been chosen to be carried out as demonstrators on the ThomX prototype
The goal of setting up such experiments is to demonstrate (i) the suitability of ThomX of performing
both imaging and therapy experiments (ii) the tenability of the source adjustable at energies
corresponding to the K edge of high Z elements routinely used in medicine (Gd I) or of those of new
experimental radiotherapy adjuvants (Au Gd nanoparticles CisPt) (iii) suitability of the source for
phase contrast imaging (iv) exploitation of the advantage of using a 2D beam with respect to the typical
laminar synchrotron X-ray beam Some imaging and therapy experiments in the biomedical field
require a wide 2D beam Using the natural divergence of the ThomX beam a sample of a few
lsquo
131
centimeters in diameter located at 10-20 meters from the ThomX interaction region can be fully
illuminated by the a conic beam with no need of optical elements Calculations have been performed in
order to show the potential of using the ThomX source in comparison with a hospital X-ray generator
(fluoroscopy unit type) as well as in comparison with the beam from the biomedical line ID17 at ESRF
Results of these comparisons are given below for each medical experiment we intend to perform with
ThomX
An imaging experiment
For imaging experiments the energy range of the beam has to around 60-90 keV According to Fig 73
the corresponding half-aperture will be 5 mrad leading to a photon flux of 61012
phs When the beam
reaches the sample stand placed on Table 2 at 20 m from the ring interaction region it will have a
diameter of 20 cm This conical beam will then have a cross section of ~300 cm2 and permit
tomographic acquisitions of a full human head using a 2D detector This wide 2D beam is a major
advantage of the ThomX source for biomedical imaging with respect to the laminar synchrotron
radiation beam The dose rate (DR) in Gys can be expressed as
(71) DR = 16 10-13
Ф[phscm2] E[keV] (microenρ)[cm
2g]
where Ф is the photon flux per cm2 E is the mean energy of the beam and (microenρ) is the mass
attenuation coefficient For an X-ray beam of 75 keV (ie the average of our 60-90 keV energy range)
the mass attenuation coefficient microenρ for soft tissues is 0028 cm2g leading to a dose rate of 67 mGys
delivered to a 300 cm2 area The corresponding expected lsquodose area productrsquo (DAP) of the ThomX
source for such imaging experiments is DAP = 20 Gyscm2 For comparison at the ESRF ID17 line a
dose rate of 800 mGys is delivered to an area of 150x2 mm2 corresponding to a DAP of 24 Gyscm
2
As far as conventional sources are concerned typical laboratory devices used in 2D fluoroscopy deliver
a DAP of 06 Gyscm2 [712]
A feasibility experiment will be performed by using a standard human head phantom (see Fig 79)
which will be mounted on a rotating stand that allows a 360deg rotation It will be possible to translate this
stand in three directions (along the X-ray beam axis and in the two other perpendicular directions) to
optimize the sample position in the beam Acquisition will be performed using the 60-90 keV pink
beam before and after the insertion of a high-Z element solution (typically iodine or gadolinium) in a
slot available in the phantom head Several images will be acquired using different concentrations of
high-Z element chosen in the range of concentrations that can be typically reached into a tumor (in
humans the high Z element is injected intravenously) The study will also aim at finding for each high-
Z element the lowest concentration that can be measured with the ThomX imaging system
Figure 79 Typical phantom featuring a human skull from X-ray absorption point of view
lsquo
132
The detector will be placed downstream of the sample A typical detector that may be used is a large
field of view 2D detector equipped with Si or CdTe sensors with ~100-200 microm pixels [713] Finally a
standard computed tomography (CT) algorithm will be used to reconstruct the tomography images and
to calculate the concentrations (in mgml) of the given high Z element in the skull The results will be
compared to previous work done at the ESRF [714]
A radiotherapy experiment
For this experiment the targets will be rats implanted in their right brain hemisphere with rodent brain
tumors (F98 9L) at an appropriate tumor stage (typically at D10 or D14 after implantation) For the
irradiation we need a pink beam with a 10 keV energy bandwidth and a median energy above the K-
edge of the high-Z element to be injected as a radiosensitizer (ie in the 45-80 keV range for both
iodine and gadolinium) The beam size will be a few centimeters in diameter Figure 710 shows the X-
ray beam energy and beam flux generated by a 69 MeV electron beam versus the photon back-
scattering angle X-rays of energy 80 keV within a 10 keV bandwidth are contained in a cone of half-
aperture 27 mrad and their flux is ~31012
phs At the sample location on optical Table 2 at 9 m from
the ring interaction region the beam has a diameter of 5 cm ie a cross section of ~20 cm2
The center of the ratrsquos right hemisphere will be placed on the rotation axis of the irradiation system and
slits will limit the beam transverse extension in order to define a precise field of irradiation Assuming
an X-ray flux (Ф) of 31012
20 phscm2 with an average energy (E) of 80 keV a mass attenuation
coefficient of 0025 cm2g (soft tissue 80 keV X-rays) Eq (71) leads to a dose rate DRe of 48 mGys
close to the tissue region where the X-ray beam penetrates Before reaching the tumor the radiation is
attenuated by the bone (~ 02 cm thickness (microρ)att = 022 cm2g ρ=192 gcm
3) and by the skull healthy
tissues (~ 15 cm thickness (microρ)att = 018 cm2g ρ = 10 gcm
3) Thus the dose rate DRt delivered at
the tumor is DRt = TDRe (where T = exp (-02219202) exp (-0181015) = 070) and is
expected to be 337 mGys The dose will be delivered while the rat undergoes a 360deg rotation The total
dose to be delivered to a tumor ranges between 10 to 20 Gy Therefore according to the expected rate
of 337 mGys the irradiation will take typically from 5 to 10 minutes with the big advantage of not
having to move the rat A dosimetry protocol (such as the one used at ID17 [715]) will be set to check
the homogeneity of the irradiation and to measure the absolute dose with a 2 to 5 uncertainty
Figure 710 X-ray energy (left panel) and X-ray flux (right panel) versus the photon back-scattering
angle for stored electrons of 69 MeV The laser wavelength is 106 microm
For humans and a tumor placed at 8 cm into the skull the transmission coefficient becomes T= 019
Therefore the expected dose rate DRt delivered by the ThomX source at the tumor is 91 mGys One
may compare with the ESRF ID17 beam-line where 800 mGys can be delivered for irradiation on a
lsquo
133
surface of 2x50 mm2 ie with T=019 a dose rate DRt of 015 Gys on 2 mm in height at the tumor
which is equivalent to a rate of 6 mGys to cover 5 cm of height with a scanning rate of 50 mms
ThomX could therefore deliver doses for human skull irradiation larger than those available at
synchrotron radiation facilities and also perform the irradiation without having to move the irradiated
sample Concerning the conventional hospital devices they deliver low doses deposited by X-rays
whose energy spectrum is broad and moreover they are not made to operate continuously for such long
time irradiation
Also the tunability of the ThomX beam will allow testing several high Z drugs containing iodine
gadolinium platinum or gold proven to be promising radiosensitizers [716]
Conformal radiotherapy
A final step in this sequence is performing conformal radiotherapy experiments ie doing both imaging
and therapy at the same time In fact one major issue of radiotherapy is to spare as much as possible
healthy tissues located around the tumor and at its border In this regard it is a key issue to adapt the
field of irradiation to tumor geometry size and positioning by performing first a tomography
Phase contrast imaging
The absorption medical imaging technique is based on the fact that different parts of a sample absorb X-
rays differently But the fact that X-ray absorption by biological material is weak constitutes also the
limitation of this technique To enhance or replace absorption-based imaging refraction diffraction and
interference of X-rays can also be used A better visibility at the edge of different regions of a specimen
can therefore be obtained by using the information contained in the phase of an X-ray wave For this a
spatially coherent X-ray source is required and was available only since the development of modern
synchrotron sources Experimentally the simplest way to visualize phase-contrast imaging is provided
by the propagation-based imaging (PBI) method In this method a spatially quasi-coherent radiation
illuminates an object which gives rise to a spatially varying phase shift in areas with spatial variations
of the refractive index As the radiation propagates beyond the sample parts of the wavefront which
have experienced different deflections on both sides of an edge of the sample produce a serie of fringes
due to the classical Fresnel-edge diffraction phenomena [717]) This gives rise to a characteristic phase
contrast pattern which is then recorded by a 2D detector placed at a proper distance downstream of the
object (see Fig 711 left panel) The actual phase-contrast signal is convolved with the detector point
spread function which has to be sufficiently narrow to avoid the smearing of the interference pattern
Additionally the phase contrast signal can be visualized if and only if the transverse coherence is
sufficient ie if Lcoh = λRs is larger than the details to be imaged (here R is the source-sample distance
and s is the source size) In contrast the requirements in terms of longitudinal coherence
(monochromaticity) are rather weak even a beam with a bandwidth of several keV does not lead to a
significant smearing of the phase contrast signal [717]
Assuming s = 70 microm R = 15 m a sample-detector distance of 5 m and a detector point spread function
of 50 microm the phase contrast signal expected when the ThomX beam illuminates a nylon wire of 300
microm diameter ranges from 10 to 3 when the X-ray energy increases from 20 to 80 keV (see Fig
711 right panel)
lsquo
134
Figure 711 Left panel The set-up for propagation-based phase contrast imaging Right panel The
simulated signal produced by a nylon wire of 300 microm diameter illuminated by the ThomX source
(simulation)
The experiment with ThomX will comprise two parts a planar radiography and a computed
tomography For the radiography three kinds of samples will be used (i) a custom-made phantom
composed of nylon wires of different diameters (ii) the ACR (American College of Radiology)
accredited phantom used in mammography quality (iii) a 4 cm thick formalin-fixed human breast tissue
obtained from a hospital bank The X-ray energy will be chosen in the 20-80 keV range For each
sample and for several energy values images will be acquired at two different sample-detector
distances in the absorption regime for comparison purposes (the detector will be in contact with the
sample) and at 5 m A silicium detector with 50 microm pixels will be used Images will be acquired for
different doses (measured with a standard PTW dosimeter) in order to obtain the signal-to-noise ratio as
a function of the dose These measurements will allow one to qualify the phase contrast imaging set-up
at ThomX
Computed tomography will be applied to (i) a meacutethyl polymeacutethacrylate (PMMA) custom-made
cylindrical phantom containing insets of different compositions and densities (water ethanol paraffin
etc) (ii) a formalin-fixed human breast tissue (5 cm in diameter) Images will be acquired using the
same parameter values as the ones chosen for the radiography Samples will be installed on a rotating
stand positioned on a double cradle that will permit an alignment with the beam and with the detector
Images will be taken during the sample rotation over 360 degrees
The final step will consist in applying standard algorithms to the images in order to calculate the phase
and to extract quantitative information about the samples This data will be compared with equivalent
images that will be acquired at the ESRF
742 Experiments using the central part of the beam
Most specimens from cultural heritage and functional materials are heterogeneous these lsquorealrsquo
materials often exhibit complex hierarchical structures involving various levels of order and
heterogeneity To understand the properties of such materials better one needs at least a description of
their structure at the atomic scale and at the macroscopicmesoscopic scale which takes into account
weak structural signals (relating to disorder minor constituents microstructure hierarchical orderhellip)
lsquo
135
This requires much probing as already demonstrated with synchrotron radiation sources A major goal
of materials science is to link global properties to local atomic features (orientation strain structural
distortion grain size) of some components in relation with neighboring domains and to deduce from
that the macroscopic properties As the functionalities of these materials often stem from the association
of different phases or constituents or of specific inter-relation between them (microstructure) highly
selective structural probes need to be used to characterize them Also in the particular domain of
cultural heritage many specimens require non-destructive methods and probes In order to understand
how they were made and to preserve them from degradation over time their physical and chemical
properties must be determined There is an increasing demand for material probing with complementary
techniques Fluorescence emission techniques excel in revealing the elemental composition of the top-
most layers of samples Diffraction is suited for studying quantitatively the phase composition the
structural details of the constituent phases and the characteristic distribution of sizes and orientations of
crystallites Tomography techniques excel in revealing the 3D mapping of the chemical andor the
density heterogeneities within a sample This is why the cultural heritage community is today strongly
involved in the development of new instruments that may characterize patrimonial artifacts by mapping
not only the chemical composition of a variety of samples but also their physical phases
Experiments described in section 741 require a large 2D beam and exploit the natural divergence of
the ThomX beam In this section diffraction scattering and spectroscopy experiments require a
focalized or a parallel beam and it should also be monochromatic or lsquopinkrsquo The optics will therefore
select the central part of the beam ie its higher energy part Let us give an order of magnitude
assuming a perfect 50 MeV electron beam a collimator of 2 mm diameter located at 2 meter from the
IP will cut the X-ray conic beam at 05 mrad leading to a flux of ~8 1010
phs with an energy of 45 keV
and an energy bandwidth ΔEE on the order of 10-3
(see Fig 73) In practice this bandwidth will be
broadened and will reach a few percent because of the actual e- beam divergence and energy spread For
experiments requiring a narrower energy bandwidth an additional selection will be performed with a
monochromator For instance selecting an energy bandwidth of 10-4
would lead to a flux on the order
of 109 phs
Since the goal of the first series of demonstration experiments is to qualify ThomX as an X-ray source
these experiments will be performed with samples previously characterized with other sources such as
synchrotron radiation sources
Diffraction and scattering
Diffraction and scattering experiments are major ways of performing materials structural and micro-
structural analyses This concerns highly ordered samples as well as powders or ill-ordered samples
Complete scattering analyses are based not only on Bragg diffraction but also on diffuse scattering in
order to look beyond the average structure and to investigate the local structure or short-range order
The crystalline quality of powders depends on the preparation methods Grain sizes vary from about 10
nm to several hundreds of microm In the case of nano-materials Bragg peaks are strongly broadened and
X-ray diffraction performed with standard laboratory sources does not provide suitable data for
structural investigations As illustrated by Fig 712 laboratory X-ray sources do not allow the
identification of minor phases in a poorly crystallized powder Figure 713 shows an example of a
diffraction pattern obtained with an excellent resolution The sample used for this analysis is an
Egyptian cosmetic powder dated to 2000-1200 BC The patterns were obtained with the ESRF
synchrotron source (bottom panel) and with a classical laboratory source (top panel)
lsquo
136
Figure 712 Powder X-ray diffraction patterns obtained by using a laboratory X-ray source Top panel
Poorly crystallized Prussian blue sample Bottom panel Prussian blue crystals From [718]
Figure 713 Powder X-ray diffraction patterns obtained by using a laboratory X-ray source (top panel)
and the ESRF synchrotron source (bottom panel) Arrays indicate minor phases From [719]
We intend to use the ThomX source to record diffraction patterns and scattering plots in order to
characterize short-range ordering and identify minor phases in such heterogeneous samples
For the first demonstration diffractionscattering experiments we will study ill-ordered Prussian blue
and white lead samples Pigments used for paintings or frescos have various compositions Since their
usage followed a definite chronology such pigments can sign an era Pigment characterization can also
help the undertaking of a restoration while a thorough knowledge of pigment synthesis through
centuries can help to apprehend the degradation mechanisms better
lsquo
137
For ordered samples diffraction will be performed with a single crystal by using a monochromatic
beam but also by using a lsquopink beamrsquo as done in the Laue method For ill-ordered samples the whole
scattering pattern will be recorded and analyzed with the Pair Distribution Function (PDF) method This
analysis technique (often called the ldquototal scattering methodrdquo) can be used alone or associated with
diffraction For experiments using the lsquopink beamrsquo Laue method the beam will be focused directly on
the sample (by using convergent refractive lenses (CRLs) or a capillary optic system as mentioned in
Section 731) To determine powder crystalline parameters diffraction analyses require the
monochromatization of the beam in order to reach ΔEE ~ 10-4
In the optic hutch (optical Table 1) the
diverging beam will first be made parallel by the CRLs or the capillary optic method and then
monochromatized in the experimental hutch (optical Table 2) by a monochromator The set-up for these
measurements needs a goniometer to align the sample and a low noise 2D-detector sensitive to high
energy photons
Fluorescent spectroscopy
X-ray fluorescence spectroscopy experiments allow one to determine the chemical composition of
materials and the mass concentrations of elements Indeed depending on the atomic number of a given
chemical element energies of the fluorescence lines emitted after an X-ray excitation are a signature of
the element An illustration of this approach is presented in Figure 714 where the fluorescence
tomographic images of a Globorotalia inflata specimen are shown and compared with a classical
radiography image
Figure 714 Fluorescence tomographic imaging (a) Set-up (b) Radiography (c) Fluorescence
spectrum measured at a given angleposition of the sample (d) 3D tomography reconstruction From
[720]
We plan to carry out fluorescence experiments since the energy of the ThomX beam can reach the
excitation energy of most chemical elements usually involved in material science for instance the
critical excitation energy of lead regarded as a heavy element is 88 keV) For this experiment the
excellent energy resolution of cooled Ge detectors will be an asset to detect the fluorescence light (the
detectors will be oriented at 90o with respect to the incident beam)
lsquo
138
K-edge and XANES spectroscopy
In K-edge experiments two images of a given element present in some sample are performed one at an X-ray
energy just below the K edge of the chosen element and the other at an energy just above Then they are
subtracted (see Fig 715 left panel) The K-edge imaging technique is used in particular in the medical and
the cultural heritage domains In XANES (X-ray Absorption Near Edge Structure) the absorption by a
material is measured over an energy range of about 100 eV on both sides of the absorption edge This
technique provides information about the local structure prevailing close to a particular chemical element (see
Fig 715 right panel)
Figure 715 Left panel K-edge subtraction imaging Right panel Principle of the XANES method
The first XANES spectroscopy experiments with ThomX could exploit the K-edge of heavy elements
such as W Pt or Pb By first selecting the central part of the ThomX beam with an energy spread of a
few 10-2
a monochromator could then provide a beam with an energy spread of 10-4
Samples will be
aligned by using a goniometer and the transmitted intensity will be collected by a low noise 2D-
detector
Combined diffractionabsorptionfluorescence tomography
A major limitation of diffraction methods comes from the fact that only average properties can be
obtained while most lsquorealrsquo materials are heterogeneous and complex Thus structural studies of such
materials are difficult to carry out A new tool for structure mapping of materials by means of X-ray
diffractionabsorptionfluorescence combined with tomography allows one to perform non-destructive
3D structure selective mappings of heterogeneous samples This way one may identify minor phases
and determine crystallite orientation and grain size Furthermore the selectivity of the method allows
one to separate and locate the origin of the diffuse or diffracted patterns to identify amorphous phases
In archaeology such complementary micro-structural analyses could shed light on the mechanical
history of artifacts
A recent experiment carried out with a micrometer beam at synchrotron radiation sources have
established the validity of such a local probe by analyzing a mixture of chalcedony and iron pigments
containing haematite [721] This experiment illustrated in Fig 716 consists in (i) performing a
structure tomography mapping directly from the diffraction data (top panel) (ii) obtaining selective
structure patterns of unknown minor phases from these maps (bottom panel) This study was realized by
the Neacuteel institute and the C2RMF and was one of the first diffraction tomography experiments
performed with a micro-beam
With the ThomX source we will use poorly crystallized samples and combine in the same experiment
and simultaneously tomography diffraction with fluorescence and absorption measurements Diffraction
lsquo
139
tomography is beam-time consuming it will be performed to demonstrate its feasibility at ThomX The
experimental set-up will be similar to the one used for multimodal analyses (cf the top panel of Fig
716) The sample will rotate and this rotation will be combined with a translation to provide a 2Θ scan
of 2D diffraction patterns obtained with a 2D detector sensitive to high energy photons The diffracted
2D fluorescence patterns will be recorded with our Ge detector while the sample absorption will be
recorded with our calibrated Si diode
Figure 716 Top panel Experimental set-up for multimodal spectroscopydiffractiontomography
analysis Insets Images of the chalcedonyndashFe mixed-pigment sample are reconstructed by combining
the various signals X-ray absorption (bottom) Fe X-ray fluorescence (top left) and chalcedony
diffraction (top right) Bottom left panel Reconstructed diffraction image based on the whole
diffraction diagram (black) compared with the calculated pattern for pure chalcedony (red) Bottom
right panel Reconstructed image that only uses the iron pigment scattering contribution (black)
compared with the calculated pattern for hematite and siderite grains (red) From [721]
75 Summary
As shown in this chapter two types of beams will be available at the ThomX compact X-ray source
(i) wide beams well adapted for imaging (conventional or phase contrast imaging) and therapy in the
biomedical field and (ii) almost monochromatic beams that can be either focused or made parallel for
diffraction and spectroscopy experiments in the field of materials science
The X-ray line is designed
- to ensure a continuous monitoring of the X-ray beam
- to measure the beam brightness and its transverse coherence
- to demonstrate the potential of this new type of compact source and identify the benefits it can
bring to the currently available X-ray sources
lsquo
140
Chapter 8 CONTROL SYSTEM
Introduction
The ThomX control system combines those hardware and software elements involved in the
management of all the facility components But more than a system dedicated to carrying out operating
processes (processes launched from the X-ray beam control room in order to undertake some specific
task such as a performing an imaging experiment or a material chemical analysis) it must also be an
advanced tool for various studies (data processing new equipment integration operating mode
evolution) and scientific experiments which will be performed with the X-ray beam line Hence the
control system has to be a versatile tool easily scalable by various users (scientists operators IT
technicians)
In the following a lsquoSub-systemrsquo is a physically connected set of hardware items sharing a global logical
function The ThomX sub-systems are as follows the linac (LINAC) the transfer line (TL) the ring
cell 1 (RI-C1) the ring celle 2 (RI-C2) the optical cavity (OC) the extraction line (EL) the section
of the X-ray line located in the Igloo (XLI) and the section of the X-ray line located in the X-ray hall
(XLH) A lsquoDomainrsquo is a set of hardware items related by a common purpose The ThomX domains are
as follows alignmentmetrology (AM) control-command (CC) diagnostics (DG) infrastruture (IF)
magnetic elements (ME)n optics (OP) pulsed elements (PE) power supplies (PS) radio-frequency
(RF) safety (SA) synchronization (SY) and vacuum (VA)
81 General principles
Some general principles drive the design of such a control system In broad terms the following criteria
will serve as guidelines
- Since monitoring is a major role of a control system comprehensive monitoring interfaces must be
provided to all users Moreover the monitoring system must take into account sub-systems specificities
dedicated GUIs must be designed for every need for instance in synoptic form to give a global system
status overview or as charts to show the time evolution of some specific parameters of a particular
device (eg pressure readings on TPG300)
- Operating processes management a control system also aims at the prioritization and sequencing
of processes It has to provide high-level functions in order to remotely launch complex operations (eg
machine startstop interlockshellip) Computing and data analysis tools which could launch such particular
procedures will be integrated or developed to maximize the automation of the machine
- Default management the facility control system must be able to manage operation defaults It will
be necessary to design graphical tools to quickly identify defaults when they occur Color display of
warnings and errors according to a criticality-based convention will be implemented in addition to the
recording of running process logs
- Safety for obvious reasons the ThomX radioprotection and safety system must standalone It should
not use any data generated by the control system Nevertheless each end-user must be able to make
sure that the ThomX facility does not present any danger whenever the accelerator is running This
information must be available in every control room through a continuous display of all the safety
equipment status (radioprotection sensors door openingshellip) A full historical record of safety-related
lsquo
141
events will be made available for analysis and a procedure of detailed acknowledgement (operatorrsquos
name action to close an incident) will be defined in order to guarantee safe operations
- Ease of use The ThomX control system is intended for various usages therefore it will concern
various users including operators who are not familiar with software development Hence it is crucial
to make it modular through relevant to sub-systems GUIs with various visualization interfaces control
buttons graphs sub-system overviews in synopsis or detailed display of subset status The definition of
new interfaces will be possible through visual development frameworks to fit new needs
82 Technical considerations
In order to guarantee a proper accelerator operation to make its use as easy as possible and to limit
costs some technical points have to be taken into account
- Distributed architecture an accelerator is a complex structure which results from various designs in
different fields The hardware part of the control system relies on dedicated IT devices (industrial PCs
programmable logic controllers) whose role is to drive sub-systems Each of these nodes will be fully
and remotely controlled by the system software from the control rooms (or from any authorized user
terminal in the ThomX network) independently of possible other issues All of these components must
communicate with each other when required by the operation needs Moreover the supervision part of
the control system must be available in different places (main control room X-ray line control room
perhaps others) The chosen system must meet these requirements and must run on several terminal
types
- Standardization of the hardware items and of the software codes is necessary in order to simplify
their integration in the control system (eg same brandmodel for all the stepper motors) Then it will be
possible to mutualize high-level controlling tools whether pre-installed or homemade Additional
benefits of this approach concern the overall maintenance the deployment of new releases and the
replacement of hardware parts
- Reliability The service continuity of the control system must be guaranteed A spare strategy must be
set up to make sure that the replacement of defective parts will be done on short notice In particular
storage memory redundancy will guarantee that none of the machine parameters or of the collected data
will be lost Periodic backup of main software systems (device databaseshellip) will be performed right
from the start Uninterruptible power supplies are foreseen to protect major elements of the control
system from electrical failures
- Upgradability Special attention will be paid to communication features when choosing hardware
devices Ethernet is now one of the major protocols and must be preferred when possible For the PLC
driven devices the fieldbus Profibus [81] is recommended since one single master interface can
manage more than 100 slave devices without data losses This product is also valuable as far as cabling
cost is concerned
- Safety Since several users may wish to perform various machine-related tasks (integration
maintenance measurementshellip) a policy must be established to control the access to the ThomX
network and its control system The management of these access permissions will be provided It will
take into account the role and the field of expertise of each project member
lsquo
142
83 The TANGO control system software
Tango [82] is the software chosen to be the backbone of the ThomX control system It is an open-
source software initially developed at the ESRF It is now used in several Institutes to control huge
accelerator facilities like the SOLEIL synchrotron In fact the SOLEIL staff is playing a major role in
maintaining and developing new Tango features
831 Software bus
An accelerator equipment is usually highly heterogeneous Thus its control system must enable various
devices to communicate with each other even if they use different protocols or communication
standards like Ethernet RS232 USB PCI DAQ Cardshellip
The first asset of Tango is the communication standardization that it provides between any type of
devices Tango implements a CORBA-based communication protocol so that a common network is
virtually created for the whole hardware If needed the parameters of a given device are thus available
to every other element of the control system This is achieved by adding an extra Tango-compliant
software layer above the hardware low-level libraries This layer is specific to a device brand and model
and is a simple executable file which can be instantiated as many times as required by the number of
devices This key element of the Tango system is called a lsquoDevice serverrsquo
832 A control system for accelerators
For basic reasons the Tango core and many of the tools developed around this software bus are open
source tools It is therefore an accessible system fully customizable for specific ThomX needs and a
system which can be maintained Since Tango is Linux and Windows compliant issues originating in
driver availability for the Linux OS can be avoided while Tango enables the use of scientific
applications which are often developed for Linux
Furthermore Tango is developed and maintained by a community which involves several accelerator
Institutes Even if Tango is not an accelerator-dedicated tool a number of developments have been
carried out or are being carried out that deal with devices currently used at such facilities Thus a large
Tango library of device servers is presently available to any new user
Finally more than a simple tool which provides a network access to a set of devices Tango is also a
framework that provides high level operational services such as configuration tool logging archiving
services and alarms permission management For developers specific tools are provided for the
generation of codes that fit in the common structure required by all device servers Other tools allow
easy GUI development As a result the integration of new devices or the upgrade of control and
supervision windows do not need in-depth coding As far as the compatibility with commercial software
is concerned dedicated bindings can be freely used eg applications developped for Matlab or
Labview can be used to perform a variety of tasks involving the ThomX devices
lsquo
143
84 Control system architecture
841 Equipment to control
On a complex machine like ThomX a wide range of devices and control signals must be taken into
account motors cameras scopes diodes or valves will deal with analog and digital inputsoutputs
complex signals transmitted through GPIB USB Ethernet or profibus interfaces
The inventory of devices to be controlled is presented in the following table Safety equipment is not
mentioned as it is a fully standalone sub-system
LINAC TL Ring OC XL
Power Supplies
Coils 2
Steerers 2 4 24 2
Dipoles 5 4 8 3
Dipole chicane 1
Quadrupoles 7 24 3
Sextupoles 12
Pulsed elements
Septum 1
Kickers 2
Synchronization
3GHz synthesizer 1
500 MHz synthesizer 1
Phase shifter 5
Programmable delays 2
Counter 1
Vacuum
Ion pump 8 4 17 4
Gauge 8 8 22 8 1
Valve 4 4 4 2
Primary pump 1
Diagnostics
BPM 1 4 12
Low frequency Signal
generator 2
ICT 1 2
Stepper motors 1 3
Camera CCD 2 3 1
RF
Klystron coil 1
Modulator 1
Amplifier 1
Canon 1
Feedback 2
Source laser
Photodiode 1
Camera CCD 1
lsquo
144
Motor 1
Mirror 2
Laser ndash FP cavity
Camera 4
CCTV Camera 10
Motor 16
Thermal control unit 2
Laser equipment 3
Scopes 3
FPGA 1
X‐ray Line characterisation
Xpad camera 1
Si PIN diode 3
Ge diode 1
Motor 13
Stepper motor 25
Others
Translation table 1 1
Slot 1
Table 81 Devices to control (TL=Transfer line OC = Optical cavity and XL = X-ray line)
842 Inputsoutputs
All devices listed above communicate with each other by digital inputsoutputs (DI DO) ie one bit
signals analog inputsoutputs (AI AO) or more complex signals as n-bit digital signals (n = 8 12 or
16) Several communication protocols mentioned in sect832 above will be used
They are detailed in Table 82
PLC InputsOutputs Bus Ethernet
connections
DAQ (ADCampDAC)
Vacuum 26 DI 13 DO 26 profibus
Diagnostics 2 gpib 30 3 x ADC 12 bits 100 kHz
Linac 2 DI 2 DO 5 AI 3 ADC 8 bits 1 GHz
Magnets 35 profibus
Feedbacks RF (x2) 10 DI 10 DO 10 AI
10 AO
USB
Ampli RF 64 DI 8 DO 32 AI 16
AO
RS232 2 4 channels ADC 12 bits
1MHz
Modulator RF 23AI 4 AO 69 DI 5 DI 5 DO
8 channels ADC 8 bits
Pulsed Elements (x3) 12 DI 3DO profibus ADC 12 bits
Laser ndash FP cavity RS232 22
Timing 2 gpib 14 15 channels DAC 16 bits
X Line 26 DI 26 DO 7
Table 82 Equipment IO to control (not complete at the time of writing)
lsquo
145
843 Front-end machine architecture PLCs and industrial PCs
The front-end architecture will comprise equipments individually dedicated to sub-systems These
machines are controlling nodes which will be located in sub-system server racks as close as possible to
the equipment to be controlled (when required)
Two kinds of front-end machines will be used
- PLCs programmable logic controllers are automation OS-based machines able to manage digital and
analog IO and to control processes carried out by devices connected to a field bus (eg Profibus) or
specialized devices They are reliable devices designed to operate in an industrial environment A
proprietary operating system runs on these machines which are powerful tools (eg automats) to
implement logic appropriate to experimentation Interconnection with the other Tango network
equipment is made through an Ethernet adapter interface
- Compact PCIs (industrial PCs) these crates can embed complex operating systems like a Linux
distribution or some Microsoft Windows Operating Systems hard drives Ethernet connections and
USB adapters Such crates are based on a PCI bus backplane to interconnect specific cards (DAC
ADC counters optoelectronics convertershellip) cPCI is a standard widely used in an industrial
environment it is compatible with numerous card suppliers These front-end machines will also allow
the running of Tango processes or specific computing tasks locally installed in a sub-system
The ThomX distributed control system will be build on these controlling nodes This structure will
allow the bandwidth optimization of the network main links and will provide cabling simplifications
The control equipment may be placed in the accelerator hall as well as in the technical rooms where
most of the electronics that must be controlled will be clustered in 19rsquorsquo standard server racks
‐ Some devices need not be controlled through these front-end machines They will be directly
connected to the control network (eg ethernet cameras or electronics for motor control) Three
ways to interconnect such devices are shown in Fig 81
‐
Figure 81 Control architecture for one sub-system
lsquo
146
These acquisition and controlling units can be Profibus PLC or a cPCI crate Each driven element will
be controlled through one of these acquisition and controlling unit
The overall control system will be composed of more than 25 front-end machines as described in Table
83
PLC cPCI Crate
Linac (wout vacuum diags modulator
magnets) 2 1
Vacuum 2
Diagnostics 2
Feedbacks 2 2
RF amplifiercavity 1 1
RF modulator 1 1
Pulsed elements 3 3
Fabry-Peacuterot cavity 1 1
X-ray line 1
Synchronization 1
Magnets 2
Total 13 14
Table 83 List of front-end machines (not complete at the time of writing)
85 IT infrastructure
- Network The ThomX local network will be organized around an IP backbone It will be based on the
Ethernet TCPIP standard in order to link user terminals front-end machines specialized servers etc
Because of the large number of devices and the requirements bearing on the bandwidth (eg for image
storage) 1 Gbitss compliant equipment will be chosen (LAN adapters switcheshellip) Depending on the
actual number of cameras an image-dedicated network could be added On the cabling side shielded
copper cables will be used as interconnection wires to limit electromagnetic interferences as much as
possible
- Data storage During the ThomX commissioning or for the operation monitoring or for the X-ray
experiments it will be crucial to store a large amount of data It is foreseen to implement the three
storage processes provided by Tango historical (long term storage at a slow rate) temporary (short
term storage at a fast rate) and snapshot (storage of parameters on operator demand or event triggered)
It will also be possible to meet the X-ray line needs for the storage of large size images during short
periods (at most for several days) This specific storage will not be necessarily on local servers it could
be convenient to move these data and store them on servers close to the final users possibly outside the
ThomX network
Two storage-dedicated servers will be installed for analyses replay of accelerator operation scenarios
and for some specific needs of the X-ray line These storage units will be specific machines which
prevent data outages or losses by redundant power supplies stored data redundancy (RAID) hard disk
hot swap etc The first of these two storage units will be a 2 To server equipped with a disk array
dedicated to the accelerator data management The second one will have a 10 To capacity it will be
available for the large size images of the X-ray line
lsquo
147
- Equipment of the control rooms Users will work in control rooms on system administration
accelerator control development and integration (device servers GUIs high level computing software
safety analysis tasks) Administration and end-user terminals should be provided in sufficient number
(see below)
The main control room will be equipped with
- 5 PCs 1 for the Tango core system 2 for operations 1 for development 1 for safety (dedicated to
visualization storage and analysis of safety related events)
- 1 server to run high-level applications and device servers
- 2 screens one displaying the ThomX status with synopses and the other one for the safety
supervision
- camera monitors for some direct controls
- 1 UPS (uninterruptible power supply)
The X-ray line control room equipment will consist of
- 2 PCs for the operation equipment and for macro development
- 1 server to run high-level applications and device servers
- 2 screens dedicated to the monitoring of the machine status to safety synopses and to the display of
safety related events
- Software needs
- Both the Linux and Windows OS must be installed in the control room PCs in order to run Tango and
many other operating processes (see the Introduction above) for development purposes and to use
configuration software Together with the Tango core and specific tools high-level software will be
installed to meet usersrsquo needs Labview and Matlab for science applications SPEC to control the
motors and for the analysis of the X-ray line data
- Data bases will run on the MySQL open-source database managing system
- An open-source http server Apache will allow the implementation of WEB interfaces for the control
system authorized users this way these users will be able to remotely visualize experiment data or
equipment status
- Finally all end-user stations will provide standard office tools
The following Figure gives an overview of the control room equipment
lsquo
148
Figure 82 Schematics of the equipment foreseen for the main control room and the X-ray line control
room
Overall the ThomX control system ndash designed to ensure harmonization and standardization and based
on open source tools ndash should be cost effective The investment cost is estimated around 200 keuro
including IT small equipment not described in this document cables a gateway to provide internet
access extra screens to display the ThomX status in the laboratory etc
lsquo
149
Chapter 9 SAFETY AND RADIOPROTECTION
91 Introduction
The ThomX facility will be sited in an existing building called lsquoIgloorsquo where a source of hard X-rays was
operated until 2003 by the Laboratoire pour lrsquoUtilisation du Rayonnement Synchrotron The ThomX linear
accelerator and storage ring will be built inside this building while the facility control room and the one of the
X-ray line will be located respectively in the D1 and D2 experimental halls The D4 hall will be devoted to
experimental tests
The ThomX operation will produce high-energy electrons and other particles such as X-rays and neutrons
which generate ionizing radiations through their interactions with matter An adequate protection against these
radiations must be ensured for people working at the facility The safety logic adopted is the so-called
lsquopositive safetyrsquo which means that any system dysfunction will immediately cause a complete stop of the
accelerator Furthermore redundant safety devices will be implemented allowing various actions that will
guarantee the staff safety such as
- Shining the beam into an area shall be submitted to prior people evacuation and to the interlocking of this
arearsquos doorways
- Access to a sensitive area shall be submitted to the prior stop of the upstream beam by using at least two
different means independent of each other
Complementary to these two procedures safety measures will be applied involving hard-wired logic safety
PLCs shielding beam dumps shutters personnel exclusion interlocks radiation monitoring of the
experimentation areas staff dosimeters and radiation hazard training of the personnel In case of dysfunction
radiological monitoring hard-wired logic or safety PLCs will act through interlocks that will break
connections to the mains voltage (Pre-Amplifier modulator or electron gun depending on the situation) in
order to stop the accelerator
92 PLC safety
The safety hard-wired logic works in association with an automaton PLC system via two completely separate
circuits A Safety Programmable Logic Controller (PLC) is a special purpose machine which is used to
provide a critical control and safety applications to automaton users This controller is an integral part of
lsquosafety instrumented systemsrsquo (SIS) which are used to detect situations involving potentially dangerous
processes When such a situation occurs the SIS is programmed to bring the process to a safe state
automatically
A safety PLC is specifically designed to accomplish two important tasks (ensured by a diversitary redundant
architrecture)
- lsquodo not fail but if this cannot be avoidedrsquo
- lsquofail only in a predictable safe wayrsquo
Hard-wired logic is a safety circuit fully independent of automatons It takes in charge in particular the
access door interlocking feedback contacts which provide a large number of checks bearing on the magnetic
field the high voltage the pre-amplifier status etc and the running control panel
The PLC safety level must be as high as the one of a positive safety hard-wired logic It manages access doors
emergency stop buttons safety rounds and reports issue authorizations to operate from a command-control
system
lsquo
150
93 Radiation monitoring
Concerning the radiation protection the ThomX facility design satisfies the following principles
- No one will receive a dose of radiation in excess of the regulatory dose limits as a result of normal
operation
- Professionnal occupation exposures will be kept As Low As Reasonably Achievable (ALARA)
- All practical steps will be taken to prevent accidents with radiological consequences and to minimize the
radiological consequences of any accident
Arrangements will be made for the monitoring of working areas such as the Igloo the bunker and the Optical
hutch For this purpose several monitoring modes will be available such as thermo-luminescent dosimeters
measurements made by the qualified person in charge of radiation protection (Competent Radiological
Protection Person) and a modular system for continuous monitoring A continuous monitoring of gamma rays
and neutrons will provide radiation dose rates and will manage alarms its measured values will be transmitted
to a central data acquisition unit Placed in the Compton room a special treatment unit will also be
implemented that will provide a local display of the dose rate signage and will manage alarms
94 Radiation protection zoning
The D1 D2 and D4 halls are not classified lsquosupervised arearsquo therefore their access is free to people who
satisfy access requirements In contrast the Igloo hall is classified lsquosupervised arearsquo
The D1 hall contains the X-ray line and technical equipment
- The X-ray line is not a classified area with the exception of the Optical hutch
- The Optical hutch is alternately classified lsquoprohibited arearsquo and lsquonot classifiedrsquo area depending on the
permission conditions issued by functional elements important to safety (shutter position permanent
radiological level)
The D2 hall contains the ThomX control room and a room which contains the laser equipment
The D4 hall is devoted to tests and mechanical assembly
The Igloo hall contains the electron accelerator located inside a bunker The Igloo is classified lsquosupervised
arearsquo therefore only workers subject to individual dosimetry are admitted However in exceptional
circumstances public access might be authorized as long as regulations and procedures are respected
The Igloo bunker on the one hand and on the other hand the basement area devoted to the facility are
alternately lsquoprohibited arearsquo during the accelerator operation and respectively lsquosupervised arearsquo and lsquonot
classified arearsquo when the facility operation has stopped
The accelerator energy is high enough to expect nuclear reactions of photons and neutrons with matter
leading to the activation of structural materials This is why it is particularly important to use appropriate low
activation structural materials or structural materials that are activated for short periods of time only
Furthermore one must ensure a nuclear waste management which will require a specific room to store and
monitor activated materials This room should be a lsquosupervised arearsquo located in the Igloo
95 Access control
951 General statement
It is a matter of absolute necessity to have appropriate procedures and devices in place to identify and screen
all persons entering the ThomX facility Consequently the LAL access control should be extended to the D1
D2 and D4 halls and to the Igloo
lsquo
151
952 Particular requirements concerning the lsquorestricted areasrsquo
The validation of door contacts as well as the validation of the position of protective shields and real-time
radiation measurement are prerequisites to access the restricted areas (ie Optical hutch and the accelerator
area) As it applies to all functional elements important to safety signage and door statements (openedclosed
lockedunlocked) from classified areas must be transmitted to the control room Local indications are
displayed alternately with the status messages The measured values of the dose rate and alarm symbols are
shown permanently
953 Access to the accelerator area and to the Igloo basement
The accelerator and the storage ring will be located in a concrete enclosure that will provide an efficient
shielding to protect people against ionizing radiations inside the igloo in line with regulations Prior to the
validation the floor load-bearing capacity must be checked by experts
The concrete containment structure will have a removable roof made of staggered concrete slabs Civil
engineering will be needed for the anchorage and supports to be used to adapt the bunker concrete blockds to
the igloo circular wall which will be part of the enclosure and to make sure that the roof of the whole
lsquoprohibited arearsquo provides a complete shielding Since doors will be made of standard steel the bunker access
will be indirect ie through (short) mazes to ensure protection against ionizing radiations
Access to the bunker and to the basement area which is part of the ThomX facility will be determined by the
equipment that guarantees peoplersquos safety in accordance with the facility operation As mentioned above
these devices ensure redundancy by using wired logic safety PLCs and also special safety door locks which
are simple but effective devices A mechanical system called lsquotrapped key lockrsquo will be implemented the key
is trapped in the lock when the door is open and it can be recovered only once the door is properly locked
Turning on the beam in lsquoprohibited areasrsquo is allowed only if the safety systems have been unlocked which
implies that the collection of access keys is complete as will be checked by the control board To meet the
safety requirements new access doors must be installed and some existing doors must be modified
954 Access to the X-ray line and to the Compton room
The D1 hall will not be a lsquoclassified arearsquo and will be open to authorized people The X-ray line sections will
be in open access during normal operation with the exception of the Optical hutch To meet the radiation
protection requirements the Optical hutch partition and its roof will be made of thick metal sheets A thick
lead shield will be installed at the end of the X-ray beam axis
The Optical hutch will be alternately classified lsquoprohibited arearsquo and lsquono classifiedrsquo area (under the worst-case
conditions it might become a lsquoregulated arearsquo) depending on permission conditions issued by safety functional
items External and internal light-signaling warning will indicate the zoning status Thus access to the Optical
hutch is firstly subordinated to the removable shutter position and secondly to the local permanent
radiological measurement values The shutter is dimensioned to protect people from high radiation rates
effectively A fail-safe closer device triggers the shutter closing automatically in case of electrical power
failure too (compressed air power supply) When a door is open the shutter command is prohibited
A local radiation detector located in the Compton room will check he radiation exposure and the dose levels
down to as low as reasonably achievable values Measured high dose rates will automatically shut down the
accelerator and activate the evacuation alarm The validation of these safety devices is mandatory to grant
access permission
lsquo
152
96 Beam dumps
The ThomX facility will comprise tow beam dumps made of concrete that will contain high energy particles
They are very important to ensure an adequate protection of people from the ionizing radiations that would be
produced if they were not used One will be aligned with the linac axis and the other one will equip the
extraction area These shields will be installed as close as possible to the radiation source to absorb ndash or at
least to moderate ndash as much as possible of the produced radiations Their size and thickness will be
sufficiently large to reduce efficiently all secondary radiation (X-rays and neutrons) produced by the stopping
of the e- beam They will be made of a lined lead shield designed to moderate back-scattered radiation placed
in a cavity made of concrete blocks or high-density polyethylene Ne peut-on pas ecirctre plus preacutecis donner des
nombres sur les dimensions et preacuteciser les mateacuteriaux utiliseacutes avec une figure
97 Interlocks
Depending on the dysfunction origin the safety system will operate and break command circuits such as the
pre-amplifier trigger the modulator high voltage the electron gun trigger or its high voltage Feedback
information from the control systems may be gradually requested according to the anomaly
98 Accelerator emergency shutdown
Accelerator Emergency Shutdown (AES) are signal and alarm devices located in specific areas A sufficient
number of AES should be installed in prohibited zones namely the accelerator area the bunker basement and
the Optical hutch For the users to spot them easily AES are devices equipped with flashing lights which are
turned on whenever the local zone is closed
If people are still present in a dangerous zone when a machine run is about to start (ie when the access is
going to be forbidden after a short while) they should signal their presence by pushing an AES This action
prevents the accelerator operation The system reactivation must follow a specific procedure
99 Control console
Placed in the control room the control console collects all access keys The accelerator may be turned on if
and only if all keys are inserted in that console
The safety circuit validation (PLC safety hard wired logic) allows one to launch the accelerator running
procedure and triggers audible warning devices for 30 seconds before operation becomes possible
910 Evacuation of dangerous areas
A specific procedure must be followed for the evacuation of the bunker and its basement Operators should be
trained in methodical evacuations with the validation of all checkpoints (roundsman function) Non
compliance with the procedure will lead to the non validation of the facility operation
A sufficient number of roundsman functions should be located in the accelerator area Also an exit path
should be marked leading to those areas that provide a possibility to regress
911 Audio system and horns
Loud-speakers and horns will be installed in the Igloo basement (in the so-called lsquorotundarsquo) and pits and
inside the bunker The sound system must be of good quality speeches and broadcast messages must be
intelligible Horns must be loud enough 80 dBA at least
lsquo
153
912 Safety notices
Local notices Posters should be put up in lsquosupervisedrsquo and lsquocontrolled areasrsquo to remind people of the
regulation requirements Such posts maintain the workerrsquos awareness whenever they are in a radiological
environment Access doors to zones with multiple classifications should be equipped with highly visible
luminous warning panels Also measured radiation levels and messages could be flashed regularly Regarding
the radiation measured level in the Optical hutch an alarm symbol must be permanently displayed that
informs of this level
Signal boards at zone entrances and within the zones will specify the real-time safety conditions display
warnings (such as presence of live voltage ongoing operation mode etc) and the access permissions (such as
open access prohibited zone etc)
913 Radiological monitoring
The control of the radiological environment will comply with the ALARA principle It will be performed by
- using bimonthly exposure dosimeters based on radio-photo-luminescence (RPL) and neutron sensitive
dosimeters
- PCR controls with live measurements
- a centralized modular system which will continuously monitor exposures feeding a data acquisition system
that can handle up to 10 sensors
This centralized system will perform dose rate and integrated dose measurements manage alarms test
measurement channels check settings and calibrations It will allow a special device placed in the Optical
hutch to display in real time local safety information
The ALARA principle is a commitment to keep radiation exposures and doses at a level as low as reasonably
achievable (social and economic factors being taken into account)
lsquo
154
Chapter 10 MANAGEMENT OF THE THOMX PROJECT
Introduction
The management of the ThomX project will have a simple structure that takes into account the three
project phases 1) The construction 2) The exploitation 3) The technology transfer Three boards will
govern the ThomX activity covering all project aspects the machine design the purchasing and the
assembly the commissioning the usersrsquo access the communication and the relationships with the
industries This management board will comprise
1) A steering committee that will supervise the project activity and define the collaboration scientific
policy
2) A scientific committee composed of the scientific coordinators of the collaboration partners It will
assess the usersrsquo projects and prioritize the beam-time allocation This committee must also define the
scientific policy of the machine exploitation and RampD of the ThomX project
3) A partnership and valorisation office This body will evaluate the cultural and the economic
potential of the ThomX innovations It will stimulate transfer initiatives and check their outcome It will
work in close contact with the steering and the scientific committees
The composition of these three bodies is given in Fig 101
Figure 101 The ThomX project committees
In parallel the communication group (see Table 101) will be in charge of all activity concerning
communication and reach-out in the ThomX project It will organise events realise the publications
relating to the project status and progressrsquo and coordinate the official announcements of the
collaboration
LAL Orsay
Achille Stocchi
CELIA Bordeaux
Philippe Balcou
UMPC
Philippe Walter
Institut Neel
Alain Schuhl
INSERM Grenoble
Franccedilois Esteve
Synchrotron SOLEIL
Amor Nadji
ESRF
Jean Susini
THALES
Yves Trinquet
Alessandro Variola
LAL Orsay
Alberto Bravin
ESRF
Geraldine Le Duc
ESRF
Jean-Louis Hazemann
Institut Neel
Philippe Walter
UMPC
Alexandre Loulergue
Synchrotron SOLEIL
Francois Esteve
INSERM Grenoble
Eric Cormier
CELIA Bordeaux
Alessandro Variola
LAL Orsay
Pierre Bordet
Institut Neel
Philippe Deblay
Synchrotron SOLEIL
Yves Trinquet
THALES
Annabelle Alves
CNRS DR4
Edward Mitchell
ESRF
Fabian Zomer
LAL Orsay
lsquo
155
The project management is organized according to a matrix structure The ThomX facility has been
divided in five sub-systems Linac Ring and transfer lines X-ray line Laser and Fabry-Peacuterot cavity
There is a coordinator responsible for the design installation and operation of each sub-system All
these sub-systems involve various technologies which have been grouped in expertise domains
Mechanics and integration RF systems Magnets Diagnostics Power supplies and Pulsed elements
Vacuum Synchronisation Controls Beam Physics Alignment Cabling Nomenclature and
Infrastructures There is a coordinator for each one of these domains These coordinators will be in
charge of the activity harmonization within each technical field and of the maintenance during the
facility exploitation (or ldquoscientific phaserdquo)
The project leader his deputy and a project engineer are responsible for the project overall
coordination Fig 102 describes the project structure
Figure 102 The project structure
Project and Quality Engineer Technical
coordinator
AVermegraves Francois Wicek
LAL Orsay
Deputy Project leader
A Loulergue
Synchrotron SOLEIL
Project Leader
A Variola
LAL Orsay
Systems
LINAC RRouxLAL Orsay
Ring and Transfer Lines A Loulergue Synchrotron SOLEIL
X line M Jacquet LAL Orsay
Laser E Cormier CELIA Bordeaux
Fabry-Perot Cavity F Zomer LAL Orsay
Groups
Mechanics and Integration MLacroix LAL Orsay
Beam Physics ALoulergue Synchrotron SOLEIL
Alignment ALestrade Synchrotron SOLEIL
RF systems P Marchand Synchrotron SOLEIL
Magnets ME Couprie Synchrotron SOLEIL
Diagnostics L Cassinari Synchrotron SOLEIL
Infrastructures BPieyre MLanglet LAL Orsay
Control PGauron LAL Orsay
Vacuum C Preacutevost LAL Orsay
Power Supplies - Pulsed Magnets P Lebasque Synchrotron SOLEIL
Synchronisation N Delerue LAL Orsay
Cabling JN Cayla LAL Orsay
Nomenclature PCornebise LAL Orsay
lsquo
156
In the following Table the support structures are presented
Support Structures
Administration Communication Quality
Administration Brigitte Renard LAL
Orsay
Procurements Anngaeumllle Mardam
Beck LAL Orsay
EDMS Support Agnes Vermes LAL
Orsay
Secretary Chloe SevestreLAL Orsay
DR04 Support Edouard Dorion
Dominique Bony LAL Orsay
Marie-Pauline Gacoin
Synchrotron SOLEIL
Claus Habfast ESRF
Sandrine Agnes Vermes
LAL Orsay
Table 101
The ThomX collaboration needs an important support from all administrative services For this an
administrative cell has been set up which is headed by the person responsible for the administration the
purchasing office the project database management the secretariat and the contact with the regional
CNRS delegation support Last but not least a quality engineer plays a major role in the project
101 Partnersrsquo complementarity
The ThomX project requires a multidisciplinary environment given the complexity of the facility and of
the set-up operation Each partner contributes its competences to reach the final goal The ThomX
collaboration was organized so as to meet the requirements of a multicultural scientific environment
where different communities are called upon to share their know-how and their knowledge to develop a
common scientific program This was already the case ndash and still is the case ndash for the MightyLaser
project In this project LAL and CELIA teams have developed a fully integrated set-up composed of a
fibre laser and a high gain optical resonator which is presently installed at ATF (KEK Japan) There the
complementarity between the two teams was crucial LAL has a solid experience in developing optical
resonators in an accelerator environment while CELIA is among the leading laboratories in fibre
amplification technology This program aims at a progressive increase of the light beam power stored in
an optical cavity in order to maximize the Compton back-scattering rate The final goal of this RampD is
to demonstrate the feasibility of a γ factory based on Compton backscattering It started as a
demonstrator for Compton-based polarized positron sources needed for the next generation linear
colliders The interest shown by other scientific communities in this technology made it clear that a low
energy Compton source would be very useful in various research fields This conclusion led to the
launch of the ThomX collaboration
The ThomX CDR has already been published and the present TDR is about to be widely circulated For
the TDR writing it was mandatory to develop close connections between the various communities
lsquo
157
participating in the ThomX project in order to merge technological developments and usersrsquo
requirements in a single document All partners have contributed to the scientific case elaboration the
study of the facility feasibility and the definition of its main components
- LAL and SOLEIL contributed the description of the Compton backscattering characteristics and
carried out the study of the accelerators and the Fabry-Peacuterot resonator
- CELIA Bordeaux Lyon provided their expertise for the laser dimensioning
- THALES provided its experience in industrial components and sub-systems
- UMPC and ESRF enriched the scientific case
This task sharing results from the multidisciplinary approach to the ThomX project where half a dozen
institutes play an important role
The writing of this TDR provided a unique opportunity of collaborating for laboratories that belong to
different fields LAL specialized in fundamental high energy physics SOLEIL and CELIA in advanced
technology in laser and accelerators LC2RMF in social sciences and compact detectors ESRF in
medical sciences In the future the diversity of this set of teams will still increase with the participation
of the Neel Institute and the GIN-INSERM in Grenoble
After a prototyping phase ThomX will operate as a mini-synchrotron whose beam line can be accessed
not only by the facility partners but also by other communities Moreover the aim of the
industrialization phase is to provide the high technology scientific and industrial with compact and
reliable sources that can be operated with little or no accelerator and laser expertise This should open
the way to a large community of CBS source users that would benefit from the complementarity
between techniques developed at synchrotron sources and Compton back-scattering facilities Imaging
techniques biological tests on cells and animals metallurgy fast chemistry lithography are examples
of future possible applications of these unique instruments
102 ThomX larger framework
ThomX is one of the EQUIPEX projects submitted for approval to the French government by the P2IO
Consortium (Physique des deux Infinis et des Origines) of the Paris-Saclay Campus This Consortium is
composed of eight laboratories or institutes (CSNSM IAS IMNC IPNO IRFU LAL LLR LPT)
some of which are among the largest French laboratories All together their staffs comprise 2000
scientists engineers and technicians working in particle physics nuclear physics accelerator science
astroparticle physics astrophysics and their interfaces P2IO represents between 20 and 90 of the
national potential in the various sub-domains Although obviously not all of the eight laboratories
participate in each EQUIPEX project each project ndash including ThomX ndash is fully supported by P2IO and
will contribute to the very ambitious common technological platform that P2IO wants to create in the
coming years in the framework of the Paris-Saclay Campus initiative Once equipped with these
EQUIPEX facilities this campus will represent a unique opportunity for French researchers and
students
lsquo
158
References
11 Geoffrey A Krafft Gerd Priebe Compton Sources of Electromagnetic RadiationlowastReviews of Accelerator
Science and Technology Vol 3 (2010) 147ndash163
12 VBesson et al NIMA 608 (2009) S75-S77
13 T Akagi et al 2012 JINST 7 P01021
14 CDR ThomX LAL RT 0928 SOLEILSOU-RA-2678 editors AVariola FZomer ALoulergue
15 PRussbueldt et al Optics Express Vol 17 Issue 15 pp12230-12245 (2009)
16 IPupeza et al OPTICS LETTERS Vol 35 No 12 June 15 2010
17 M Y Shverdin et al 2009 OSAIPNRANLOSL 2009
18 CVaccarezza et al IPAC Conference proceedings New Orleans USA2012 TUOBB01
19 AVariola IPAC Conference proceedings San Sebastian Spain 2011 WEOAA01 1903-1905
21 S Doumlbert ldquoIntegration of the PHIN RF Gun into the CLIC Test Facilityrdquo Proceedings of EPAC 2006
Edinburgh Scotland 2006 MOPLS129
22 R Roux S Cavalier M Bernard G Bienvenu MJoreacuteBLeblondBMercier BMoutonCPrevost A
Variola ldquoPHIL a Test Beamline at LALrdquo Proceedings of EPAC08 Genoa Italy 2008 WEPP078
31 J Payet Beta Code CEA SACLAY
32 JB Murphy et al Longitudinal Wakefield for an Electron Moving on a Circular Orbit Vol 57
Part Acc 1997
33 K Bane M Sand The Short-Range Resistive Wall Wakefields SLAC-PUB 95-7074
34 E Forest Phys Tech Part Photon Beam
35 J Zang et al lsquoDipole fringe field effects in the Thom-X ringrsquo Proc IPAC13
36 R Nagaoka et al Recent Studies of Geometric impedance At SOLEIL PAC2006
37 Z Huang RD Ruth Phys Rev Lett 80976 (1998)
38 S K Mtingwa African Physical Review 2 (2008) 001
39 P Chen G Horton-Smith T Ohgaki AW Weidemann K Yokoya
NuclInstrumMethA355107-1101995
310 K Ohmi Phys Rev E 55 7550 (1997)
311 S Haifets D Teytelman Phys Rev Spec Top Acc and beams 8 064402 (2005)
312 E Weihreter Compact synchrotron light sources Series On Synchrotron Radiation Techniques
And Applications - Vol 3 World Scientific
313 P Lebasque et al Synchrotron SOLEIL lsquoEddy current septum magnets for booster injection and
extraction and storage ring injection at Synchrotron SOLEILrsquo EPAC 2006 Edinburgh Scotland
314 P Lebasque et al Synchrotron SOLEIL lsquoOptimisation of the coating thickness on the ceramic vacuum
chambers of SOLEIL storage ringrsquo EPAC 2006 Edinburgh Scotland
315 P Lebasque et al Synchrotron SOLEIL lsquoFour matched kicker systems for the SOLEIL storage ring
injection a full solid-state solution of pulsed power supplies working at high currentrsquo EPAC 2006
Edinburgh Scotland
316 F Marhauser E Weihreter (BESSY) ldquoImpedance Measurements of the HOM-Damped 500 MHz
Metrology Light Source Cavityrdquo BESSY Gmbh Germany 08 January 2007
317 M Langlois P Sanchez M Cornelis F Perez (ALBA) ldquoMeasurements on the RF cavity for the ALBA
storage ringrdquo Proceedings of EPAC08 Genoa Italy
318 R A Rimmer (Lawrence Bekeley Lab) et al ldquoDevelopment of a high-power RF cavity for the PEP-II B
factoryrdquo DE-AC03-76SF00098 (LBL)
lsquo
159
319 P Fernandes R Massarino R Parodi ATarditi (INFN Italy) A Massarotti (Sincrotrone Trieste Italy)
rdquoThe Design of the RF Cavities for ELETTRArdquo IEEE 1989
320 M Svandrlik A Fabris C Pasotti (Sincrotrone Trieste) rdquoImprovements in curing coupled bunch
instabilities at ELETTRA by mode shifting after the installation of the adjustable higher order mode frequency
shifter (HOMFS)rdquo IEEE 1998
321 W Bruns web site httpwwwgdfidlde
322 P Marchand et al Development of high RF power solid state amplifiers at SOLEIL Proceedings of the
IPAC2011 San Sebastian Spain p 376
323 C Pardine et al Status of the 476 MHz 50 kW solid state amplifier Proceedings of the IPAC2010
Kyoto Japan p 3972
41 YAG(CE) data sheet Saint Gobain Crystals httpwwwdetectorssaint-gobaincomYAG28Ce29aspx
42 SPI Suppliesreg YAGYAP Comparison Table
httpwww2spicomcatalogscintillsintillator_properties_comparison_tablehtml
43 K Nakamura et al (Particle Data Group) J Phys G 37 075021 (2010)
44 Wartski et al Detection of optical transition radiation and its application to beam diagnostics IEEE
TransNuclSci20544-5481973
45]Saturation of YAG Beam Profile Monitors Measured at ATF A Murokh J Rosenzweig V Yakimenko X
J Wang
46 Radiation Detection by Cerenkov Emission in Optical Fibers at TTF E Janata M Koumlrfer Tesla-Report
2000-27
47 CERN-2009-005 p273
48 CERN-2009-005 p272
51 J Bonis et al 2012 JINST 7 P01017 doi1010881748-0221701P01017
61 J Bonis et al Non-planar four-mirror optical cavity for high intensity g ray flux production by pulsed laser
beam Compton scattering off GeV-electrons JINST SISSA Vol 7 January 2012
62 R V Pound lsquolsquoElectronic Frequency Stabilization of Microwave Oscillatorsrsquorsquo Rev Sci Instrum 17 490ndash
505 (1946)
71 httpwwwjjxraydkhome
72 RW Alkire et al Nucl Inst and Meth A 350 (1994) 13-16
73 B Lengeler et al J Synchrotron Rad 6 (1999) 1153-1167
74 RL Owen et al J Synchrotron Rad 16 (2009) 143-151
75 MS Nogueira et al Radiation Protection Dosimetry 111 (2004) 105-110
76 M Krumrey et al AIP Conf Proc 705 (2004) 861-865
M Gerlach et al Metrologia 45 (2008) 577-585
77 httpwwwcanberracompdfProductsUltra-LEGe-SS-M3843pdf
78 K Sakaue et al Rev Sci Instrum 80 (2009) 123304
79 httpsaleshamamatsucomassetspdfparts_FF9892-31_-32_TMCP1039E03pdf
710 O Chubar et al Proc DIPAC (2001) CT09
711 D Paterson et al Opt Com 195 (2001) 79-84
712 M Mahesh RadioGraphics 21 (2001) 1033-1045
713 K Medjoubi et al JINST 6 C01080 (2011)
httpwwwvariancomusxrayproductsdigital_radiographypaxscan_productshtml
714 H Elleaume et al Physics in Medicine and Biology 47 (2002) 3369-3385
lsquo
160
715 Y Prezado et al Medical Physics 38 (2011) 1709
716 I Martiacutenez-Rovira and Y Prezadoa Medical Physics 38 (2011) 4430
717 L Samain Thesis Liege university (ULG) 2012
718 P Martinetto Thesis Joseph Fourier university (UJF) 2000
719 P Bleuet et al Analytical Chemistry 29 (2010) 518-527
720 P Bleuet et al Nature Materials 7 (2008) 468-472
81 httpwwwprofibuscomncdownloadspecifications-standardsdownloadsprofibus-standard-dp-
specificationdisplay
82 httpwwwtango-controlsorgDocumentstango-kernel
lsquo
161
Table of content
INTRODUCTION 2
1 The ThomX project 2
2 Scheme and options 3
3 General layout of the machine 4
4 Machine parameters 6
5 Summary of the TDR main technical points 8
Chapter 1 THE THOMX INTERACTION REGION 9
11 Electron beam and laser pulse characteristics in the interaction region 9
12 Expected flux and its characteristics 10
13 Position stability 11
14 Timing stability 12
15 Energy stability 13
Chapter 2 INJECTOR 14
21 Injector specifications 14
22 Technical description of the linac components 16
221 Photo‐injector 16
222 Solenoids 17
223 Laser 18
23 Acceleration section 20
231 The RF source 21
232 RF network 22
24 Beam dynamics 23
241 Nominal operation 23
242 Alternative operation modes 24
Chapter 3 TRANSFER LINE and RING 28
31 Transfer line 28
32 Extraction line 31
33 Storage ring 31
331 Optics and lattice 31
332 Non‐linear dynamics 35
333 Collective effects feedbacks 37
334 CBS and IBS effects 42
335 Scattering on the residual gas 46
336 Ion effects 47
337 Electron beam injection and extraction 48
338 Orbit corrections 50
341 Dipole magnetic design 53
342 Quadrupole magnetic design 54
343 Sextupole magnetic design 56
344 Corrector magnetic design 58
345 Injection dipole 59
lsquo
162
35 Power supplies 60
351 Pulsed magnets 60
36 Storage ring RF system 65
361 HOM impedance thresholds as regard to beam instabilities 66
362 Cures to the HOM induced instabilities 66
363 RF power source 70
364 Low level RF system and longitudinal feedback 71
365 RF equipment status 73
366 RF system conclusion 73
37 Transverse feedback 74
371 Specifications 74
372 Electronics 75
373 Kicker 76
38 Vacuum 77
381 Transfer line vacuum 77
382 Ring vacuum 78
Chapter 4 INJECTOR TRANSFER LINE AND RING DIAGNOSTICS 80
41 Charge measurement 80
411 Current transformers 80
412 Faraday cups 81
42 Transverse profile measurements 81
421 Diagnostics stations 81
43 Emittance measurement 84
44 Beam position measurement 85
441 Beam Position Monitors (BPM) 86
442 Orbit feedback system 90
443 Tune measurement and feedback 90
444 Energy measurement 90
45 Bunch length measurement 90
451 Cerenkov radiation 90
452 Synchrotron radiation 92
453 Transport of the Cerenkov and synchrotron radiations to the diagnostics hutch 93
454 Streak camera for the bunch length measurement 93
46 Bunch imaging using synchrotron radiation 94
47 Beam loss measurement 94
48 Beam dumps 94
Chapter 5 SYNCHRONIZATION 95
51 Overall scheme 95
52 Specifications 96
Chapter 6 OPTICAL SYSTEM DESIGN 97
61 Feedback system and results on a Fabry‐Perot cavity locking 97
611 Electron and FPC pulse synchronization 98
612 Laser parameters to be stabilized 99
lsquo
163
613 PDH Measurement technique 101
614 Actuators 103
615 The digital feedback system 107
62 High average power laser system and its coupling to a high gain FP cavity 110
621 The MightyLaser oscillator 111
623 Stretcher and compressor 115
624 ThomX development 115
63 Mechanical design 118
631 Optical cavity vacuum 123
Chapter 7 X‐RAY BEAM LINE AND EXPERIMENTS 124
Introduction 124
71 Available extracted X‐ray beam 124
72 Global view of the X‐ray lne 124
73 Chraracterization of the X‐ray beaugrave 126
731 Optical table 1 126
732 Beam transport 128
733 Optical table 2 128
74 Demonstration experiments 130
741 Experiments using the full X‐ry beam 130
742 Experiments using the central part of the beam 134
75 Summary 139
Chapter 8 CONTROL SYSTEM 140
Introduction 140
81 General principles 140
82 Technical considerations 141
83 The TANGO control system software 142
831 Software bus 142
832 A control system for accelerators 142
84 Control system architecture 143
841 Equipment to control 143
842 Inputsoutputs 144
843 Front‐end machine architecture PLCs and industrial PCs 145
85 IT infrastructure 146
Chapter 9 SAFETY AND RADIOPROTECTION 149
91 Introduction 149
92 PLC safety 149
93 Radiation monitoring 150
94 Radiation protection zoning 150
95 Access control 150
951 General statement 150
952 Particular requirements concerning the lsquorestricted areasrsquo 151
953 Access to the accelerator area and to the Igloo basement 151
954 Access to the X‐ray line and to the Compton room 151
lsquo
164
96 Beam dumps 152
97 Interlocks 152
98 Accelerator emergency shutdown 152
99 Control console 152
910 Evacuation of dangerous areas 152
911 Audio system and horns 152
912 Safety notices 153
913 Radiological monitoring 153
Chapter 10 MANAGEMENT OF THE THOMX PROJECT 154
Introduction 154
101 Partnersrsquo complementarity 156
102 ThomX larger framework 157
lsquo
5
Figure 2 ThomX layout
The ring design ensures a four-fold symmetry based on a double bend achromat (DBA) optics It
includes 8 dipoles 24 quadrupoles and 12 sextupoles for chromatic corrections The bunch storage
time is too short (20 ms) and the beam energy is too low for synchrotron damping to play a role in
regard to the Compton back-scattering recoil effect nor in regard to collective effects such as CSR IBS
and ion instabilities For this reason an lsquoELETTRArsquo type RF cavity (12) will be used not to restore
energy but to bunch the electrons and to perform a longitudinal feedback Transverse position
diagnostics and feedback will be provided by a strip-line and by BPMs A synchrotron radiation
chamber will be used for transverse and longitudinal optical diagnostics The required ultra-vacuum
will be maintained by ion pumps distributed along the ring and by two dedicated pumps in the Fabry-
Peacuterot resonator
In the ring lattice two of the achromatic lines have been shortened by removing all the quadrupoles (see
Fig 3) so that the Fabry-Peacuterot cavity (13) can be easily integrated between two dipoles By minimizing
the beta functions it was possible to locate the collision region (14) in such a short section
Furthermore this design provides the possibility of implementing a second interaction region in the
future
lsquo
6
Fig 3 Layout of the interaction region and of the optical cavity integration
4 Machine parameters
The ThomX machine is composed of four main systems the injector the storage ring the laser and the
Fabry-Peacuterot resonator As mentioned previously we foresee some flexibility in the machine operation
so that the X-ray energy can be varied For this all four systems are dimensioned to withstand an
operating energy up to 70 MeV Nevertheless to characterize the machine a baseline configuration has
been chosen at 50 MeV In Tables 1 2 3 and 4 the parameter values which are given below correspond
to this baseline configuration
Table 1
Injector
Charge 1 nC
Laser wavelength and pulse energy 266 nm 100 microJ
Gun Q and Rs 14400 49 MWm
Gun accelerating gradient 80 MVm 5 MW
Normalized rms emittance 44 π mm mrad
Energy spread 04
Bunch length 43
lsquo
7
Table 2
Table 3
Laser and FP cavity
Laser wavelength 1030 nm
Laser and FP cavity Frep 3568 MHz
Laser power 50 ndash 100 W
Laser pulse energy 14 ndash 28 microJ
Fabry-Perot pulse energy 28 mJ
Fabry-Perot pulse length (rms) 5 ps
FP cavity finesseGain 3000-30000 1000- 10000
FP waist 70 microm
Power circulating in the FP cavity ~ 007 ndash 07 MW
Ring
Energy 50 MeV (70 MeV possible)
Circumference 168 m
Crossing-Angle (full) 2 degrees
βxy IP 01 m
εxy just after injection 5 10-8
m
Bunch length just after injection (rms) 4 ps
Bunch length at the end of a 20 ms storage
cycle
50 ps (rms)
Beam current 1784 mA
RF frequency 500 MHz
Transverselongitudinal damping time 1 s 05 s
RF Voltage 300 kV
Revolution frequency 1784 MHz
σx IP (just after injection) 70 microm
Tune xy 317 174
Momentum compaction factor αc 00136
InitialFinal relative energy spread (with IBS
and Compton back-scattering)
0406
lsquo
8
Table 4
Source
Photon energy cut-off 46 keV (50 MeV) 90 keV ( 70 MeV)
Total Flux 1011
-1013
photons
Bandwidth (with diaphragm) 1 - 10
Divergence 10 mrad (1γ) without diaphragm 50 MeV
5 Summary of the TDR main technical points
After the publication of the CDR much work has been put in simulations design and technical choices
The goal of this TDR is to provide a reference document in view of the machine construction as well as
a more precise planning and a more precise costing
The first chapter deals with the machine performances and the tolerances to be met so that the collisions
between the electron bunch and the laser pulse do take place in the conditions specified in the CDR
Then the main systems are described and our technical choices are presented This last section is
subdivided as follows
1 Linac
2 Beam transfer and injection section ring
3 Laser and Fabry-Peacuterot cavity
4 X-ray user line
A special attention is given to the integration of each system and to the global integration of the ThomX
machine The lsquoOptical systemrsquo chapter includes a short report on recent results obtained in the laser and
Fabry-Peacuterot cavity RampD program both in the laboratory and on a beam line at KEK-ATF (Japan)
Special chapters are dedicated to diagnostics the control of the main systems and their synchronization
safety and radioprotection The last chapter presents the project management
lsquo
9
Chapter 1 THE THOMX INTERACTION REGION
11 Electron beam and laser pulse characteristics in the interaction region
The ThomX injection frequency has been fixed at 50 Hz (20 ms period) by taking into account the
Compton scattering recoil effect which continuously deteriorates the beam longitudinal emittance This
effect is all the more important as synchrotron damping is strongly suppressed since the beam energy is
low Because of this lack of damping the Compton scattering recoil effect and eventual beam
instabilities must be controlled by longitudinal and transverse feedback systems
We stress the fact that in ThomX the beam does not reach a permanent state during a storage cycle its
properties are continuously varying To describe the Compton interaction dynamics we define a set of
nominal beam characteristics which are those prevailing just after the injection of a fresh e- bunch
Simulations have been carried out to characterize the X-rays emitted during a single crossing of the
electron bunch and the photon pulse (stored in the optical cavity) as a function of the collision
geometric parameters
These nominal collision parameters are as follows
Parameter value unit
Crossing angle 2 degree
Electrons energy 50 MeV
Electrons energy spread 03
Number of electrons 625 109
Fabry Perot pulse energy 28 mJ
Repetition rate 1784 MHz
Laser pulse transverse dimension (rms) 35 microm rms
Electron bunch transverse dimension (rms) 70 microm rms
Laser pulse duration (rms) 5 ps rms
Electron bunch length at injection 4 ps rms
Table 11 Nominal collision parameters
To extend these results to other geometric configurations one has to take into account the parametric
dependence of the Compton collision rate The X-ray flux is the product of the Compton cross-section
and the luminosity It is proportional to the electron bunch intensity and to the laser pulse power The
geometry dependence is given by the luminosity formula which involves the geometric 6D size and the
crossing angle Assuming Gaussian bunches and neglecting the hourglass effect the ring luminosity is
given by the following expression
Eq 11
L = NeNγ f cosφ
2π
1
σ ye
2 +σ yγ2 σ xγ
2 +σ xe
2( ) cos2φ + σ ze
2 +σ zγ2( ) sin2φ
lsquo
10
where σeγxyz represent respectively the two transverse dimensions and the longitudinal one of the
electron bunch and of the laser pulse f is the collision frequency Ne and Nγ are the number of
electrons per bunch and photons per pulse Φ is half the crossing angle
The X-ray energy cut-off is higher for head-on collisions and for photons which are exactly back-
scattered In this case the photon energy gain is 4 γ2
This calculation provides a useful reference to assess the efficiency of other configurations but it
remains an approximation since exact results must take into account the beam evolution during a
storage cycle and should be obtained from a full revolution by revolution simulation
12 Expected flux and its characteristics
In nominal conditions the expected flux is 78 1012
X-rayss whether one uses Eq 11 or the monte-
carlo simulation code CAIN Figure 11a shows the energy spectrum of the scattered photons and its
cut-off at 45 keV As shown in Fig 12b most photons are emitted very close to the interaction region
axis within a small cone with a 2 mrad opening angle They may be selected by using a small
diaphragm The energy spectrum of the X-rays emitted in a cone of 225 mrad opening angle is shown
in Fig 12a with a FWHM energy spread of 25 keV
Figure 11 Results of simulations performed with CAIN using the parameters listed above a)
Energy spectrum b) Transverse size of the source c) Angular emission distribution d) Horizontal (x-
xrsquo) phase space of the scattered X-rays
lsquo
11
The spot size at the source is dominated by the electron bunch of 7 10-5
m as illustrated in Fig 11b
The dimansions are assumed to be the same in the horizontal and vertical plane The total divergence
of the source is around 15 mrad given by the electron beam energy (ie the 1γ parameter) with an rms
size of 4 mrad (Fig 11c) The emittance of the photon source shown in Fig 11d is 03 mm mrad The
energy-angle correlation and the collimation effect are illustrated in Fig 12 As expected the high-
energy X-ray flux is peaked with a 1γ opening angle The diaphragm which is assumed for this
simulation selects emission angles and thus photon energies resulting in a ~ 10 monochromatic
photon beam
Figure 12 Results of simulations performed with CAIN using the parameters listed above
Left panel Energy spectrum of photons emitted in a cone with an opening angle of 225 mrad
Right panel Number of photonsmradsup2s versus energy and angle (the color bar indicates the photon
number)
13 Position stability
To optimize the X-ray flux a precise transverse alignment must be acieved in the interaction region It
can be performed either by steering the electron beam or by adjusting the optical table position These
procedures will be monitored by means of a double BPM which is integrated in the Fabry-Peacuterot cavity
vacuum chamber exactly at the interaction point The resolution of this technique is a few microns A
simulation has been performed to calculate the flux reduction resulting from a possible mismatch
between the electron bunch orbit and the laser pulse trajectory Figure 13 shows the number of X-rays
produced per bunch crossing and per keV as a function of a transverse displacement of the optical
cavity axis with respect to the electron orbit from zero to 140 microns
lsquo
12
Fig13 Number of back-scattered photons and their energy as a function of a transverse alignment
error This figure clearly shows that the losses entailed by a displacement comparable to the BPM
sensitivity are negligible The color bar indicates the number of X-rays produced at each crossing
14 Timing stability
A precise synchronization between the electron bunch and the laser pulse at the interaction point is
another important requirement A timing mismatch introduces a longitudinal shift between the two
bunches at the IP This can be due to an error in the location of the Fabry-Perot cavity with respect to
the e- storage ring or to a jitter in the synchronization system Fig 14 shows the variation of the X-ray
flux as a function of a longitudinal displacement
The flux is not so much reduced as long as the jitter is less than 10 ps This number fixes the
synchronization tolerance and thus the performance of the longitudinal feedback
Fig 14 Flux and energy spectrum of back-scattered photons as a function of the relative
synchronization of the beams (longitudinal displacement)
lsquo
13
15 Energy stability
The nominal ThomX injection energy is 50 MeV Phase rotation in the RF source or phase mismatch
will generate a jitter in the injection energy Such a jitter entails synchrotron oscillations which will be
damped by the longitudinal feedback However before this feedback has stabilized the longitudinal
oscillations the collision conditions will not be nominal This situation has been simulated the results
are illustrated in Fig15 One can see that it is in the higher part of the X-ray energy spectrum that the
loss is more important
Fig 15 Flux and spectrum of the back-scattered photons as a function of the e- energy
lsquo
14
Chapter 2 INJECTOR
The most important components of the linac injector are the electron gun and the acceleration section
which brings the beam to the desired energy Other components are the laser used for the e- gun the
magnetic elements used to transport the beam the diagnostics the vacuum system the waveguides and
the radio-frequency (RF) source Each component is detailed in the following First of all the injector
specifications are given since they lead to the choice of some of the linac components
21 Injector specifications
The project goal namely to produce a high flux of X-rays of energy 50 keV leads to the following
specifications for the linac
Energy 50 MeV
Charge 1 nC
Number of bunches per RF pulse 1
Emittance (rms normalised) lt5 mm mrad
Energy spread rms lt1
Bunch length rms lt5 ps
Average current 50 nA
Repetition frequency 50 Hz
Table 21 Nominal linac parameters
First of all one notices that the low average current 50 nA does not justify the use of cold technology
(ie of a superconductor) for the acceleration structure But there is still a large choice of possibilities
for an RF structure operating at room temperature For economic reasons and because of the long
experience accumulated in the field we have chosen an RF structure operating at 3 GHz This
frequency is a European standard
Furthermore the above specifications led us to choose a photo-injector for the electron source since a
low emittance beam is required Again to minimize risks we chose to build the same gun as the one we
constructed for the CLIC lsquoTest Facility 3rsquo at CERN [21] This gun has been running successfully for 4
years To increase the current per bunch we will use a metallic magnesium photocathode which can
deliver more than 1 nC with a laser pulse energy of a few tens of a microJ at a wavelength of 260 nm The
gun is made of 2 12 copper cells magnetically coupled to the waveguide To get 1 nC with an
emittance lower than 5 mmmrad an accelerating field of 80 MVm is required which means an RF
power of 5 MW in a 3 micros pulse The electron beam energy output by the gun is around 5 MeV
We will use an industrial laser to extract the electrons from the cathode by photoelectric effect The
main specifications of this laser are the following ones Wavelength 260 nm energy per pulse 100 microJ
pulse duration around 5 ps It must be synchronized with the RF frequency and triggered at 50 Hz This
laser will be installed in an optical hutch located outside the concrete shielding which will enclose the
accelerator Therefore its beam must be transported with dielectric mirrors and lenses over a distance of
about 20 meters
Concerning the acceleration section SOLEIL a partner of the project will lend us a lsquoLILrsquo section This
section was made by LAL for the pre-injector of LEP at CERN It has been kept as a spare for the
SOLEIL linac The ThomX project will provide an other section in case SOLEIL needs one The
section length is about 46 m To reach a final energy of 50 MeV the energy gain in the section must be
lsquo
15
45 MeV The latter value implies an electrical field of 125 MVm and an RF power of about 10 MW
However the accelerator should reach higher energies in order to produce X-rays beyond 50 keV Since
the maximum targeted X-ray energy is 90 keV the linac design should allow a beam energy of 70
MeV corresponding to an RF power of 20 MW Thus taking into account the RF system efficiency the
specification for the total RF power is set at 35 MW
This specification lead us to choose a klystron producing 35 MW in a 45 micros pulse To feed the
klystron a modulator is needed that provides high voltage pulses of typically 240 kV through a high
voltage transformer soaking in an oil tank The two technologies which can be used for this component
are discussed in detail below
For the electron beam steering two coils and two magnetic correctors are required The coils are placed
around the gun Opposite sign currents circulate in these coils (-310 A 336 A) so that the magnetic field
vanishes at the cathode location Its amplitude is 023 T roughly 7 cm downstream from the cathode
Concerning the magnetic horizontalvertical correctors once a good mechanical alignment is achieved
experience has shown that beam trajectory errors are not larger than a few mm between the gun and the
acceleration section Hence the correctors must produce a beam deflection of up to 10 mrad For the
model operating at LAL [22] two bipolar power supplies of 10 A per corrector are adequate
For the diagnostics three different instruments will be installed between the RF gun and the
acceleration section An Integated Current Transformer (ICT) will measure the charge per bunch
Beyond this device a beam position monitor (BPM) will measure the transverse positions (x y) of the
electron beam We will use a BPM based on striplines which was designed for SOLEIL The third item
comprises a vacuum chamber equipped with an arm which can be translated by compressed air in order
to introduce a YAG screen to intercept the beam The YAG crystal fluoresces when ionized by the e-
beam The emitted light shows the beam transverse shape A short optical transport line and a CCD
camera allow the measurement of the transverse dimensions LAL designed and constructed the
diagnostics chamber It has already been successfully tested in the PHIL facility A drawing showing
the linac layout is shown in Fig 21
lsquo
16
Figure 21 Linac layout In the upper figure the different components of the LINAC are illustrated
starting form the gun and subsequently passing trough the stripline the ICT for current measurements
the laser introduction chamber the diagnostics station the steerers and the accelerating section
22 Technical description of the linac components
221 Photo-injector
It is a copy of a gun built by LAL for the CLIC Test Facility 3 of CERN the so-called lsquoprobe beam
gunrsquo[22] Simulations have shown that the performances of this gun fulfill the ThomX specifications
We are thus benefiting from all the technical drawings which already exist and all the fabrication
processes are mastered by LAL from the machining to the brazing This RF gun is comprised of 2 12
cells The output cell is magnetically coupled and the other end is short-circuited The main parameters
are given in Table 22
Frequency 299855 MHz
Rs 50 MΩm
Q 14500
β 1
E 80 MVm
PRF 5 MW
Table 22 RF characteristics of the photo-injector at 30degC in vacuum
The photo-cathode will be made of magnesium It has the big advantage of producing the specified
charge without requiring an in-situ photo-cathode preparation chamber which is a heavy and expensive
lsquo
17
installation Preliminary tests performed at LAL have demonstrated the possibility to obtain good
quantum efficiency with such cathodes
The main difficulty expected in operating this device is the laser cleaning of the photocathode surface
when polluted by an exposure to air To perform this cleaning a laser intensity of 160 microJmm2 is
required together with an RF electric field of at least 50 MVm The magnesium disk holder is not
designed yet it could be press fitted screwed or brazed
222 Solenoids
The gun is equipped with two solenoids one is the focusing coil while the other is the bucking coil
which cancels the magnetic field at the photo-cathode location They have been modeled by usning
POISSON A schematic is shown in Fig 22
Figure 22 Schematic of the gun with its coils
Dimensions
For both solenoids the inner diameter is 160 mm and the outer one is 500 mm The copper part is 30
mm long for the bucking coil and 56 mm for the focusing coil
The conductor used for these magnets is a standard one with a 6x6 mm2 square shape It is cooled by
water circulating in a 4 mm diameter channel One can easily calculate the current the resistance the
dissipated power and the temperature rise assuming that the cooling water is supplied at 20degC and 8
bars Values are provided for each lsquopancakersquo since the conductors are wound by columns of two The
bucking coil Bbc has two pancakes and the focusing coil Bfoc four
Bbc Bfoc units
Number of coils 84 168 MHz
Number of pancakes 2 4
Current -274 326 A
P 23 33 kW
R 306 31 mΩ
Flow 164 164 lmn
ΔT +20 +28 degC
Table 23 Solenoid electric and cooling parameters lsquoNumber of coilsrsquo refers to one pancake
Two power supplies are required one for Bbc (400A 30 V) and one for Bfoc (400A 60 V)
lsquo
18
223 Laser
The laser extracts a bunch of electrons from the photo-cathode surface by photo-electric effect Table
24 specifies its characteristics
Parameter value unit
l 260‐270 nm
Emax 100 plusmn 05 microJ
Beam transverse quality M2 M2 lt 11
Gaussian Pulse width 5 ps
Transverse profile square
Pointing stability 10 of beam diameter
Oscillator frequency 74963750 externally triggered with jitter lt 1 ps MHz
Amplifier repletion rate 50 externally triggered Hz
Table 24 Laser specifications for the RF gun photocathode
The laser architecture is shown in Figure 23
Figure 23 Architecture of the laser proposed by industry
An oscillator provides a signal at the specified frequency It can be either a fiber or a TiSaphire laser
Then the pulse is stretched amplified (with one or two amplifiers) recompressed and converted in the
UV range
The laser will be installed on an optical table in a hutch whose temperature and hygrometry will be
regulated
224 Laser injection chamber
The laser injection chamber is located at a distance of 8011 mm downstream of the photo-cathode It
contains a metallic aluminum mirror placed 30 mm away from the accelerator axis It provides an 80
reflectivity The angle of incidence of the laser beam on the photocathode is 214deg This mirror is fixed
To adjust the laser beam position on the photo-cathode and its incident angle we will use a motorized
mirror with a remote control placed on an optical table just beside the laser injection chamber
The size of this optical table will be 1x1 m2 On it there will be
Oscillator Stretcher Preamplifier Amplifier Compressor
nJ microJ
mJ
Frequency
shifter
Clock
Syncro
Line syncro
Shaper
lsquo
19
- 2 dielectric mirrors
- 2 beam splitters
- 1 UV photo-diode to monitor the laser intensity
- 1 CCD camera to observe the laser spot on a screen (in paper or lanex) facing the photo-cathode
- An optical beam expander to adjust the laser spot size on the photo-cathode
This is the minimum number of optical items that will be put on the table some changes may be made
depending on the very final geometry of the accelerator and laser transport lines The set-up design can
be finalized during the last stage of the accelerator construction since it involves standard components
only
225 Beam steering coils
The e- beam trajectory will be adjusted by sets of two steering coils which produce dipolar magnetic
fields over short distances along the X and the Y axes One such device will be located at the gun exit
and the other about 800 mm downstream Only very small position and angle errors must be corrected
Figure 24 shows the LAL design for such a coil Experience with the PHIL photo-injector at LAL [22]
showed that an integral of Bmiddotdl of 52 G m is sufficient The latter figure has been validated by
calculations made by a private company With a current of 5 A a Bmiddotdl of 58 G m was obtained that
results in a 100 mrad deviation of a 5 MeV beam Thus a 100 mm lateral displacement of the beam can
be produced at the entrance of the acceleration section
Figure 24 Schematic of the beam steering coils Dimensions are in mm
All together the steering coils need 4 bipolar DC power supplies Their maximum current is 10 A
lsquo
20
23 Acceleration section
The acceleration section is shown in Fig 25 Its characteristics are given in Table 25
Figure 25 Picture of the SOLEIL LIL acceleration section
Length 45 m
Number of cells mode 135 2π3
Frequency 299855 at 30degC MHz
Rs 63divide74 MΩ
Q 14800
Attenuation 74 dB
Energy gain 15 MW 60 MeV
τ 135 micros
Ez max 182 MVm
Table 25 Characteristics of the acceleration section
In a second stage of the ThomX project a call of tender will be issued for a new acceleration section
that should be shorter in order to make the machine more compact
lsquo
21
231 The RF source
Figure 26 shows the RF source schematic
Figure 26 Schematic of the RF source
The RF timing and its synchronization are described in chapter 5 A clock signal is sent to the linac
which uses it to trigger a 3 GHz synthesizer Its output is fed to a preamplifier which brings its power to
350 W The preamplifier is triggered at 50 Hz Then the 3 GHz signal is amplified by a klystron whose
gain can reach 53 dB to produce a 35 MW RF wave as required by ThomX This power is distributed
via a waveguide network which is described below
To drive the klystron one needs a modulator delivering a pulsed high voltage (HV) whose
specifications are given in Table 26 The klystron cathode must be heated by a power supply that
provides a current rising slowly from 0 to 25 A and a voltage rising from 0 to 30 A in 20 minutes A
precision of 1 on the current value is required The klystron is generally equipped with focusing coils
and their power supply whose standard parameters are 40 A 120 V
Repetition rate 50 Hz
Plateau length 99 gt 3 micros
Plateau ripples lt plusmn 05
Pulse duration at 50 6 micros
Rise time 10 divide90 lt 05 micros
Fall time 90 divide10 lt 1 micros
Pulse to pulse stability lt 05
Voltage range 0divide300 kV
Peak current 285 kV 300 Hz
Table 26 Modulator specifications
The classical and well-proven modulator technology relies on the high voltage (40 kV) loading of a
delay line made of inductances and capacitances The delay line is switched by a thyratron A new
technology is presently emerging based on solid-state amplifiers Each amplifier produces a 100 V
lsquo
22
pulse Thus to produce a 40 kV pulse roughly 400 amplifiers are put in a rack and their output pulses
are added The performances of both technologies are similar
232 RF network
The RF power is transported and distributed to the electron gun and the acceleration section by a
waveguide network shown in Fig 27
Figure 27 Schematic of the RF waveguide network
The WR 284 waveguide network is pressurized at two bars with SF6 between ceramic windows at the
klystron output and at the acceleration structure entrance A 5 dB bidirectional coupler splits the power
in two channels In the branch which runs toward the electron gun there is a 10 dB attenuator and a
circulator to protect the klystron from the RF power reflected by the electron gun In the branch running
toward the acceleration structure there is a 10 dB attenuator and a phase shifter to adjust the relative
phase between the gun and the acceleration section Various RF loads (five in total) ensure the system
safety by absorbing of RF power of the reflected waves
lsquo
23
The network also includes three couplers to measure the incident and the reflected RF power One is
placed at the klystron power exit one is placed at the gun entrance and the last one is placed at the
acceleration section entrance The RF gun is equipped with a loop that measures the stored RF energy
All of these loops used for measurements are weakly coupled at -70 dB The signals are measured with
crystals or C boxes
24 Beam dynamics
241 Nominal operation
The beam dynamics was simulated with the PAMELA tracking code for the ThomX nominal
parameters ie a bunch charge of 1 nC at 50 MeV The 50 MeV can be obtained with an electric field
of 80 MVm in the RF gun and 125 MVm in the acceleration section It is clear that a different set of
field values can lead to the total of 50 MeV In particular one may wish to operate the gun at a higher
field in order to improve the beam quality We will operate the linac with this possible improvement in
view
The beam will be accelerated on crest Thus the coil magnetic fields are the only parameters savailable
to minimize the transveres and the longitudinal emittances Simulations have shown that for precise
magnetic field values (see Table 27) the e- beam emittances are optimized as illustrated in Fig 28
20
15
10
05
00
σxy
(m
m)
6005004003002001000
z (cm)
20
15
10
5
0
em
it (
mm
mra
d)
6005004003002001000
z (cm)
20
15
10
05
00
ΔE
E (
)
6005004003002001000
z (cm)
46
44
42
40
38
36
34
32
30
σz (p
s)
6005004003002001000z (cm)
lsquo
24
Figure 28 Beam characteristics (rms values) as a function of the distance along the accelerator (z
coordinate) Upper left panel Transverse size upper right panel normalized emittance lower left
panel Energy spread lower right panel Bunch length
The beam parameter values at the linac end are given in Table 27
Beam parameters value unit
Energy 50 MeV
Beam size 17 mm
Normalised Emittance 44 π mm mrad
Energy spread 04
Bunch length 43 ps
Table 27 Beam parameter rms values at the linac end
The accelerator parameters which should ensure the above beam characteristics are given in Table 28
RF structure parameters value unit
Ez in the gun 80 MVm
PRF in the gun 5 MW
Ez in the section 142 MVm
PRF in the section 14 MW
Solenoid parameters value unit
Bz max 2500 G
Ibc -274 A
Ifoc 326 A
Laser parameters value unit
Energy impinging on a Mg photocathode 10 microJ
Square transverse profile diameter 2 mm
Variation of the intensity from axis to border 14
Table 28 Summary of the linac parameters
242 Alternative operation modes
Concerning the injector other operation modes can be envisaged One at least will occur namely an
operation at 70 MeV in order to extend the X-ray energy range up to 90 keV Such an option has
attractive applications In this mode the electron beam performances should remain exactly the same as
the ones prevailing at 50 MeV (cf Table 27) For the accelerator parameters the only change is the RF
power required for the acceleration section it must reach 195 MW instead of 14 MW at 50 MeV
The operator can also be tempted to change the RF phase The one used in the simulations gives the
minimum emittance A phase shift of -10deg of the RF gun with respect to the laser could lead to an
energy spread reduction by a factor 2 at the expense of an emittance degradation Whatever the goals
the linac operator must be aware of the high sensitivity of the beam parameters to the RF phase This is
illustrated in Fig 29 An error of 5deg in the acceleration section phase results in almost a doubling of the
energy spread
lsquo
25
Figure 29 Energy spread at the linac end as a function of the RF phase in the acceleration section all
other parameters keeping their nominal values
The RF electrical field in the gun also plays an important role in the beam performances The results of
simulations of the emittance variation as a function of the gun electric field are presented in Fig 210 If
this field reaches 120 MVm the emittance is reduced by 20 with respect to the 80 MVm case
However the possibility to operate the gun at such a high gradient can only be assessed experimentally
Figure 210 Injector beam emittance as a function of the electric field in the electron gun (Gaussian
laser beam 1 nC e- bunches)
The injector performances as a function of the bunch charge are summarized in Table 29 Three modes
will be available in order to reduce the effect of collective instabilities at injection
Parameters Mode 1 Mode 2 Mode 3 Unit
Q 1 05 01 nC
σxy 165 15 08 mm
εxy 47 33 11 π mmmrad
σz 44 4 33 ps
σEE 04 03 02
Table 29 Linac beam performances (rms values) as a function of the bunch electric charge
As one can see in the above Table reducing the bunch charge by a factor 10 leads to an emittance
reduction by a factor 4 Although the X-ray flux is reduced the source brightness ie the ratio of the
flux to the divergence is improved by a factor 16 As a consequence a higher number of X-rays can be
20
18
16
14
12
10
08
06
04
02
δΕ
Ε (
)
151050-5-10
φ (deg)
10
9
8
7
6
5
ε (πmmmrad)
1401201008060
E0 (MVm)
lsquo
26
transported to a sample (number per unit area) Whatever the eventual benefits to the X-ray production
the advantage for the accelerator to operate at a lower charge per bunch is clear the reliability of the
accelerator is much enhanced the laser energy requirement is reduced and it becomes also possible to
use a copper photo-cathode instead of a magnesium one easier to handle But the main advantage lies
in the ring operation Indeed a 1 nC bunch whose length is only 4 ps induce a strong wakefield
stemming from the interaction between the beam and the vacuum pipe Such a wakefield perturbs the
beam and may cause oscillations of the bunch centroid possibly instabilities and thus a reduction of the
X-ray production rate
25 Photo-injector and acceleration section vacuum
Simulations of the photo-injector vacuum have been performed assuming a pressure of 3 10-8
Pa at the
cathode location and requiring a vacuum time constant (VS) that ensures an easy gun conditioning This
constraint must be respected to maintain a 10-7
Pa pressure at the acceleration section entrance where the
system will be baked to obtain an outgassing rate τ = 5x10-10 Pa m s-1
(N2)
Pressure in the line of injection
The use of a semi-conductor cathode (Cs2Te) puts severe constraints on the vacuum and requires a preparation
and a transfer chamber To avoid these complications some tests have been made on a metallic cathode (Mg)
The first attempt ran into difficulties (it took 12 h to reach a gradient of ~ 60 MV m-1
) we observed pressure
rises and sparks In presence of only the RF power we observed hydrogen CO and CH4 outgassing (as the
main components) A slight reduction in the quantity of water and oxygen was noticed due to chemical re-
composition When the laser beam was introduced to extract electrons a pressure increase by a factor of ten
was observed The 24th
25th
and the 26th
peaks also significantly increased for the HxCy compounds but no
Mg was measured
lsquo
27
Gas analyzes during the conditioning of cathode Mg
On the basis of simulations and experimental results we identified the need for the following items
‐ 2 ion pumps 40 l s-1
(upstream of the cathode amp injector) with their baking systems and power
supplies
‐ 2 ion pumps 55 l s-1
(for the RF network) with their baking systems and power supplies
‐ 1 ion pump 150 l s-1
(for the laser introduction chamber) with its baking system and power supply
‐ 1 ion pump 300 l s-1
(for the acceleration section) with its baking systems and power supply
‐ 1 gate valve manually actuated DN 100 (injector- upstream) and a baking valve
‐ 1 gate valve electro-pneumatically actuated DN 63 (injector - downstream) and a baking valve
‐ 1 gate valve electro-pneumatically actuated DN 40 (injector- downstream) and a baking valve
‐ 3 Pirani gauges (before the injector injector laser introduction acceleration section)
‐ 3 cold cathode gauges (after the injector injector laser introduction acceleration section)
‐ 2 power supply gauges
‐ 1 residual gas analyzer (RGA)
‐ 1 turbo pump (DN 63 ~ 80 l s-1
)
‐ 1 rotary vane pump (~ 12 m3 h
-1)
‐ 4 angle valves manually actuated for pre-pumping DN 63
‐ 2 T with RF grids
lsquo
28
Chapter 3 TRANSFER LINE and RING
31 Transfer line
The transfer line (TL) has to transport the beam from the linac to the ring and to match the optics of
these two devices In addition a set of beam diagnostics needed for an efficient injection in the ring are
localized along the line as sketched in Fig 31 The transfer line main characteristics are summarized in
Table 31
Figure 31 Layout of the injection and extraction transfer lines
A particular attention is paid to ensure a versatile optics that allows the matching of different linac
beams to different ring optics Simulations have been carried out using the BETA code [31] The
transfer line optics is optimized in order to accommodate some emittance degradation which may arise
from collective effects
Screen station + dump
Linac emit + pulse length diags
Screen + Scraper
Linac energy diags
Screen station
TL emit diags
Extraction line dump
lsquo
29
Figure 32 Transfer line optics including the ring The vertical line indicates the ring entrance point
The nominal transfer line optics is shown in Fig 32 from the linac to the ring together with the ring The
nominal input bunch Twiss parameters are x = z = 34 x = z = 42 for Q = 1 nC bunch length = 4
ps (rms) and an optimized normalized emittance of about 5 πmmmrad The strong bend produced by the four
dipoles of the transfer line increases the risk of emittance degradation under collective effects but simulations
show that the emittance increase is only of about 20 including coherent synchrotron radiation (CSR [2]) and
3D space charge effects (see Fig 33)
Figure 33 Emittance increase due to chromatic (cont lines) CSR (dashed) and space charge (dot dashed)
effects for 1 nC and 4 ps (rms) bunches at 50 MeV along the TL and the first revolution in the ring
The magnetic elements are identical to those of the ring
lsquo
30
Parameter value unit
Length 14 m
Nominal energy 50 MeV
Beta max x z 70 40 m
Maximum dispersion 1 m
Dipole numberfamily 4 1
Quadrupole number 7
Biplane corrector number 5
Table 31 Transfer line parameters and components
Besides ensuring the matching between the linac and the ring the transfer line must provide the measurement
of the beam emittances the beam energy and the energy spread Care has been taken to locate the diagnostic
stations in a way that ensures accurate measurements of these important parameters
The emittance measurements will be done by means of the lsquothree-gradientrsquo scheme (The pepper-pots
alternative is also foreseen) For instance with an adequate refocusing that produces a double waist (vertical
and horizontal) at the screen it is possible to scan both planes at the same time by varying only one
quadrupole gradient (see Fig 34) Other scan variations are also possible The following figures show the
optics retuning as well as the beam spot size variation at the screens of both stations the one at the linac end
and the one downstream of the U-turn
Figure 34 Optics retuning and beam spot size scan as observed on the screen (simulation)
The electron energy and its spread are measured in between the dipoles by a dedicated optics Note that it is
also possible to retrieve the bunch length by measuring the energy spread as a function of the linac phase
(three-phase scheme)
lsquo
31
32 Extraction line
To dump the residual beam at the end of the storage time (typically from 20 to 50 ms) an extraction line
composed of one small dipole and two quadrupoles is also foreseen This line will allow the measurement of
the beam properties after it has circulated in the ring The injectionextraction devices are symmetrically
implemented in the ring injection section
33 Storage ring
331 Optics and lattice
Thom-X must produce a high X-ray flux of 1011
divide1013
phs in a stable way The flux depends on the
characteristics of the electron bunch and of the laser pulse Assuming a 1 nC electron bunch charge at
50 MeV and a 20 to 30 mJ laser pulse ( 124 microm) the optics constraints at the interaction point (IP)
are as follows
- an rms size of the electron beam less than or equal to 70 microm in the transverse plane requiring
low beta functions of 01 m with an rms geometric emittance of 50 nmrad injected from the
linac (5 mmmrad normalized)
- a vanishing dispersion at the interaction point in order to avoid a beam size increase resulting
from an e- energy spread increase as well as to avoid potential synchro-betatron coupling
resonances
- a bunch length less than 20-30 ps to ensure a good overlap with the laser pulse (the crossing
angle will be 2deg) and long enough to reduce collective effects stemming from the pipe
impedance CSR bunch internal scattering etc
Besides these specific IP constraints there are two general layout constraints namely
- a ring circumference as short as possible with enough straight sections to accommodate the
interaction region the RF the injection and possible other equipment as RF and transverse
feedback systems
- the need for some flexibility as far as the optics and the working point are concerned
The ring layout and its optics are based on a two-fold symmetric lattice with four achromatic arcs and
four straight sections two short ones (02 m) and two long ones (16 m) This layout preserves the ring
compactness and provides an adequate interaction region in one of the two straight sections between
two adjacent dipoles (cf Fig 35) The main ring characteristics are summarized in Tables 32 and 33
lsquo
32
Figure 35 Ring layout
The laser pulse frequency is the harmonic 2 of the e- bunch revolution frequency in the ring ie 357
MHz This leaves open the possibility of storing two bunches in the ring in order to increase the
Compton flux but at present this operation mode is not taken into account because of injection
constraints The optical cavity mirrors are located outside the ring vacuum chamber thus enabling a
mirror substitution without affecting the ring In addition the extraction of the X-rays can be performed
closer to their source than in the case where the optical cavity is inserted in a long straight section
The two long free straight sections accommodate the injectionextraction devices for one and the RF
and feedbacks for the other
The ring is composed of 8 dipoles 24 quadrupoles and 12 sextupoles Its circumference is 168 m long
the revolution frequency is 1785 MHz The RF frequency is set at 500 MHz ie at the 28th
harmonic
of the ring revolution frequency The nominal optical functions have been optimized with the BETA
code [31] They are plotted in Fig 36 The dispersion vanishes in the long and in the short straight
sections (RF injection and interaction regions) The beta functions decrease down to 01 m at the IP to
provide a 70 microm focus assuming that the injected beam has an emittance of 50 nmrad (5 mmmrad
normalized) in both the vertical and the horizontal planes
RF and feedback section
Injection extraction section
IP 1
Interaction Point (IP)
lsquo
33
Ring parameters value unit
Circumference 1680 m
Nominal energy 50 MeV
Betatron tunes nx nz 317 174
Beta max xz 11 11 m
Dispersion max 08 m
Beta xz dispersion IP 01 01 0 m
Momentum Compaction Factor 0014
RF frequency 500 MHz
RF harmonic 28
RF voltage 300 kV
Synchrotron frequency 360 kHz
Period Revolution frequency 56 1784 nsMHz
Natural chromaticities -32 -82
Dipole nb family 8 1
Quadrupole nb families 24 6
Sextupole nb families 12 3
Corrector nb families 12 12
Table 32 Ring parameters
Max field 70 MeV Bore length Gap Diameter
Dipole 07 T 2765 mm
(R=352 mm)
40 mm
Quadrupole 5 Tm 150 mm 41 mm
Sextupole 15 Tm2 60 mm 42 mm
Correctors 510-4
Tm
Table 33 Main magnet characteristics
Taking into account the constraints imposed by the optical functions the chromaticity and the dynamic
aperture the operating point was chosen with tunes νx=317 and νz=174 There is still enough
flexibility to vary these tunes by plusmn 02 without spoiling the main ring performances The momentum
compaction factor can also be changed over a large range in order to scan the best working point
regarding the beam dynamics and the X-ray flux
Figure 36 Nominal beta functions and dispersion over one super-period
lsquo
34
The RF acceptance for the nominal momentum compaction factor (MCF) and a larger one including the
non-linear MCF and the vacuum pipe limit are plotted in Fig 37
Figure 37 RF acceptance Left panel nominal lattice Right panel for a MCF twice as large as the
nominal one VRF= 300 kV
RF acceptance nominal r56 = - 02 m r56 = - 04 m
Linear (RF bucket) 10 10
Non linear -32 +18 - 68 +4
Pipe limit 25 25
Table 34 RF acceptance for two different values of the momentum compaction factor
The two following plots (Fig 38) show (i) the vacuum chamber occupation by the beam at injection
when the emittance is 50 nmrad and the energy dispersion is 03 and (ii) at the end of the storage
cycle when the emittance reaches 300 nmrad and the energy dispersion reaches 06 (see section
438) The orbit components (misalignment errors) are not included since they are negligible once
corrected The choice of the pipe aperture (40 mm in H by 28 mm in V full aperture) takes into account
several constraints the beam pipe occupation the magnet bore radius the vacuum and the coherent
synchrotron shielding [32]
b a
lsquo
35
Figure 38 Transverse occupation (3 rms) of the vacuum chamber at injection (left) and at the end of
the storage cycle (right)
332 Non-linear dynamics
The natural horizontal and vertical chromaticities are respectively -32 and -68 Chromaticity
corrections are achieved by means of 12 sextupoles (3 families) located in the dispersive sections
Without performing a specific optimization the dynamic aperture is large enough about 50 times the
rms transverse size of the injected beam (see Fig 39) the quadrupole fringe fields being taken into
account Furthermore this dynamic aperture remains about the same for off-momentum particles (up to
plusmn3) compared to particles having the nominal energy The fringe field of the dipole magnets
introduces high order non-linear components Since the Thom-X dipoles are short with a large gap
their magnetic design (see section 34) has been carefully worked out in order to maintain the dynamic
aperture
Pole number 6 8 10 12 18 20 28 30
Inner dipoles +10 0 +3
Outer dipoles -16 -1 -6
Quadrupoles +10 +5 -5
Sextupoles +10 -10
Table 34 List of multipole tolerances for each kind of magnets expressed in 10-4
relative field variation
at 18 mm radial position
Figures 39 and 310 show the change of the dynamic aperture with and without considering the
multipole (the fringe field of the quadrupoles is also taken into account in these plots) These
Dynamical Apertures (DA) have been calculated with two different codes BETA and TRACYIII They
lsquo
36
both give very similar results In addition the TRACYIII code dedicated to long symplectic tracking
performs Frequency Map Analysis (FMA) giving more detailed information concerning the particle
diffusion process
Figure 39 On-momentum DA and FMA for the nominal ThomX lattice Beta code left and TracyIII
code (right) sextupoles only (top) and including magnet multipole tolerances (bottom)
lsquo
37
Figure 310 Off-momentum DA and FMA for the nominal ThomX parameters Beta code left and
TracyIII code (right) sextupoles only (top) and including magnet multipole tolerances (bottom)
333 Collective effects feedbacks
At low energy and thus low beam rigidity the electron bunch is sensitive to electromagnetic self-
perturbations that act revolution after revolution in the ring Several instability sources can be
identified
‐ Interaction with surrounding metallic items such as the resistive walls (RW) the RF cavity and
various discontinuities such as bellows BPMs and flanges HOMs etc
‐ Space charge forces
‐ Coherent synchrotron radiation (CSR)
These interactions affect the bunch energy and the bunch longitudinal time-profile leading to
distortions emittance growth bunch oscillations or even above some threshold instabilities and beam
losses Similar interactions may also affect the transverse beam dynamics but to a lower extent The
two relevant parameters for the instability threshold are the bunch charge and its length The strength of
the interaction is proportional to the bunch charge and varies inversely with the bunch length
The two plots in Fig 311 show the longitudinal wakefield over one revolution at 50 MeV generated by
a 1 nC Gaussian bunch with an rms length of 12 and 6 mm (4 and 20 ps) Five different contributions
are compared in this figure
‐ the shielded CSR following the Murphy and Skrinsky [32] formulation with a beam pipe full height
of 40 mm a dipole field of 05 T and a curvature of 0352 m
‐ the longitudinal space charge (LSC)
‐ the resistive wall (RW) following the K Banes and M Sands formulation [33] with a full beam
pipe height of 40 mm and an electrical conductivity of 14 106
‐ the vacuum vessel broadband component including 12 BPM 12 flanges and 12 bellows based on
the synchrotron SOLEIL type This potential has been calculated by means of the HFSS finite
elements code [34]
‐ the 500 MHz ELETTRA type cavity broadband component This potential has been calculated by
using the HFSS finite element code
They exhibit a very strong CSR wake field (up to few per mil) with the shorter bunch case
Figu
re
311
Lon
gitu
dina
l
wak
e
field
over
one
lsquo
38
turn generated by a 1 nC Gaussian bunch at 50 MeV for a bunch length of 4 ps (left panel) and 20 ps
(right panel)
The ThomX low energy operation (50 MeV) practically suppresses the natural damping effect of the
synchrotron radiation and complicates the bunch stabilization Each kind of perturbation grows
irreversibly implying that feedback systems in the three planes are essential to cope with the induced
oscillations of the bunch center-of-mass In addition to these collective effects the unavoidable
injection jitters have also to be taken into account To calculate these effects 6D tracking simulations
that include the longitudinal impedance as well as the large mismatch between the linac and the ring
optics in the longitudinal phase space have been performed (see Fig 312)
Large longitudinal oscillations
without feedback at injection
induced by collective effects
Longitudinal oscillations damped by the
RF phase feedback system operating at
F~360 kHz
Damping lt 50 micros
Figure 312 Tracking results (50 MeV
1 nC bunch) over the first 3000
revolutions just after injection without
and with the RF phase feedback system (VRF = 300 kV)
Large oscillations start very quickly after injection when the bunch is still short (4 ps) and are of the
same order of magnitude as the ones induced by the energy-phase jitter that affect the linac injection
This effect is the one to address in the design of the longitudinal feedback As far as the transverse plane
is concerned the growth times characterizing various instability sources are listed in Table 35 The
revolution-by-revolution feedback strength indicated for each instability source is the one needed to
suppress the oscillations
Source Type Growth time Revolution-by-
revolution kicker
strength
Beam pipe
geometry
TMCI
Head-Tail
-
160 micros
gt10 nrad
Resistive Wall
600 micros gt 2 nrad
Ions lt 100 micros gt20 nrad
Table 35 Transverse instability growth time and feedback kicker strength for a 1 nC bunch at 50 MeV
lsquo
39
In contrast with what prevails in the longitudinal phase space the main transverse oscillation source is
the injection position jitter Since the non-linear sextupole components are large this jitter may produce
a large emittance increase as a consequence of the phase space painting process (see Fig 313)
Figure 313 Sketch of the transverse phase space painting process
Simulations show that a very fast increase of the transverse emittance can result from steering errors at
injection in about 500 revolutions or 25 micros To overcome this effect a fast transverse feedback is
needed it has to damp the oscillation in about 100 revolutions or ~5 micros The strength of the revolution-
by-revolution feedback has to be on the order of 2 microrad This strength is far larger than the ones
estimated to cope with the transverse instabilities For this transverse feedback a dedicated strip-line
acting in both planes is foreseen in the RF section (see Fig 314)
Figure 314 Simulated transverse emittance increase induced by a 05 mm injection jitter with and
without feedback
After N
turns
lsquo
40
Phase space Specifications Device
Horizontal amp vertical Fr Frequency = 178 MHz (F0)
Strength = 2 microrad
Dedicated strip-line for
both planes
Longitudinal Frequency = 360 kHz (Fs)
Strength = 105 rads
Main RF phase loop
Table 36 Summary of the feedback specifications
Other collective effects such as the intra-beam scattering (IBS) which is important at low energy or the
Compton recoil effect play an important role during the 20 ms storage time Their dynamics differ from
an instability dynamics but they also increase the electron energy spread and the bunch emittance mdash
and thus reduce the X-ray flux
To summarize three different contributions to the e- bunch dynamics must be dealt with
‐ at injection jitters which may affect either the injected bunch energy or its trajectory or the precise
timing of its injection in the ring generate oscillations which may be large and which must be
quickly damped by feedbacks in all three dimensions (x y z)
‐ during a transition period corresponding to the first ~1000 revolutions strong collective effects
enhanced by a possible synchronization error between the linac and the ring may lead to a complex
turbulent regime in the longitudinal plane prior to ring matching (elliptical phase space density
profil)
‐ during the 20 ms storage period IBS and CBS produce a progressive increase of the bunch size in
all three directions This evolution cannot be stopped by a feedback system In contrast RF HOMs
and other perturbations may generate instabilities that should be controlled by feedbacks
To take into account collective effects a dedicated code has been developed including a collection of
wake field as well as non-linear dynamics It is based on a 6D multi-particle symplectic tracking
restricted to perfect hard edge drift quadrupoles and dipoles of finite length Other higher order
elements as sextupoles are treated in the thin length approximation The RF cavity is simply treated as
a longitudinal cosine kick The mapping through elements is given by the exact Hamiltonian solution
[34] for the drift sections and the dipoles (including their small curvature and dipole edge effects) The
quadrupole mapping is restricted to small radial angles It has been compared to various codes
(TRACYIII ELEGANT BETA and MADX) by performing simulations of the SOLEIL and the
ThomX rings with special care given to the dipole edge effects [35]
The transverse beam size at IP and the bunch length are plotted in Fig 315 over the first 10 000
revolutions or 056 ms with a 1 nC bunch charge This study shows that a small positive chromaticity in
the horizontal plane is necessary to stabilize the horizontal emittance During the very first revolutions
the tune dependent emittance behavior has not been fully investigated nor understood It appears that
this behavior follows from the bunch length mismatch between the linac and the ring The fact that the
linac produces very short bunches of 4-5 ps (rms) leads to a complex turbulent dynamics (see Fig
316) The effect of this mismatch disappears progressively over the first 1000 revolutions then the
bunch length reaches 20-25 ps (rms)
lsquo
41
Figure 315 Red RMS beam size and bunch length at IP Magenta bunch centroid position over the
first 10 000 revolutions (056 ms) for a 1 nC bunch The beam energy is 50 MeV
Figure 316 Turbulent dynamics in the longitudinal phase space that us taking place just at injection
These plots show the phase space density after 100 and 3000 turns
lsquo
42
334 CBS and IBS effects
The electron beam dynamics that prevails in a ring where Compton back-scattering is taking place is
similar to the one resulting from synchrotron radiation (SR) in a synchrotron light source CBS gives
rise to random excitation in the transverse and the longitudinal planes In the transverse plane the
emittance turns out to remain close to the one at injection [37] In contrast the CBS excitation strongly
affects the dynamics in the longitudinal phase space It leads to a substantial energy spread increase
over 50 ms storage time (see Fig 317a) resulting in a significant bunch lengthening
Based on the lattice parameters the calculation of the Touschek lifetime leads to a value on the order of
10 s this effect can thus be neglected
Figure 317 a) Energy spread increase and b) Radial (x xrsquo) relative emittance increase during a 100 ms
storage time Blue curves CBS effect only red curves IBS effect only black curves (almost on top of
the blue ones) CBS and IBS combined effect The IBS calculation is based on the Mtingwa formalism
[38]
In contrast the multiple Touschek effect (lsquoIBSrsquo) plays a significant role in the radial phase space (x xrsquo)
as shown in Fig 317b After 20 ms this horizontal emittance is increased by a factor ~ 4 from 50 to
200 nmrad and the relative energy spread by ~20 from 03 to 036 as can be seen in Fig 317a
Figure 317 shows that the Compton back-scattering process plays a role by far more important than the
IBS does It also shows that because of the large emittance increase produced by the CBS the bunch
density decreases by a large amount making the IBS much less effective (see Fig 317b where the
curve showing the combined effects lies below the curve illustrating the IBS effect by itself)
When the electron beam energy is increased up to 70 MeV the IBS effect on the horizontal emittance
becomes less important During a 20 ms storage cycle this effect is reduced by a factor 2 On the other
hand the energy spread degradation increases by more than a factor two compared to the 50 MeV case
This change is due to the higher energy of the back-scattered photons In contrast if the electron beam
energy is decreased down to 25 MeV the energy spread is reduced by a factor two while the horizontal
emittance increases by a factor seven compared to the 50 MeV case
Assuming an optimal overlap between the e- bunch and the laser pulse in the interaction region and
neglecting the e- bunch size variation across this region (the lsquohourglassrsquo effect) as well as the variation
of the laser beam envelope near its waist (characterized by the Rayleigh length) the X-ray flux is given
by the following formula (see Eq 11)
The nominal storage time is 20 ms only
lsquo
43
where σth is the Thomson cross section Nx is the number of photons per laser pulse f is the e- bunch
revolution frequency ϕ is half the collision angle σxyze σxyzγ are respectively the three dimensional
bunch rms sizes for the electron (e subscript) and for the laser pulse (γ subscript) In fact because of the
increase of the e- bunch size during the storage time the flux is reduced by roughly a factor two at the
end of a cycle as shown in Fig 318
Figure 318 X-ray flux as a function of the storage time These results take into account the energy
spread increase and the emittance degradation due to IBS only (blue curve) and to the combined CBS
and IBS effects (black curve)
To calculate more precisely the X-ray flux produced by Thom-X the code CAIN [39] (cf sect 12 above)
has been included in the previous 6D tracking code including the non linear effects of the lattice
magnets as well as collective effects The X-rays characteristics are computed at each ring revolution
The intra-beam scattering effect is calculated also at each e- bunch revolution based on the Bjorken and
Mtingwa formalism [38] To reduce the computing time the code extrapolates the IBS effect whenever
there is only a little change from one revolution to the next (this method reduces the computing time by
a factor 4) This code runs on linux OS Using the computer farm of the IN2P3 computing center in
Lyon it takes about 15 days to simulate a full Thom-X cycle (20 ms ie 400 000 revolutions) when the
e- bunch is modeled by 500 000 macro-particles
The way the total X-ray flux decreases during the first 10 000 revolutions is shown in Fig 319 This
decrease is due to the e-
beam quality degradation mainly due to the CBS effect Also shown in this
figure is the X-ray flux evolution as a function of the storage time for three different lower cut-offs in
the X-ray energy spectrum Figure 320 shows a typical angular distribution of the X-ray emission
Figure 321 plots the bunch longitudinal phase space for the first revolutions The macro-particles in
green are those which have been hit by photons at the last crossing while those in blue have been hit
before the others are in red
( ) ( ) φσ+σ+φσ+σσ+σ
φfNNσ=N
2
γzez
2
γxex
2
γyey
γeth
x22222
sincos
2πcos
lsquo
44
Cross comparison and benchmarking of this code against other codes developed for ThomX (6D
tracking without Compton effect or Compton + IBS without tracking) are in progress and indicate a
good agreement between the codes
Figure 319 Left panel Number of X-rays produced as a function of the X-ray energy at the end of the first
10 000 first revolutions (in red) and during the last 10 000 revolutions before extraction (in blue) Right
panel Number of X-rays produced summed over 2500 revolutions as a function of the revolution number
Figure 320 X-ray intensity as a function of the emission angle in mrad (horizontal axis) and their
energy in eV (vertical axis) The color bar indicates the number of X-rays
lsquo
45
Figure 321 Upper nine panels Evolution of the e- beam longitudinal phase-space after injection The macro-
particle positions are plotted along the horizontal axis while their energy deviations are plotted along the
vertical one Green electrons which have collided with photons at the last crossing blue electrons which
have collided before red electrons which have not yet undergone any collision These plots reveal a turbulent
dynamics Lower nine panels Time evolution of the beam longitudinal phase space as a result of a turbulent
regime from the initial lsquobanana shapersquo at injection to a Gaussian distribution after a few ms (colors indicate
the electron number-density red corresponds to the most populated region)
lsquo
46
Figure 322 Evolution of the X-ray energy spectrum when the e- beam energy is increased from 475 to 505
MeV Left panel The X-ray energy spectrum is plotted vertically for seven e- beam energies Colors
characterize the X-ray intensity Right panel Each curve shows the total number of X-rays above some lower
energy cut-off Green cut-off set at 20 KeV blue cut-off at 30 KeV red cut-off at 40 KeV
335 Scattering on the residual gas
Coulomb scattering of the electrons on the residual gas molecules induces an increase of the circulating
beam emittance The amplitude of this effect is proportional to the average of the beta functions and to
the gas partial pressure Moreover it varies as the square of the atomic number of the residual gas
atoms and is inversely proportional to the electron energy With a low beam energy of 50 MeV it is
therefore essential to keep a low pressure to minimize the time-averaged emittance Fig 323 shows the
growth of the transverse emittance during the 20 ms storage time in the nitrogen case and in the
hydrogen one Assuming that one deals with nitrogen at a pressure of 8 10-7
Pa the emittance increases
by a factor of two (see Fig 323) while the degradation is negligible in the dihydrogen case We
conclude that a pressure lower than 3 10-7
Pa must be maintained to keep the emittance growth at an
acceptable level
Figure 323 Emittance increase due to Coulomb scattering assuming that the residual gas is composed
of a) 100 nitrogen b) 100 dihydrogen
lsquo
47
336 Ion effects
The beam-ion instability [310 39 312] originates in the ionization of the residual gas molecules Ions
can subsequently be trapped by the electron beam electrostatic potential We recall that the on-axis
potential is inversely proportional to the beam size
When the trapped ion number density reaches some threshold beam instability is triggered Besides this
effect trapped ions damage the beam quality by shifting the tunes increassing the emittance and
lowering the beam lifetime since the local pressure is increased
For the Thom-X ring the critical mass above which ions are trapped is on the order of one in units of
the proton mass Thus all ions whatever their mass can be trapped in our case
Once trapped ions oscillate transversally in the potential well Their oscillation frequency depends on
the electron beam size and on the ion species (see Fig 324 1c) Because of the e- beam bunching
when the ion oscillation frequency becomes larger than the beam betatron frequency a lsquofast ion
instabilityrsquo may develop the e- bunch is submitted to the electric field of the oscillating ions eventually
resulting in an exponential growth of the oscillations of all particles (electrons and ions) This instability
arises above some value of the neutralization factor which depends on the lattice
The neutralization factor and the growth time of either the vertical or the horizontal oscillations are
shown in Fig 324 as a function of the horizontal beam size for several ion species For a horizontal
beam size averaged over the ring of 05 to 1 mm (which is about what is expected) the growth time is
very short (less than one micros) but for this would happen for a neutralization factor much larger than one
obviously this is excluded To keep under control the other ion harmful effects mentioned above one
may prevent ion accumulation over many storage cycles by introducing gaps between short series of
injections or by clearing the ions by an electrostatic potential that cancels out the potential well
produced by the e- beam A simple solution consists in polarizing the beam position monitors (BPMs)
thus avoiding the insertion of electrodes along the ring This scheme should be effective since ions will
drift longitudinally as the kicks they receive at each e- bunch revolution has a longitudinal component
The amplitude of these kicks along the z direction is shown in Fig 325 where the BPMs are
represented by circles They are placed near accumulation points where the longitudinal kick vanishes
ie where ions will converge A difficulty may arise at s =125 m This point is opposite to the IP in the
lattice at this point the e- beam has small transverse dimensions and a vacuum pump is placed Indeed
with a small beam size the neutralization factor threshold is very low (10-3
) and the growth time
constant is on the order of one micros This potential difficulty can be overcome by adding an electrode in
the short section between the two dipoles to clear the ions at this accumulation point
Ion clearing with polarized BPMs or gaps introduced in the injection sequence should keep other ion
effects at an acceptable level (more work is needed to quantify this statement)
Defined as the total number of ions divided by the total number of electrons
lsquo
48
Figure 324 a) Threshold neutralization factor b) growth time and c) ion oscillation frequency for the
following fractional becirctatron tunes 1) 064 and 2) 017 for different ion species The dashed lines show
the e- bunch betatron frequency σx is the mean beam size
Figure 325 Longitudinal kick given to on-axis ions at each e- bunch revolution
337 Electron beam injection and extraction
Originally the linac beam was injected in the ring by using only a septum followed by a fast kicker
Given the high repetition rate (50 Hz) radiation safety issues and a possible noise in the X-ray
experimentation area lead to set up an e- beam extraction line The symmetric septum magnet located in
lsquo
49
the middle of one of the long straight sections is used in a double pass mode for injection and
extraction It is flanked by two identical kickers one downstream for the injection and one upstream for
the extraction (Fig 326)
Figure 326 Sketch (top panel) implementation (middle panel) and simulation (lower panel) of the
injectionextraction process
The main characteristics of the septum and the kicker magnets are listed in Table 37 Since in both
processes the beam runs through quadrupoles some margins with respect to the nominal deflection
values must be provided to cope with the fact that quadrupole fields are not uniform
Kicker Septum
Deflection 15 mrad max (11 nominal) 170 mrad (40 mTm 70 MeV)
Pulse duration lt 56 ns half sine 130 micros full sine
Effective length 200 mm
250 mm
Overall length 450 mm 500 mm
Table 37 Main characteristics of the pulsed devices
Septum center
x =14 mm
xp=9 mrad + 170 mrad
Kicker center
x =0 mm
xp=0+11 mrad
2 fast kickers
Inj Ext
1 slow septum
lsquo
50
338 Orbit corrections
A set of 12 beam position monitors (BPMs) and 12 correctors in both planes are implemented along the
lattice in order to control and correct the orbit An additional BPM located at the IP is fixed on the
table that supports the laser cavity The main error sources are listed below Uncorrected and corrected
orbits are illustrated in Fig 327
rms value dBB dL dX dZ dS tilt
Dipoles 10-4 01 mm 02 mm 01 mm 02 mrad
Quadrupoles - - 02 mm - -
Dipoles 10-4 01 mm 02 mm 01 mm 02 mrad
Quadrupoles - - 02 mm - -
Table 38 Main magnet misalignment errors
Uncorrected orbit with typical magnet alignment errors
rms value xmax (mm) zmax (mm)
Max 7 3
IP 05 03
rms value xʹmax (mrad) zʹmax (mrad)
Max 12 4
IP 6 3
lsquo
51
Corrected orbit
rms value xmax (microm) zmax (microm)
Max 200 140
IP 200 30
rms value xʹmax (microrad) zʹmax (microrad)
Max 600 300
IP 350 60
Figure 327 Uncorrected and corrected closed orbits Red lines horizontal projection blue lines
vertical projection
Quadrupoles and sextupoles are fixed on girders with a maximum error of 50 microm The magnetic field
center error relative to the magnet yoke being of the same order the expected total alignment errors are
~100 microm in each plane The girders supporting all the elements will be aligned as solid blocks Since
lsquo
52
dipole misalignments have a large effect on the orbit dedicated additional support tools will allow the
readjusting of the dipole positions relative to the girder
One needs a maximum corrector strength of 05 mrad when this strength is computed by assuming just
one sigma alignment errors to cope with alignment errors which would be twice as large corrector
strengths of 1 mrad are thus necessary
Figure 328 Orbit bumps (red horizontal blue vertical) that can be produced at the IP
In addition to this standard orbit correction procedure a dedicated orbit scan at the IP will be very
useful to ensure an optimal overlap between the e- bunch and the laser pulse in the interaction region
(see Fig 328) To provide a scan range of up to 1 mm or 1 mrad the corrector strength must be further
increased by 1 mrad Thus the corrector maximum strength will be 2 mrad
The orbit correction will be performed by a slow feedback operating around 01 The correctors are in
the sextupoles magnets (combined magets) The orbit feedback correction will also control the RF
frequency in order to keep the orbit length at the ring design value A specific synchronization scheme
that ensures the proper injection timing and that maximizes the electron-photon collision rate is
described in chapter 5
Position amplitude scan at IP location up to 1 mm
Angle amplitude scan at the IP
location up to 1 mrad
lsquo
53
34 Magnets
Table 39 describes the magnets that will equip the transfer line the ring and the extraction line
Magnet type Dipoles Dipoles for inj Quad Sext Corr
Number 4+8+2 2 7+24+2 12 4+12+1
PS number 3 2 33 3 34
Strength 0180 T m 5 Tm 40 Tmsup2 05 mTm
Length 2765 mm 150 mm 60 mm 30 mm
Table 39 ThomX magnet parameters
341 Dipole magnetic design
The dipole bending angle is 45deg A magnetic field of 05 T is needed at 50 MeV Since their radius of
curvature is very small (0352 m) special attention has been paid to the field profile between the magnet poles
and to the pole edge shape where large multipole components are present Lateral shims and pole edge
specific profiles have been optimized with the TOSCA3D code to minimize the impact of the multipoles on
the beam dynamics
Characteristics unit value
Gap mm 40
Field at centre T 063
Overall dimension mm3 350 x 400 x 400
Current A 263
Voltage V 177
Pole width mm 100
Pole height mm 100
Yoke length mm 2765
NI Aturns 13000
Coil area mmsup2 67x40
Current density Ammsup2 116
Number of turns turns 48
Transverse size of the conductor mmsup2 6x6
Cooling channel mm 4
Water pressure drop bar 7
Temperature increase degC 47
Flow rate (per dipole) lmn 86
Electric power (per dipole) W 3100
Table 310 ThomX dipole parameters
lsquo
54
Figure 329 Drawing of a ThomX dipole
342 Quadrupole magnetic design
The quadrupole design (see Fig 330) was also performed with the TOSCA software The magnetic field
is produced by four coils carrying a DC current The conductor is wrapped around four poles which
concentrate and define the quadrupole magnetic field map
Figure 330 Quadrupole magnetic design
A quadrupole can be manufactured in four identical parts each one comprising one pole and one quarter
of the yoke or the yoke and the four poles can be manufactured separately The quadrupole will be
equipped with special shims which will be adjusted during the magnetic measurement to ensure a good
alignment of the magnetic axis and low harmonic components The ThomX quadrupoles are air-cooled
to reduce the infrastructure cost This is possible since the maximum current density is 15 Ammsup2
Figure 331 shows the magnetic field intensity versus the radial distance from the quadrupole axis The
maximum gradient is 5 Tm Figure 332 shows the magnetic field intensity at a distance of -20 mm
from the magnet axis versus the longitudinal coordinate z The maximum magnetic field is 01 T
lsquo
55
Figure 331 Magnetic field (in T) versus the distance (in mm to the magnet axis
Figure 332 Magnetic field intensity (in T) versus z (the longitudinal coordinate in mm)
lsquo
56
The main parameters of the ThomX quadrupoles are given in Table 311 Each quadrupole is powered
by a 10 A 32 V DC power supply thus the released heat power remains below 32 W A dedicated
magnetic bench equipped with a rotating coil will be used for the quadrupole magnetic measurements
This bench has been developed for the magnetic measurements of the SOLEIL storage ring
quadrupoles
Parameters unit value
Bore mm 41
Gradient Tm 5
Current density Ammsup2 15
Overall dimensions cm3 19x19x18
Current A 10
Voltage V 35
NI Aturns 810
Pole width mm 20
Pole height mm 50
Coil dimensions mmsup2 15x36
Number of turns turns 81
Wire dimensions mmsup2 5x125
Yoke length mm 150
Yoke thickness mm 20
Chamfer mm 5 (not optimised yet)
Table 311 Main parameters of the ThomX quadrupoles
343 Sextupole magnetic design
The ThomX sextupole design (see Fig 333) was also performed with the TOSCA software The magnetic
field is produced by six coils that carry a DC current The coils are wrapped around six iron poles which
concentrate the field and define the sextupole field map These magnets are combined function and are also
equipped with additional coils to produce the corrector dipole fields
Figure 333 Sextupole magnetic design
lsquo
57
Each sextupole yoke is manufactured as a single piece and the six poles are screwed on this yoke Then a coil
is wound around each pole During the design phase a chamfer was calculated with TOSCA to guaranty a low
harmonic content The sextupoles will be air-cooled to reduce the infrastructure cost The maximum current
density is 15 Ammsup2
The magnetic field intensity versus the distance to the magnet axis is shown in Fig 334 The maximum
sextupolar strength is 40 Tmsup2
Figure 334 The ThomX sextupole magnetic field versus the distance to the magnet axis
Fig 335 shows the variation of the magnetic field at a distance of -20 mm from the sextupole axis versus the
longitudinal coordinate z The maximum magnetic field is 0016 T
Figure 335 ThomX sextupole magnetic field versus z (the longitudinal coordinate)
Table 312 gives the main parameters of the ThomX sextupoles They will be powered by a 10 A 05 V DC
power supply
lsquo
58
Parameters unit value
Bore diameter mm 42
Sextupolar Strenght Tm2 40
Field 20 mm T 0016
Current density Amm2 25
Overall dimension cm3 19x19x8
Current A 10
Tension V 05
AmpereTurn At 100
Pole width mm 20
Pole height mm 40
Coil dimension mm2 6x40
Number of turns Tours 10
Conductor dimension mm2 2x2
Yoke length mm 60
Yoke thickness mm 20
Chamfer mm 15
Table 312 Main characteristics of the ThomX sextupoles
A dedicated bench equipped with a rotating coil will be used for the magnetic measurements of the sextupoles
This bench has been developed for the magnetic measurements of the SOLEIL storage ring sextupoles
344 Corrector magnetic design
Simple window frame correctors based on an already existing SOLEIL design will be used for the orbit
corrections of the two transfer line
Figure 336 Magnetic design of the window frame correctors
lsquo
59
345 Injection dipole
The two injectionextraction dipoles compensate the septum deflection with a 170 mrad deflection (cf sect 337
see Fig 326) They are also air-cooled Their design and their technical characteristics are illustrated in Fig
336 and Table 313
Figure 336 Magnetic design of the injection dipole
Parameters unit value
Gap mm 40
Field at the magnet centre T 0224
Current A
10
Voltage V 6
Pole width mm 60
Pole heigth mm 70
Yoke length mm 150
AI (per coil) At 3600
Coil cross section area mmsup2 60x60
Current density Ammsup2 09
Number of turns tours 360
Conductor size mmsup2 10 x 1
Pressure drop of the cooling water bar NC
Temperature increase degC NC
Power (per dipole) W 120
Table 313 Parameters of the injection and extraction dipoles
lsquo
60
35 Power supplies
In order to provide a high operation flexibility ThomX will be equipped with the following power supplies
ndash One power supply for the ring dipoles (the latter will be connected in series) one for the TL dipoles
and one for the DL
ndash Individual power supplies for all the other magnets (quadrupoles sextupoles and correctors)
All the power supplies will have a 16 bit-resolution and a plusmn20 ppm reproducibilitystability over 24 hours The
technology is a switched-mode one in order to achieve good efficiencies (gt 85)
Magnet Nb PS Nb mag current
(A)
voltage
(V)
type load
R
(mOhm)
L
(mH)
Dip ring 1 8 300 170 Unipol 565 48
Dip TL 1 4 300 85 Unipol 280 24
Dip DL 1 2 300 45 Unipol 150 12
Dip inj 2 1 12 18 Unipol 1330 180
Quad 33 1 12 6 Unipol 500 10
Sext 12 1 12 3 Unipol 250 08
Corr ring 24 1 plusmn12 plusmn3 Bipol 250 08
Corr TL DL 10 1 plusmn12 plusmn3 Bipol 250 08
Table 314 Overview of the ThomX power supply equipment
351 Pulsed magnets
3511 Layout
As discussed above the e- beam injection in the storage ring and its extraction will take place in the
same straight section (see Fig 337)
Figure 337 Injectionextraction section layout
Eddy current septum magnet
(under vacuum) Fast Kicker Fast Kicker
lsquo
61
The first injection step involves a thin septum magnet It bends the incoming bunch trajectory to bring it
inside the ring vacuum chamber close to the stored beam orbit Then a fast kicker compensates for the
residual trajectory angle and brings the injected bunch on-axis
Extraction is realized by another fast kicker followed by the same septum magnet as the one used for
injection This kicker deflects the bunch trajectory so that it enters into the septum channel in turn the
thin septum bends the bunch trajectory which enters then into the extraction channel that brings the
bunch into a beam dump
In this configuration the same thin septum magnet is used for both the bunch injection and the bunch
extraction This requires a symmetrical design with two flanges one dedicated to the injection and the
other to the extraction The magnetic gap is wide enough to accommodate the two bent trajectories
When operating at a 50 Hz injection rate the septum current pulse consists of a slow full sine whose
same positive half sine will be used successively for the lsquoold bunchrsquo extraction and the for the lsquonew
bunchrsquo injection with a suitable delay between the two steps
The two fast kicker magnets are identical their length has been reduced as much as possible because of
the very limited space available The current pulse in the kickers will have a half-sine shape One must
achieve a rise time (from zero current to peak current) and a decay time (from peak current to zero)
each less than a ring revolution period Thus the current pulse can have a maximum duration of two ring
revolution periods Consequently their charging voltage reaches high values even though most of the
deflection is performed by the septum magnet
The design of these three devices ensures that they can operate with a beam energy of 50 MeV
(nominal) or 70 MeV (maximum) and at a 50 Hz repetition rate
3512 Slow septum magnet and its pulsed power supply
The main constraints on the septum design are as follows This septum must produce a large trajectory
deflection (160 mrad nominal peak value 170 mrad maximum peak value) to bring the injected beam
very close to the ring straight section axis with no significant disturbance of the stored beam The
magnet mechanics should preserve the ring vacuum chamber geometry to keep its impedance as low as
possible Furthermore its stray field must remain very low within the vacuum chamber where the stored
beam circulates as the septum excitation is done by a full sine pulse whose width (130 micros) is very long
compared to the ring revolution time (56 ns)
An eddy current septum magnet installed in vacuum will fulfil these constraints The effective septum
blade thickness can be as small as 3 mm thus ensuring the closeness of the stored beam orbit and the
desired bunch trajectory at the output of the septum yoke An internal chamber connected to the magnet
yoke extends the ring vacuum chamber and preserves the impedance budget The injected bunch
deflected by the septum magnet field penetrates through a small hole in this internal chamber Such a
septum magnet is very asymmetric in the transverse plane it is driven by a copper loop that wraps the
steel yoke
In order to contain the magnetic flux the whole yoke is enclosed in a copper box which carries the eddy
current On the side facing the stored beam orbit the copper box thickness is reduced so that it
constitutes the thin septum blade As a result of the electric shielding provided by the eddy current
circulating in the copper septum such a structure is very effective to avoid a large magnetic stray field
outside the magnet gap The fact that the excitation pulse is a full sine reduces the duration and the
lsquo
62
amplitude of the stray field (the slow delayed stray field tends to be the sum of the two opposite half
current arcs this feature brings about a reduction of the stray field)
An additional magnetic shielding is required to strongly reduce the residual strayfield This shield is
provided by a 05 mm thick sheet of high permeability mumetal that will completely surround and
enclose the internal chamber (thus shielding the stored beam orbit from the strayfield)
Such a design that combines an eddy current shield a full sine pulse excitation and the use of a high
permeability magnetic material reduces the stray field to a very low level (a few 10 ppm of the main
pulsed field) [313]
Assuming a septum opening of H x V = 30 x 12 mm2 and an active length of 250 mm (the length was
chosen as short as possible because of constraints coming from the septum implementation in the
straight section) the septum magnet inductance is estimated to be 16 microH Connexions and cables
increase this inductance up to 23 microH
This design leads to the following parameters for the septum magnet and its power supply (values for a
70 MeV ring operation)
Septum deflection field integral field peak current charging voltage
170 mrad 40 mTm 160 mT 1500 A 226 V
Table 315 Septum main parameters
The pulsed power supply will be based on the lsquoclassicalrsquo resonant discharge circuit scheme composed
of a storage energy capacitor (190 microF) a fast high-current thyristor switch and a transmission line to
the magnet comprising a few low impedance coaxial cables
A 300V-20A power supply is foreseen It will be connected to the storage energy capacitor through a
current-limiting power resistor
3513 Fast kicker magnet and its pulsed power supply
The two fast kicker magnets must be identical Because of the very limited space available their length
has been reduced as much as possible The final numbers are 200 mm and 450 mm for the active length
and for the overall length respectively They will be located outside the vacuum vessel with their
lsquowindow framersquo rectangular ferrite yokes surrounding the ceramic vacuum chamber (see Fig 338)
Figure 338 Front view of the kickers
The two ceramic sections of the vacuum chamber will have a flange-to-flange length of 450 mm Their
aperture will be the same as the one in the ring standard sections but with a racetrack transverse shape
Both extremities of each ceramic chamber include a smooth shape transition and a short bellow that
lsquo
63
allows for RF finger contacts These bellows can accommodate thermal dilatations especially during
bake-out The ceramic chambers will be coated with a metallic deposit that will carry the bunch mirror
current and thus avoid an increase of the chamber impedance This coating needs to be very thin It will
consists of a titanium layer with a thickness le0050 microm The thickness has been calculated so as to
avoid the deterioration of the fast pulse shape and to avoid the temperature rise due to the mirror current
[314]
Figure 339 Time profile of the kicker pulse versus the titanium layer thickness
The kicker dimensions follow from
- the aperture needed by the stored beam H x V = 40 x 28 mm2
- the required deflection kick nominal 11 mrad maximum 15 mrad
- the beam energy nominal 50MeV maximum 70MeV
- the short revolution period of the stored e- bunch Trev = 56 ns
- the very short length of the injected bunch from 4 to 25 ps (rms)
With a ceramic thickness of 6 mm the effective magnet aperture must be H x V = 55 x 55 mm2 These
parameters determine the current and voltage pulse characteristics
The kicker magnets will be powered by a circuit delivering half-sine pulses [315] In fact because of
the e- bunch short revolution period (56 ns) the field pulse duration has to be less than twice the ring
revolution period (less than 112 ns) and thus must a have very fast rise time and decay time (both less
than a ring revolution period) On the other hand with such a very short bunch length (4 or 25 ps (rms)
whether one deals with the incoming bunch or the extracted bunch one may assume that the pulses do
not have to present a marked flat top
These considerations lead to the following kicker parameters
00000E
-2
17000E
-10
34000E
-10
51000E
-10
68000E
-10
85000E
-10
10200E
-9
11900E
-9
0000E+00
2000E-01
4000E-01
6000E-01
8000E-01
1000E+00
1200E+00
Thom-X injection kicker pulse field attenuation relating to Titanium thickness
Bext(t)
Bint(t) 002microm
Bint(t) 004microm
Bint(t) 005microm
Bint(t) 006microm
Bint(t) 008microm
Bint(t) 01microm
Bint(t) 02microm
Time
Fie
ld a
mp
litu
de
lsquo
64
Kicker beam energy deflection field integral
Operation at Enominal 50 MeV 15 mrad 25 mTm
Operation at Emax 70 MeV 15 mrad 35 mTm
field peak current estimated voltage
92 mT 406A 10 kV
175 mT 771 A 18 kV
A specific RampD program is necessary to guarantee that these constraints will be fulfilled by the power
supply and by the magnet Preliminary studies were based on the standard circuit used for such a pulsed
power supply (see Fig 340) involving a high voltage (HV) capacitor to store the energy in series with
a HV switch and a load whose inductance is close to the one of the kicker magnet In this circuit the
HV switch is a very fast MOSFET switch followed by a HV fast recovery diode module
Figure 340 Power supply circuit for the ThomX kickers
These first studies lead to unsatisfactory preliminary results
- The BEHLKE Ultra-Fast 17 kV 480 A MOSFET module can switch ON in a very short time less
than 8 ns Then the pulse shape is normally determined by the passive component parameters in the
circuit provided that the negative current is quickly interrupted either by HV diodes in series or by
a turn Off control of the HV switch
- But the different types of HV diodes used are not able to switch OFF the pulsed current quickly
enough Even the lsquofast recovery diodesrsquo have a reverse recovery time too much long to stop cleanly
the pulse at its zero crossing Thus it was necessary to work on other topologies and to find better
suited components
Further simulations were performed to see how the circuit could be improved It turned out that
controlling the switch both at the closing and at the opening can improves the shape of the current
pulse A prototype using the same Behlke MOSFET modules and including a coaxial transmission line
was built and tested A plot of the current pulse in the kicker coil that was obtained is shown below
lsquo
65
This plot was obtained with the following prototype circuit
As one can see more work is still needed to improve the shape of the current in the coil (namely one
should reduce the level of reflections improve the current pulse shape and work on the peak current to
meet the 70 MeV requirements) We are also looking for an other supplier who could provide controlled
HV switches ensuringr both ON and OFF fast operation
Mechanical and magnetic studies of the kicker magnets are ongoing
36 Storage ring RF system
The choice of 500 MHz for the RF frequency is a compromise between several constraints the cavity
fundamental and high order mode (HOM) impedances space requirements the availability of RF power
sources and other components The required accelerating voltage of 500 kV can be provided by only
one 500 MHz single-cell cavity with about 40 kW of RF power As shown in the following the choice
of the cavity design is mainly dictated by the need for a strong damping of the HOM
lsquo
66
361 HOM impedance thresholds as regard to beam instabilities
When a cavity mode is excited in resonance it generates beam oscillations with a rise time
approximately given by one of the two following expressions
(1τ)l = Ib α fm Rl (2 Qs Ee) in the longitudinal case and
(1τ)t = Ib βt fo Rt (2 Ee) in the transverse case
where fm is the HOM resonant frequency Rl [Ω] and Rt [Ωm] are respectively the longitudinal and the
transverse HOM impedances Qs = fs fo is the synchrotron tune (fs and fo are the synchrotron and the
revolution frequencies) βt is the beta function amplitude at the cavity location α is the momentum
compaction factor Ib is the average beam current and Ee is the beam energy in eV
Assuming typical values for the cavity HOM impedances (Rl fm = 01 to 1 MΩ GHz Rt = 1 to
10 MΩm) and the machine parameters listed in Table 315 the instability growth time is around 10 micros
in both the longitudinal and the transverse cases
Orbit length C 168 m
Electron energy E 50 MeV
RF frequency fRF 500 MHz
Revolution frequency fo 1786 MHz
Harmonic number h 28
Momentum compaction factor α 0015
β-function amplitude at the cavity location βxz 3 m 4 m
Relative energy spread σE E 40 10-3
Peak acceleration voltage VRF 300 (500) kV
RMS bunch length σs 27 (21) ps
Synchrotron frequency fs 358 (462) kHz
Bunch charge Qb 1 nC
Number of stored bunches Nb 1
Maximum current Ib 1786 mA
Table 315 Ring parameters used to estimate the rise time of the HOM induced instabilities
In a low energy ring like ThomX radiation damping is so weak (the damping time is ~ 1 s) that a
stationary state is not reached during the beam storage time (20 ms) On the other hand to ensure a
good ring operation it is sufficient to make all instability rise times much longer than this storage
period This can be achieved by attenuating the HOM impedances very strongly typically by a factor
104
362 Cures to the HOM induced instabilities
There are essentially two ways of coping with such HOM induced instabilities either by performing a
strong de-Qing of the HOM resonances or by tuning their frequencies away from the beam spectral
lines in order to prevent resonant excitation
In practice HOM de-Qing is achieved by extracting the cavity HOM power and transferring it towards
external dissipative loads But in this approach it is difficult to reach attenuation factors higher than
102 - 10
3 over a large frequency range and this technique requires heavy and cumbersome equipment
close to the cavity (coaxial lines with coupling antennas or ferrite loaded waveguides) The DAMPY
lsquo
67
cavity designed at BESSY [316] and which is used in ALBA [317] and the PEP II cavity developed
at LBNL [318] are two variants of this technique
The other method which consists in controlling the HOM frequencies is better suited to a small
circumference machine like ThomX where the beam spectral linesrsquo spacing (~ 18 MHz) is very large
compared to the HOM bandwidth (this bandwidth is of a few tens of kHz for Qo equal to a few 104) As
long as the HOM density is not too high and the modes can easily be tuned far enough from the beam
spectral lines (δf gtgt fm Qo) it should be possible to reduce their effective impedances (the ones lsquoseenrsquo
by the beam) down to tolerable levels ie Reff asymp Rs (2 Qo δf fm) 2 ltlt Rs
We are investigating the potential performance of this technique by applying it to an ELETTRA-type
cavity (see Fig 344) [319] which combines three frequency tuning means The HOM frequencies can
be precisely controlled by an appropriate choice of the temperature of the water cooling the cavity while
the fundamental frequency is maintained constant by a mechanism which deforms the cavity
longitudinally In addition a movable plunger tuner also called HOM frequency shifter (HFS) located
in the cavity equatorial plane provides an additional degree of freedom [320]
Tables 316 a and b list the characteristics of such a cavity fundamental and HOM resonances They
have been calculated with the 3D GdfidL code [321] One finds 18 significant HOM (9 monopoles and
9 dipoles) with resonant frequencies lower than the cavity cut-off frequency Figures 345 a and b
represent the corresponding HOM spectra fm Rl(f) and Rt(f) folded over a single base band of
18 MHz The horizontal dashed line in the graph represents the impedance threshold corresponding to
an instability rise time of 20 ms the beam storage time Although they remain somewhat critical these
results tend to confirm that it should be possible to find acceptable operating conditions as long as the
HOM frequencies can be shifted by a few MHz by combining temperature and HFS tuning
lsquo
68
Figure 344 Drawing and photo of the ELETTRA cavity
lsquo
69
Figure 345 (a) Longitudinal and (b) transverse HOM spectra folded over a single base band of
18 MHz for the ELETTRA-type cavity
τL = 20 ms
lb
os
I
eEQ
τα
2
ttob
o
fI
eE
τβ
2
τt = 20 ms
lsquo
70
a) Longitudinal modes fm [MHz] Q0 Rs Q0
[Ω]
Δfm (T)
[MHz]
Δfm (HFS)
[MHz]
L0 (fundamental) 499132 44164 765 000 000
L1 958618 48158 336 037 -100
L2 102781 57861 036 080 369
L3 14087 52562 220 174 021
L4 151528 69088 783 112 086
L5 155129 66375 850 240 721
L6 186912 55785 097 142 -092
L7 19495 81752 008 183 143
L8 203985 62729 086 400
L9 206896 87300 530 445 667
Table 316 Main characteristics of the monopole (a) and dipole (b) modes computed with GdfidL for
an ELETTRA-type cavity Δfm (T) and Δfm (HFS) are the effective temperature and HFS tuning ranges
assuming a constant fundamental frequency
It is worthwhile mentioning that in any event fast longitudinal and transverse feedback systems are
required to damp the beam oscillations resulting from injection transients (see section 333) They will
contribute to the HOM damping as well
363 RF power source
An RF power of about 40 kW is required to generate the desired voltage of 500 kV in a single 500 MHz
ELETTRA-type cavity For this frequency there exists a wide choice of power sources based either on
vacuum tubes (klystron IOT diacrode) or on solid-state technology The low operating voltage and
high modularity of the solid-state version offer significant advantages compared to vacuum tubes
Experience acquired at SOLEIL after few years of operation has definitely demonstrated all the benefits
of this technology [322] Figure 346 shows the 35 kW - 352 MHz solid-state amplifier used for the
SOLEIL booster synchrotron It consists of a combination of 147 modules Each one has a power of 330
W a gain of 11 dB and an efficiency of 62 This amplifier was commissioned about 7 years ago
During a running period of more than 35 000 hours it tripped only once and this trip was caused by a
human mistake
b) Transverse modes fm
MHz]
Qo Rsperp Qo
[Ω]
Rsperp[MΩm] Δfm(T)[MHz] Δfm(HFS)
[MHz]
T1 73701 50267 335 2599 04 075
T2 74606 52566 92 7557 155 285
T3 115202 47196 264 3006 049 -312
T4 125565 55386 51 7428 068 725
T5 130249 71674 36 7039 097
T6 169981 69325 103 2542 188
T7 173650 91039 165 5463 gt 2
T8 177337 59409 32 7061 gt 2
T9 183617 77697 55 1643 gt 2
T10 206142 88484 59 2254 gt 2
T11 222475 99092 11 5079 gt 2
T12 225933 56641 13 3484 gt 2
lsquo
71
This technology has already been extended to 476 MHz in the framework of a SOLEIL-LNLS
collaboration aimed at realizing two 50 kW amplifiers for the LNLS storage ring [323] More recently
the SOLEIL team has developed modules which can deliver up to 650 W at 500 MHz with a gain of
17 dB and an efficiency of 62 while still reducing the thermal stress [322] About a hundred of such
modules will be combined to obtain the 50 kW required by ThomX
Figure 346 35 kW - 352 MHz solid-state amplifier of the SOLEIL Booster (147 amplifier modules
and DCDC converters bolded on 8 water-cooled dissipaters)
364 Low level RF system and longitudinal feedback
The proposed low level RF (LLRF) system is conventional As shown by the synoptic diagram of Fig
347 it consists of three relatively slow loops (frequency amplitude and phase) aimed at controlling the
cavity voltage with an accuracy of about plusmn 02 in amplitude plusmn 02deg in phase and a response time
around 1 ms This scheme is suitable for compensating the pressure and temperature drifts as well as
the power supply ripples
Such a system is not fast enough to deal with the beam phase oscillations at the synchrotron frequency
fs on the order of 400 kHz that originate in injection transients These oscillations can produce an
undue emittance blow up and a phase jitter at the interaction point Beam dynamics simulations show
that preserving the beam quality requires a damping of these oscillations within a few tens of a micros (see
section 333) As the radiation damping is negligible during the storage time a longitudinal feedback
600 W 280 Vdc 28 Vdc converter
Power combination
330 W amplifier module
lsquo
72
(LFB) must be implemented It can be performed by a dedicated broad-band longitudinal kicker that
acts on the beam Since ThomX is operating with a single bunch we propose to use the main cavity as a
longitudinal kicker despite its narrow bandwidth of 25 kHz ltlt fs This difficulty can be overcome by
making the cavity effective bandwidth much larger than this value in fact larger than fs We will use a
high gain direct RF feedback (RFFB) that allows the operation of a fast phase feedback (ϕFB)
Assuming a cavity-to-amplifier distance of about 10 m the loop delay will be less than 150 ns Then
one expects an RFFB gain Go larger than 50 which leads to an effective bandwidth larger than 1 MHz
Simulations show that a LFB that combines such a RFFB with a ϕFB (see Fig 348) can damp injection
phase errors of up to 10 degrees within a few micros ie faster than the above specification
Figure 347 Basic LLRF system scheme with lsquoslowrsquo frequency amplitude and phase loops
lsquo
73
Figure 348 Schematics of the longitudinal feedback system combining a phase feedback with a direct
RF feedback
365 RF equipment status
For the ThomX commissioning we will use an ELETTRA cavity The design of all the components of
the 50 kW solid-state amplifier (600 W modules combiners power supplieshellip) was validated in 2011
The next two milestones are first the validation of a 9 kW unit (combining 16 modules) and secondly
the commissioning of the complete amplifier on a dummy load
Several versions of the LLRF system including LFB are under investigation either fully analogue (see
Fig 349) or mixed analogue for the fast feedback loops and digital for the slower ones (see Fig 350)
or even fully digital based on the new generation of fast ADC DAC and FPGA
366 RF system conclusion
For the RF system of the ThomX storage ring we plan to use a 500 MHz single-cell cavity of the
ELETTRA-type Maintaining at a tolerable level the impact of the RF cavity HOMs on the beam is a
challenge However this goal should be achievable with a proper control of the HOM frequencies It
can be done by combining three different means of tuning the cavity temperature control longitudinal
elastic deformation of the cavity and motion of a plunger The fast longitudinal and transverse feedback
systems - which in any event are required to damp the beam oscillations resulting from injection
transients - will contribute to the damping of the HOM excitations as well
In order to provide the required accelerating voltage of 500 kV the cavity will be powered by a solid-
state amplifier of the lsquoSOLEILrsquo type able to deliver a power of up to 50 kW The cavity voltage will be
controlled with an accuracy of plusmn 02 and plusmn 02 degree by means of conventional amplitude and phase
loops (either analogue or digital) with a few kHz bandwidths In addition a fast phase feedback system
combined with a high gain direct RF feedback will provide the longitudinal damping which is required
to preserve the beam quality
lsquo
74
Figure 349 Complete LLRF system longitudinal feedback included (analogue version with phase and
amplitude modulation ndash the option with IQ modulation is also considered)
Figure 350 Complete LLRF system longitudinal feedback included (mixed analogdigital version)
37 Transverse feedback
371 Specifications
Table 317 summarizes the instability growth time constants and the corresponding kicker strength
requirements for the main types of instability It shows that for Thom-X the betatron oscillations due to
the injection jitter in position and angle are the most critical ones They require a feedback damping
time constant of 5 micros or less (cf chapter 3)
lsquo
75
Source type growth time constant kicker strength
Beam pipe
geometry
TMCI
Head-Tail
-
160 micros
gt10 nrad
Resistive Wall 600 micros gt2 nrad
Ions lt 100 micros gt20 nrad
Injection Jitter 5 micros 2 microrad
Table 317 Overview of expected instabilities (1 nC 25 ps rms at 50 MeV)
372 Electronics
To cope with these instabilities we will use a SPring-8 based high performance digital transverse
feedback system which consists of a wideband detector (a BPM) an RF front-end an FPGA based
processor a power amplifier and a stripline kicker (see Fig 351) This system is capable of detecting a
coherent transverse motion and applying a counter kick to damp it bunch-by-bunch and turn-by-turn
with one or two bunches stored in the ring
Figure 351 Spring-8 based digital feedback system
Two BPMs (one for each plane) will provide the beam position information The RF front-end will
extract the betatron oscillation information which will be sampled by ADCs From the sampled data the
FPGA will compute a suitable kick to damp the oscillation The signal processor output will be
amplified by a 10 W wideband amplifiers before being fed to the kicker
In the RF front-end (see Fig 352) a baseband down-converter will be implemented in order to amplify
the beam signal and to minimize the jitter effect for the same reason the digital kick will also be
extended over four RF periods
lsquo
76
Figure 352 Front-end of the Thom-X RF electronics
For the commissioning phase a digital system (rather than an analog one) will be used to benefit from
the tuning flexibility provided by the digital parameters of the FIR filter Once the machine tune will be
fixed a more suitable low cost analog system will be installed
373 Kicker
The kicker is under development It consists of a stripline composed of four electrodes connected to
vacuum feedthroughs on each side (see Fig 353) The amplifiers will be connected to the downstream
ports whereas upstream ports will be used for signal detection and they will be equipped with 50 Ω
loads The same kicker will be used to act on the beam in both planes
Electrode line impedances must be adapted (50 Ω) Electromagnetic 3-D simulations are being
performed to optimize the bandwidth and shunt impedance (ie to maximize the stripline efficiency)
and also to minimize the kicker impedance to wakefields
Figure 353 Schematics of the Thom-X stripline kicker
lsquo
77
38 Vacuum
381 Transfer line vacuum
Concerning the design of the transfer line vacuum system three main constraints have been taken into
account
- Pressure of 10-7
Pa at the acceleration section output
- Ion pump operating at 10-6
Pa
- Ring pressure of 10-8
Pa
Simulations have been performed assuming that two 03 m3 s
-1 triode pumps will be installed one at the end of
the acceleration section and the other at the septum The vacuum chamber is a DN40 The septum output
conductance is 018 m3 s
-1 Without baking the outgassing rate would be σ τ = 2x10
-8 Pa m s
-1 and the
pressure at the ring entrance would be about 10-7
Pa Thus the septum and its environment must be baked to
bring τ down to 5x10-10
Pa m s-1
and obtain a pressure of 10-8
Pa at the ring entrance
Pressure in the line of transfer
The vacuum equipment envisaged for the transfer line is the following one
‐ 3 ion pumps 150 l s-1
with their baking systems and power supplies
‐ 1 ion pump 500 l s-1
with their baking system and power supply
‐ 4 Pirani gauges
‐ 4 cold cathode gauges
‐ 2 power supplies gauges
‐ 2 gate valves electro pneumatically actuated DN 40 (acceleration section output ring entrance beam
dump)
‐ 1 turbo pump (DN 63 ~ 80 l s-1
)
‐ 1 rotary vane pump (~ 12 m3 h
-1)
‐ 4 angle valves manually actuated for pre-pumping DN 63
lsquo
78
382 Ring vacuum
Concerning the ultra-vacuum to be maintained in the ThomX ring one must take into account first the
pipe relatively low specific conductance c = 452x10-3
m3 m s
-1 and secondly the pipe specific
outgassing rate θ τ = 108x10-10
Pa m2 s
-1 Also the pumpsrsquo effective pumping speed is reduced by the
T port with the grid Seff = 19x10-2
m3 s
-1 In a first stage the ionic pumps chosen for the simulations
are 0040 m3 s
-1 diode pumps This corresponds to S = 0030 m
3 s
-1 at 10
-8 Pa and with a grid pumping
port whose conductance is 00362 m3 s
-1 to an effective pumping speed Seff = 0019 m
3s
-1 The pressure
has been estimated by modeling a quarter of the ring including half of the non-baked optical cavity ie
in the most unfavorable case
Figure 354 Pressure distribution in the vacuum chamber
The calculated average pressure is 306x10-8
Pa It can be improved by increasing the grid pumping port
conductance by replacing the DN35 with a DN63 Monte-Carlo simulations show an improvement of ~
16 in efficiency
The entire ring must be baked Geometrical and temperature constraints (respectively a distance of only
07 mm between the magnet coils and the vacuum chamber a baking temperature of 150 degC for the
chamber while keeping the coilsrsquo temperature below 80deg C) are strict An ultra-thin baking system has
been constructed tested and validated
Figure 355 The vacuum chamber ultrathin baking set-up
lsquo
79
The following is the vacuum equipment needed for one quarter of the ring
- 3 ion pumps (40 l s-1
) with their baking equipment and their power supplies
- 1 residual gas analyzer (RGA)
- 2 turbo-pumps (DN 63 ~ 80 l s-1
)
- 2 rotary vane pumps (~ 12 m3 h
-1)
- 2 Pirani gauges
- 2 cold cathode gauges
- 1 gauge power supply
- 1 DN 63 gate valve electro-pneumatically actuated with an HF feed-through (one on each side of
the RF cavity and similarly for the septum) and the baking equipment
- the vacuum chamber baking equipment
- 3 angle valves manually actuated for pre-pumping DN63
- 3 T with the grid
lsquo
80
Chapter 4 INJECTOR TRANSFER LINE AND RING DIAGNOSTICS
Several diagnostic devices will be implemented in the ring to measure the electron beam parameters
listed in Table 41
Parameter unit value
Energy MeV 50
Average current mA 1784
Bunch charge nC 1
Energy dispersion 03 at injection to 06 at storage end
Emittance x mrad 5 e-8 at injection to 30 e-8 at storage end
Normalized emittance x mrad 5 e-6 at injection to 30 e-6 at storage end
Emittance y mrad 5 e-8
Normalized emittance y mrad 5 e-6
Bunch length ps-rms revolutions 1 ~ 4
revolutions 100-200 ~ 25
revolutions 200-300000 ~ 50
RF frequency MHz 500
Revolution frequency MHz 1784
Period ns 56
Injection repetition rate Hz 50
Ring circumference m 1680
(a) Ring parameters
Magnetic field T 05
Radius of curvature m 0352
R51 m 0 to -025
R52 m 0 to -045
R56 m 07 to -07
Beta x m 06
Beta y m 6
Eta x m 02 - 03
Transverse dimension x mm (rms) 06
Transverse divergence x mrad 2
Transverse dimension y mm (rms) 054
Transverse divergence y mrad 2
(b) Parameter values at the location of the bending magnets
Table 41 ThomX beam parameters
41 Charge measurement
411 Current transformers
The beam charge will be measured non-destructively with commercial in-flange high resolution
current transformers (ICT) Three devices will be installed
- one in the linac (LIDGICT010) located before the accelerating sections
- one at the transfer line entrance (TLDGICT010)
lsquo
81
- one at the transfer line exit (TLDGICT020)
In-flange ICTs are directly mounted in the beam line They are UHV compatible can be baked up to
150deg C (185deg C on request) and they are available for many pipe diameters (also with an elliptical
aperture or any other aperture shape) We will use the CF6rdquo model its internal diameter is 604 mm and
it is 60 mm long This device includes a ceramic gap a shield and a wall current bypass Bellows are
not required
The signal from each integrating current transformer will be processed by a BCM (beam charge
monitor) A built-in calibration generator provides an on-line check of the measurement reliability An
Integrate-Hold-Reset (BCM-IHR) device is suited for pulse repetition rates ranging from 10 kHz down
to single pulses and for bunch charges up to 2 nC The acquisition will be performed by using a 100
kHz 12 bit ADC
412 Faraday cups
Interceptive absolute beam charge measurements will be performed by Faraday cups There will be two
of them placed inside the two beam dumps (see Fig 2 of the Introduction) They should provide a
better resolution for small electric charges
42 Transverse profile measurements
421 Diagnostics stations
Five diagnostics stations will be installed to measure the transverse beam profile as shown in Fig 41
Diagnostics station 1 (DS1 ndash LIDGSST010) will be located after the RF gun at the beginning of the
linac Diagnostics station 2 (DS2 ndash TLDGSST010) will be located between the end of the linac and
the linac beam dump Diagnostics station 3 (DS3 ndash TLDGSST020) will be placed in the middle of the
transfer line Diagnostics station 4 (DS4 ndash TLDGSST030) will be located at the end of the transfer
line and Diagnostics station 5 (DS5 ndash ELDGSST010) will be located before the ring dump
lsquo
82
Figure 41 Location of the diagnostics stations (see text for the nomenclature)
The ThomX station design draws from the station built by LAL for PITZ (itself drawn from a SOLEIL
design) It is illustrated in Fig 42
Figure 42 3D view of a ThomX diagnostic station
Each station will have a motorized arm on which several screens will be mounted A stepper motor will
move this arm and insert the screens laterally The following screens will be available
- 1 YAG(Ce) screen in all stations
- 1 optical transition radiation (OTR) screen in all stations
lsquo
83
- 1 target to allow an online calibration of the camera in all stations
- one Cerenkov screen for bunch length measurements in station DS2 (described in more detail
below)
The YAG(Ce) and OTR screens and the calibration target will be tilted at 45deg with respect to the beam
axis so that the emitted or reflected light will travel downward and exit the vacuum through a DN60 CF
fused silica window A CCD camera will be located below the station to image the screens The
Cerenkov screen will be titled so that its light exits at the top of the diagnostic station Shutters will be
used to close either the Cerenkov window or the YAG(Ce)OTR window when not in use to prevent
measurement noise due to background photons Several LEDs located just after the bottom window will
be used to illuminate the inside of the chamber when needed
At the beginning of the Linac at station DS1 the beam size will be about 05-07mm (rms) in both
planes at the end of the Linac at DS2 the beam size will be 15 mm (rms) in both planes In the middle
of the transfer line at DS3 the beam size is expected to reach 25 mm (rms) in x and slightly less than
1mm in y At the end of the transfer line at DS4 it will be about 1 mm in both planes After the
extraction at DS5 the beam size will be about 05-07mm (rms) in both planes Given these beam sizes
we chose a screen diameter of 25 mm for all stations
Screen photon yield
The YAG(Ce) screen will be 100 microm thick Such screens have a light yield of 8 photons per deposited
keV [41] The YAG radiation length is 35 mm [42] assuming the usual model for energy deposition in
matter [43] each electron will deposit about 143 keV in the screen this corresponds to the emission of
~1100 photons Thus the total number of photons produced by an e- bunch crossing the screen will be
71 1012
We recall that YAG(Ce) emission occurs in all directions
The second screen will be of the OTR type The screen support design allows either the use of a 100 microm
aluminized silicon wafer or of an ultra-thin aluminized mylar film The silicon wafer has the advantage
of giving a better image quality while the mylar film allows a beam injection through the screen without
significantly affecting its quality The OTR yield within a cone of half-aperture θ for photons whose
wavelengths range between λ1 and λ2 is given by [44]
Thus for a standard CCD camera whose sensitivity ranges between 400 nm and 800 nm and assuming
an acceptance cone of 003 radian this expression gives ~0002 photon per electron at 50 MeV (station
DS2-5) A similar yield is reached at 5 MeV with an acceptance of 03 radians (station DS1)
To image the screens a lens will be located 500 mm away from them A small telescope made of two
lenses will be used to focus the light on the CCD camera At least one of the telescope lenses will be
movable to provide the needed dynamic range A Gigabit Ethernet camera with at least 600 x 400 pixels
will record the images The camerarsquos square pixels are about 74 x 74 microm2
With such a configuration one expects that a 50 MeV bunch passing through the YAG(Ce) screen will
emit 71x1012
photons and assuming that the CCD chip has an average efficiency of 50 1010
counts
will be recorded by the camera taking into account the solid angle covered by the lens
lsquo
84
The same bunch passing through the OTR screen will emit 82 107 photons within the lens acceptance
Thus 3 106 counts will be recorded by the camera
For a 5 MeV bunch (station DS1) the YAG(Ce) screen will emit 2 1012
photons and 109 counts will be
recorded by the camera The OTR screen will emit 82 107 photons within the lens acceptance and 3 10
6
counts will be recorded by the camera
Assuming a transverse beam size of 15 mm one expects 8 106 counts per pixels with a YAG(Ce)
screen and 25 105 counts per pixel with an OTR
If the 50 MeV beam is defocused (for example during a quadrupole scan) to produce a 5 mm spot size
these numbers become 8 105 for the YAG(Ce) screen and 2 10
4 for the OTR
Number of OTR
photons emitted
Number of
OTR
photons
recorded by
the camera
Number of
YAG
photon
emitted
Number of
YAG
photons
recorded
DS1 82 107 3 10
6 2 10
12 10
9
DS2-5 82 107 3 10
6 71x10
12 10
10
Assuming a YAG Cerium dopant density of 018 we get a Cerium density of 25 1019
cm-3
[45] and
a 15 scintillation efficiency It follows that the scintillating site density is 37 1018
cm-3
Therefore the
100 microm YAG(Ce) screen will saturate when the photon density reaches 36 1014
mm-2
This last figure
translates into an electron density of 32 1011
mm-2
for an e- beam energy of 50 MeV In these
conditions the YAG(Ce) screen will saturate if the beam is focussed on a spot smaller than 20 microm This
is five times less than the smallest beam foreseen at ThomX
Additional targets will be used to calibrate the cameras when necessary They will be made of stainless
steel plates with etched grids Each square of the grid will be 2 by 2 mm2 In some of these squares
smaller lines of known thickness will be etched Images of these targets will allow one to check the
whole optics
To minimize a possible radiation deterioration of the cameras lead-glass windows will be inserted
between the vacuum windows and the focusing lenses
43 Emittance measurement
The beam emittance will be measured with the quadrupole scan technique In this technique a triplet
(set of three quadrupoles) is used to find the minimum transverse beam size in each plane A fit of the
beam size variation versus the quadrupole strength enables one to retrieve the projected transverse
emittance
The sets of triplets and the diagnostic stations implemented along the ring allow an emittance
measurement at three different locations at the exit of the linac with DS2 in the transfer line with DS4
and in the ring with DS5 by using the extraction kicker The beam emittance measurement with DS5
lsquo
85
will provide a check of the turn-by-turn emittance growth thus bringing precious information on the
transverse beam dynamics
Pepper-pot measurements are also foreseen The advantage of this technique with respect to a
quadrupole scan is that it consists in a single shot measurement independent of machine operation
fluctuations
A pepper-pot set-up comprises a plate with evenly distributed holes Such a plate is placed in the beam
path together with a scintillating screen located downstream and an imaging system A pepper-pot
mounted on a movable arm could be located at the end of the linac between the last dipole and the
diagnostic station DS2 and another one could be located just before station DS5
At station DS2 the beam size will be about 15 mm (rms) and the geometric emittance about 5 10-8
m
rad Therefore the maximum beam divergence will be about 50 microrad By adjusting the quadrupoles one
may focus the beam down to a 20 microm spot and increase its divergence up to 250 microrad To measure the
transverse emittance one would have to position this focal point two meters before DS2 (or DS5) and
to position a pepper-pot mask with 100 microm holes or slits one meter upstream of DS2 (or DS5)
44 Beam position measurement
The beam position will be measured by a set of BPMs placed all along the ring Their locations are
shown in Fig 43
lsquo
86
Figure 43 Location of the BPMs along the ring (note the BPM located at the IP has been suppressed
since this drawing was made)
441 Beam Position Monitors (BPM)
Two types of BPMs will be used short circuited striplines in the linac and the transfer line and buttons
in the storage ring
Compared to button BPMs striplines generate a higher signal but need more space and contribute more
to the wake-field generating impedance For these reasons striplines are preferred for the linac and the
transfer line where we need single pass measurements and where wakefield is not an issue In the
storage ring button BPMs will be chosen for their small size and small contribution to the ring
impedance The lower signal produced by the small electrodes will be compensated by the high
repetition rate of the stored beam (averaging is possible)
BPM electronics detection frequency on the storage ring is usually equal to the RF frequency as it is the
only common frequency component present in the BPM data whatever the filling pattern is Even
though only one bunch will be stored in ThomX the same approach is chosen Therefore the storage
ring BPM electronics detection frequency is 500 MHz
Since it is more convenient to adopt the same electronics for all the BPMs (linac transfer line and
storage ring) stripline BPMs on the Linac and transfer line are optimized to give a signal with the
highest possible component at 500 MHz Their electrode length is thus adapted to this frequency which
means that it has to be equal to λ4 (150 mm 500 MHz) where λ is the RF wavelength
4411 BPM for the linac
The linac BPM equipment will be as follows
- Number of BPM 1
- Type short circuited stripline
- Electrode length 100 mm
- Required resolution 100 microm for a single pass measurement and a 1 nC bunch charge
Figure 44 Linac BPM
lsquo
87
Due to lack of space in the linac section where a stripline must be installed the electrode length cannot
be 150 mm but must be reduced to 100 mm Transfer impedance calculations show that at 500 MHz
the signal amplitude will be reduced by 13 compared to the optimal length of 150 mm (see Fig 45)
This amplitude reduction is acceptable
Feedthroughs will be soldered on CF16 flanges
Figure 45 Stripline transfer impedance versus frequency for various electrode lengths With an
electrode length of 100 mm the signal is reduced by 13 compared to its maximum value for a
frequency of 500 MHz
4412 BPMs for the transfer line
The transfer line will be equipped as follows
- Number of BPM 4
- Type short circuited stripline
- Electrode length 150 mm
- Required resolution 100 microm for a single pass measurement and a 1 nC bunch charge
lsquo
88
Figure 46 Drawing of the BPMs that will equip the electron transfer line
Feedthroughs will be soldered to CF16 flanges
4413 BPMs for the storage ring
The specifications relevant to the ring BPMs are as follows
- Number of BPM 12
- Type button
- Button diameter 107mm
- Required resolution ~1 microm 50 Hz and for a bunch charge of 1 nC
- Absolute precision plusmn 250 microm
In order to provide additional BPM signals for other applications (see below) 4 BPM blocks will be
equipped with a double set of electrodes In that case only 4 electrodes (among 8) will be used for
position monitoring The button size and the beam pipe size remain unchanged
Figure 47 Simple and double BPM blocks for the ThomX storage ring
The button positions and dimensions have been determined by simulations with Poisson Superfish (an
electrostatic simulation software) An evaluation of the impedance due to the button discontinuities has
shown that their effect will be negligible
Feedthroughs will be soldered directly to the BPM block They are bakable Connectors will be SMA
males to prevent any deformation during baking
lsquo
89
Alignment
BPM blocks will have an external mechanical reference This reference will be used for BPM alignment
during the installation phase and to position precisely the block in a calibration bench
Calibration
Storage ring BPMs will be calibrated by using a dedicated bench An antenna inside the block will be
fed with a 500 MHz wave of constant amplitude Calibration is done by measuring the signal output by
the 4 buttons A precise position of the antenna is not needed as position errors can be eliminated by
repeating the measurement with the BPM block turned by 180deg and averaging the results
The second halves of the blocks equipped with two sets of buttons will be dedicated to
‐ longitudinal (beam phase) feedback (one set of BPMs)
‐ transverse feedback (one set of BPMs)
‐ ion clearing To suppress ions which may generate beam instabilities BPM buttons will be
polarized with a high voltage (max plusmn200V) This electric field will deflect the ion trajectories
toward the vacuum chamber
Electronics
Electronics will be the same for all BPMs (whether equipping the linac the transfer line or the storage
ring)
Data rate
Revolution-by-revolution beam position measurements would be best But because of the high
revolution frequency (178 MHz) it might be difficult to get electronic devices that handle the
corresponding data rate Then one may work with averages over two turns This data will be provided
on demand and the acquisition will be triggered The buffer size should accommodate data collected
during at least 20 ms to cover full storage cycles Revolution-by-revolution data will be used for
machine physics studies and tune measurement The position measurement resolution must be better
than 100 microm
Slow acquisition
Slow data acquisition will be used for slow orbit correction and slow orbit feedback It will be obtained
by averaging the fast data flow with a sampling rate of ~10 Hz A data rate of 50 Hz would provide just
one measurement per storage cycle The resolution aimed at for this measurement is one micron or
better
Acquisition must be synchronous with the machine revolution frequency Moreover modules must be
synchronized together
Connections and cables
The four cables that carry the signals of each BPM should be put together in a strand Their electrical
lengths will be matched so that phase differences are kept below 13deg 500 MHz The cable attenuation
should not differ much from one BPM to another If the cable lengths cannot be the same for all BPMs
cable sections will be chosen in order to ensure the same attenuation for all cables To minimize both
lsquo
90
attenuation and cost cable lengths below will be kept below 30 m The LMR240 cable is an example
of what could be used its attenuation is ~6 dBm 500 MHz over a distance of 33 m
As radiation close to the machine can damage the cable insulator short (~1 m) pigtails (that can be
easily replaced if needed) will be inserted between the BPMs and the long cables
442 Orbit feedback system
A feedback system will keep the beam orbit stable within plusmn1 microm It will use the position measurements
provided by the 12 ring BPMs The correction rate will be comprised between 01 Hz and 10 Hz
Correction signals will be applied to the 12 correctors located along the ring The orbit feedback system
will be a matlab based application
443 Tune measurement and feedback
The tune will be obtained from the revolution-by-revolution data provided by the BPMs There is no
need for any kicker to excite betatron oscillations as the beam transverse position will naturally oscillate
after each injection (ie every 20 ms) If necessary a tune feedback will be implemented to keep the
tunes constant
444 Energy measurement
In the middle of the transfer line ie downstream of the dipole located at the end of the linac the
dispersive function is non-zero Therefore the beam energy and its spread can be deduced from the
electron horizontal distribution shown by the YAG screen in DS3 The same measurement will be
performed with DS4 using the dispersion induced by the extraction magnet
45 Bunch length measurement
The bunch length will be measured at two different places at the end of the linac and in the ring
Depending on the observation point two different physical processes are used to produce visible light
pulses which are then transported by dedicated optical lines through the ThomX shielding wall to a
double-sweep streak camera located in the experimental hall
451 Cerenkov radiation
Cerenkov radiation (CR) produced in a target placed at the end of the linac will be used to measure the
e- bunch length Cerenkov radiation is emitted when a charged particle passes through a medium at
velocity greater than the speed of light in that medium It is emitted at an angle θcr ndash with respect to the
particle momentum ndash determined by n(λ) the medium index of refraction and β the particle velocity as
follows
Above 10 MeV the emission angle is almost independent of the particle velocity Ncr the number of
radiated photons whose wavelengths range between λ1 and λ2 summed over a path of length d along the
charged particle trajectory is
( )
minus
minus
λnβλλπαd=N
cr 22
21
11
112
( )
minus
minus
λnβλλπαd=N
cr 22
21
11
112
( )( )λβn
=θcr
1cos
lsquo
91
where α=73 10-3
is the fine structure constant
Two types of radiator are usually used to produce Cerenkov radiation an aerogel or a solid target
Aerogels dramatically spoil the vacuum and therefore have to be mounted in a separate vacuum vessel
to maintain ultra-high vacuum conditions Solid targets are UHV compatible This is why they are
preferred
At the end of the linac when the bending magnet is off the beam goes straight into a beam dump
Before the dump several types of targets can be inserted in the beam trajectory for diagnostic purposes
For the production of Cerenkov radiation a sapphire target is adequate This materialrsquos index of
refraction is 177 With a 50 MeV beam the Cerenkov angle is θcr=5563deg Assuming that the window
thickness is 025 mm 16 photonse- are created with a wavelength ranging between λ1 = 300 nm and λ2
= 800 nm Their spectrum is given in Fig 48 For a 1 nC electron bunch 1011
photons are created The
energy deposited in such a target is negligible
Figure 48 Spectrum of the Cerenkov radiation emitted by a 50 MeV e- beam passing through of
sapphire window 025 mm thick
For simplicity the sapphire target is oriented so that the Cerenkov radiation exits in the vertical
direction For a Cerenkov angle θcr = 5563deg the window angle with respect to the beam axis must be θP
= 11475deg (see Fig 49)
8x10-8
6
4
2
0
In
ten
sity
[au
]
800700600500400300200
Wavelength [nm]
lsquo
92
Figure 49 Orientation of the sapphire target used to observe the Cerenkov light emitted by the e- beam
at the linac end
At the target location the beam dimensions will be σxy= 14 mm (rms) It will produce a light spot
whose dimensions will be σx = 14 mm (rms) and σy = 154 mm (rms) in the sapphire To intercept the
whole light beam and to leave some margin for the alignment we will use a target whose diameter will
be equal to 2 x 6 x σy ~ 20 mm
The Cerenkov radiation will exit the vacuum chamber via a UHV viewport made of fused silica (FuSi)
with a 90 transmission over the 300-800 nm wavelength range The viewport is mounted on a CF63
flange that provides a 68 mm clear aperture At the exit of the viewport a FuSi lens is used to focus the
light This collimated beam is then transported via a set of planar mirrors across the shielded area and
then through the ThomX shield to a dedicated hutch in the experimental hall (see sect 453)
452 Synchrotron radiation
Most of the synchrotron radiation (SR) emitted when the electrons circulate in a bending magnet (BM)
will be in the far infrared but visible light will also be present The cut-off wavelength of this radiation
corresponds to ~077 eV Thus the X-ray content is negligible This last feature makes the extraction of
the visible part of the spectrum easy The spectral flux per unit bandwidth per unit surface ΦBM
[Photons01bwmmsup2] is shown in Figure 410
Figure 410 On-axis spectral flux 1 m downstream of a bending magnet Photon energy in eV The
visible spectral range corresponds to [400 nm - 800 nm] or [155 eV - 31 eV]
The radiation is extracted at the exit of the BM ndeg5 through a porthole which provides a 63 mrad x 32
mrad aperture at 15deg with respect to the 0deg incident angle in the dipole The maximum power density is
lsquo
93
4 microWmmsup2 in the horizontal plane of this extraction porthole thus no dedicated absorber is required
(But for the porthole region the radiated power will be dissipated in the vacuum chamber wall) The
radiation will exit the vacuum chamber through a UHV viewport made of MgF2 (this material offers
the highest transmission coefficient over the 200-800 nm range) The synchrotron radiation distribution
at 5 eV one meter downstream of the bending magnet is around 6 mm (FWHM) So the spectral flux
integrated over a collecting area of 6 mm x 63 mm is about (around 5 eV)
Φe [Phs] = ΦBM [Phs01bwmmsup2] x 6 mm x 63 mm x 1000
Since photons of lower energy thus with a higher divergence will also contribute to the total spectral
flux this value gives a low limit estimate of the real total spectral flux Figure 411 shows the Φe
spectral flux
As was the case for the Cerenkov radiation a FuSi lens will be used at the exit of the viewport to focus
the beam The collimated radiation will then be transported via a set of planar mirrors across the
shielded area and then through the ThomX shield to a dedicated hutch in the experimental hall (see next
section)
Figure 411 Spectral flux Φe in a 63 x 32 mrad2 aperture 1 m downstream of a bending magnet
453 Transport of the Cerenkov and synchrotron radiations to the diagnostics hutch
Both the Cerenkov light and the synchrotron radiation have to be transported to the diagnostics hutch
located in the experimental hall Starting from different locations both radiations will be transported via
a set of planar mirrors to a lsquomeeting pointrsquo inside the shielded area A flip mirror (ie a double-
orientation mirror) will enable the selection of one of the radiations The one selected will be further
transported again with a set of planar mirrors through the shielding wall to the diagnostics hutch
Inside the diagnostics hutch the last lens will focus the radiation on the streak camera entrance The
lens aperture and its focal length will be adapted to the selected incoming radiation All the line
components and their parameters are chosen to minimize spatial aberrations and to maximize the
number of photons entering the streak camera
454 Streak camera for the bunch length measurement
We plan to use a double-sweep streak camera to measure the bunch length and to visualize its evolution
over short time scales Since the bunch length may vary from 4 to 20 ps (rms) a 2 ps (FWHM)
resolution streak camera is required
lsquo
94
The time resolution of the bunch length measurement depends on the radiation type (CR or SR) which
is used and involves several contributions the electron transverse beam size at the light emission point
chromatic effects etc All these contributions will be minimized as much as possible to maintain the
effective resolution as close as possible to the one of the streak camera
46 Bunch imaging using synchrotron radiation
The synchrotron radiation transported to perform the bunch length measurement (see section 45) can
also be used to image the bunch while it circulates For this purpose a beam splitter could be installed
at the radiation exit port the main part of the radiation would be sent to the streak camera while a small
fraction would be directed to a CCD camera mounted nearby A focusing lens placed between the
viewport and the CCD would enable the imaging of the stored beam
In the orbit section where the bunch emits the synchrotron radiation used for the above measurements
the e- beam transverse size is about 05 mm (rms) Since the size of a CCD chip is typically 5 mm the
lens gain should be about 10 Such a set-up would enable one to follow the evolution of the beam size
in real time (we will use a CCD whose acquisition rate is about 10 Hz) It would be a non-destructive
and cheap diagnostic Similar setups are currently used on storage rings to measure the electron beam
emittance assuming that the beta function is known
47 Beam loss measurement
Particle losses will be measured by using optical fibers [46] When a charged particle crosses a glass
fiber Cerenkov radiation is emitted within the fiber and propagates along the fiber It can be detected
by a photomultiplier that collects the light at the fiber end The measurement eg with an oscilloscope
of the pulse production (in the fiber) time enables one to recover the particle loss location
Two fibers will be installed One will be used to monitor e- losses along the linac and the transfer line
The second fiber will be used to monitor particle losses along the ring Each one of these fibers will be
connected to a fast photomultiplier (PMT) The PMT time resolution will determine the accuracy with
which the losses can be localized
A fused silica fiber has a yield of 227 photons per cm of a relativistic electron track [47 48] Therefore
a 5 mm thick fiber will have a yield of ~100 photons per relativistic electron which crosses it Such a
signal should be easily detected by a PMT
As it may be difficult to place the fiber close to the beam pipe in some parts of the ring the fiber may
have to be located at a distance d away from the beam axis Then the number of lost particles which will
cross the fiber will be reduced by a factor dsup2
48 Beam dumps
The ThomX set-up will include two particle dumps to stop the e- beam in such conditions that the
radioactivity then generated is minimal The first dump will be located at the linac exit The second
dump will be located downstream of the injectionextraction section The latter dump can be used to
stop the beam either at the exit of the transfer line or after any number of bunch revolutions in the ring
Both dumps are based on the SOLEIL design and can handle the 35 W power deposition (1 nC 70
MeV 50 Hz repetition rate)
lsquo
95
Chapter 5 SYNCHRONIZATION
51 Overall scheme
An ultra-low noise 500 MHz synthesizer will be used as the main clock for ThomX The 10 MHz
output of this synthesizer will be used to drive a second synthesizer (the so-called lsquolinac synthesizerrsquo)
operating at 3 GHz The RF output of the latter synthesizer will be used for the linac operation It will
be split in two lines The first one will be used to synchronize the gun laser oscillator (after appropriate
down conversion of the frequency) The second line will be used to drive the gun and the linac RF
The RF output of the main synthesizer (500 MHz) will be used for the ring RF it will be split in three
lines The first line will drive the ring RF (after appropriate phase shifting) The frequency of the second
line will be divided by 14 this line will synchronize the Compton cavity As in the MightyLaser
experiment [51] the laser used to inject power in the Compton cavity will be synchronized with that
cavity by using the PDH method The frequency of the third line will be divided by 28 and will
therefore be synchronous with the electron revolution frequency This line will be used to synchronize
programmable delays delivering turn-by-turn triggers and injectionextraction triggers To ensure that
injection is always taking place at a zero crossing of the mains the programmable delays will also be
synchronized with the mains A counter will reduce the mains frequency to the operations frequency
(when lower than 50Hz) The gun trigger will be generated when the mains will be at a zero crossing
and the ring RF at a preset phase that takes into account the time it will take for the electrons to travel
from the linac gun to the ring accelerating cavity
To ensure that the e- bunches will always be injected at a triple zero (50 Hz 500 MHz and 3 GHz) we
need to specify the tolerance on the 50 Hz zero crossing We may take 1 (ie 02 ms) for this
tolerance this means that the coincidence between the ring RF (~500 MHz) and the linac RF (~3 GHz)
must occur at a frequency greater than 5 kHz Therefore by choosing a heterodyne frequency (ie the
frequency difference between the linac RF divided by 6 and the ring RF) greater than 5 kHz we ensure
that there will always be a common zero crossing between the ring RF and the linac RF within 1 of
the mains zero crossing This sets a constraint on the ring radius of 10-5
ie 168 micrometres on a
168 m long orbit (see Table 2)
The RF bucket must be positioned with an accuracy of 3 ps in the ring RF Thus the phase noise
between the linac RF and the ring RF must be lower than 3 ps when integrated between 50 Hz and
500 MHz Most good-quality synthesizers achieve such a performance
The information collected by the ring diagnostics will be used to adjust the exact value of the frequency
(around 500 MHz) generated by the main synthesizer so that it matches the exact ring length This
information will also be used to adjust the phase between the injection trigger and the ring
To reach the correct signal level each RF line may have to be amplified An isolator will be added to
avoid reflections that may degrade the overall performance of the system
To ensure long-term low frequency synchronization a Network Time Protocol (NTP) server will be
available to allow all computers working at ThomX to synchronize their internal clock
lsquo
96
52 Specifications
Synthesizer
The main synthesizer will be used to generate a 500 MHz adjustable signal Its phase noise jitter
integrated between 50 Hz and 20 MHz must be less than a few picoseconds It must support a frequency
modulation of at least 100 kHz Typical mid-range synthesizers meet such specifications
Measurements done at LAL in collaboration with SOLEIL have shown that the addition of an external
frequency reference does not necessarily improve the phase noise of these synthesizers in the frequency
range of interest
The linac synthesizer specifications are the same ones except that it must be able to generate a
frequency of 3 GHz
Programmable delay
Two low jitters units with 8 channels each will be used
Phase shifter
The phase shifters must be voltage controlled and cover 360deg (two 180deg units)
Figure 51 Synchronization layout
lsquo
97
Chapter 6 OPTICAL SYSTEM DESIGN
Since the writing of the ThomX CDR three directions have been pursued improvement of the feedback
system improvement of the laser amplification and mechanical design of the optical resonator The
results are described in the following three sections
In the first section the ThomX feedback system is described During the past year a strong RampD
program has been developed on the locking of a TiSapph picosecond laser beam on a two-mirror high
finesse cavity at OrsayLAL Very good results in operating for the first time a 30 000 finesse cavity
in the picosecond regime were obtained The feedback techniques used for this task are directly usable
for ThomX and are reported in this section A 3000 finesse four-mirror cavity at KEK was also installed
and operated and we succeeded in producing γ rays off the ATF electron bunches (Mightylaser project
[61]) Hardware damages were however caused by the Japanese 2011 earthquake At the time of
writing this document we are recovering all our equipment to reach our final goal locking a 30000
finesse four-mirror cavity to produce a high γ ray flux at ATF Thanks to a contact with the SYRTE
laboratory we have acquired a new laser oscillator with a 1030 nm wavelength that we are testing and
that we plan to use for ThomX With this oscillator by embedding an intra-cavity EOM we are able to
increase our feedback bandwidth by more than one order of magnitude
The second section illustrates the laser amplification developments done at CELIA First a non-all-
connected fiber amplifier for the Mightylaser project was set up that allows us to extract an average
power of 50 W This amplifier was used to produce γ rays at ATF A new setup has been designed and
built to reach both the reliability and the power level required by ThomX An important activity is
devoted to the construction of a fully all-connected amplification chain As mentioned above the new
laser oscillator and the new laser amplification chain are being integrated in order to fill a 30000 finesse
high finesse cavity to conclude the ANR Mightylaser project
In the third section the details of the implementation of the cavity in the electron ring and the cavity
vacuum system are described with the opto-mechanical system that will be used for a precise alignment
of the cavity mirrors
61 Feedback system and results on a Fabry-Perot cavity locking
To produce a high X-ray flux the ThomX facility produces high intensity laser pulses and collides them
with electrons stored in a ring Hence the locking system (see figure below) has a double goal
- Stabilize the synchronization of the Fabry-Perot cavity (FPC) photon pulse with the electron bunch
- Stabilize the synchronization of the laser pulses with the FPC to obtain amplification
For this last point as will be seen in section 62 firstly the laser beam will be amplified with a CPA
technique using optically pumped fibers and then it will be amplified again by accumulation during
many round trips into a high finesse FPC (the target is a gain of 10000 which leads to a finesse of
30000)
This section describes the following topics related to the stabilization of the laser pulses relative to the
FPC
- Parameters to be stabilized repetition rate and carrier-envelop phase (CEP)
- Error signals the Pound-Drever-Hall (PDH) measurement technique
lsquo
98
- Actuators dynamic range and bandwidth
- Behavior of the digital feedback system (DFS)
General scheme for the locking system Detection and actuation elements are shown in red
The results on the locking of a TiSapph laser oscillator on a 30000 finesse cavity will also be
illustrated
Figure 61 Locking scheme for the synchronization of the electron bunch and the FPC pulse for the
ATF experiment at KEK
611 Electron and FPC pulse synchronization
This kind of stabilization has already been achieved for the MightyLaser experiment at KEK (ATF) a
1785MHz repetition rate A digital feedback system was used (i) to acquire two clocks one from the
laser and one from the master oscillator of the electron ring (ii) to perform a digital phase detection
(iii) to build a digital integrator filter to remove phase drifts and (iv) to react on a PZT inside the FPC
to change its length according to the measured phase drift (see Fig 61)
lsquo
99
612 Laser parameters to be stabilized
We will choose a laser with a repetition rate corresponding to twice the e- bunch revolution frequency
in the ring (two laser pulses per electron bunch revolution) around 500MHz14 = 357 MHz A phase
noise measurement on a similar product operating 1785 MHz was already carried out to characterize
the repetition rate phase noise and the drift (see Fig 62 and Fig 63)
Figure 62 Repetition rate phase noise spectrum Figure 63 Repetition rate drift measured
X-axis offset frequency from the carrier in Hz during about one hour X-axis repetition
Y-axis single side band phase noise power rate drift in Hz Y-axis time duration in
density in dBcHz seconds
The global shape of the phase noise in 1f will permit to use three feedback actuarurs a high bandwidth
low dynamic one (EOM for repetition rate and AOM or pump current for CEP) a middle
bandwidthmiddle dynamic one (PZT for repetition rate) and a high dynamiclow bandwidth one
(mirror motor and isochronic wedge motor)
For this kind of laser the harmonic content (index m) is well known and can be represented either in the
time domain or in the spectral domain (see Fig 63 and Fig 64)
( )2
CEL REP CE REPf m m f f m f
φ
π
Δ = + = +
lsquo
100
Figure 64 Time domain and spectral representation of the laser pulses
Since one is dealing with two parameters (the repetition rate and the CEP) one needs two different
sources of information to measure them The standard method to measure these two parameters consists
in using the Pound-Drever-Hall (PDH) technique [62] which gives the position of the laser harmonics
relative to the FPC resonances
( )FPC CAVf m m f=
One of the parameters is deduced from the whole beam signal using a central photodiode the second
one is derived from a part of the spectrum selected by a grating and a slit and measured by the grating
photodiode
If the repetition rate and the FPC round trip frequency are close enough the error signal coming from
the central photodiode (index m0) is proportional to the difference between the laser harmonics and the
corresponding resonances of the FPC
1 0 0
2
CEREP CAVm f m f
φε
π
Δ prop + minus
Frequency
domain
lsquo
101
As the locking system cancels the error the central laser harmonic m0 is perfectly aligned on the FPC
resonance but all other laser harmonics (m=m0+Δm) are slightly detuned from the corresponding FPC
resonances This introduces a correlation between the CEP and the repetition rate
( )( )
1 0 0
2 0
0 0
02
1 12
CEREP CAV
CEREP CAV REP CE
m f m f
m mm m f f f f
m m
φε
π
φε
π
Δ = rArr + =
Δprop minus minus = minus = minus
As a result of these equations the signal received by the PDH system located after the grating (ε2) is
directly proportional to the offset frequency (fCE) and then to the CEP Thus if we change the CEP of
the laser we should clearly see on the grating photodiode PDH signal ndash which measures the laser-FPC
detuning on the border of the spectrum (for an m index far from m0) ndash a linear relation between the error
signal and the CEP
This method well known for femtosecond lasers has been tested in our laboratory in the picosecond
regime Figure 65 shows our results (submitted to Applied Optics) on the locking of a 76MHz
TiSapph picosecond laser oscillator on a 30000 finesse cavity
Figure 65 Top experimental measurement of the CEP value of a TiSapph laser oscillator Bottom
measurement of the corresponding grating photodiode signal
In Fig 65 one sees that when the CEP is measured close enough to zero the grating photodiode
exhibits a linear relation with the CEP otherwise the photodiode is saturated (due to the PDH signal
saturation) In our experiment the ratio between the cavity reflected power and the incident beam
power was ~20 which leads to a value of 80 for the coupling of the incident beam power to the
cavity
These results demonstrate for the first time that a picosecond laser oscillator can be locked to a 30000
finesse cavity (implying a power enhancement factor of 10000) with a power coupling of 80
613 PDH Measurement technique
Having shown that we are able to measure independently both parameters of interest (the repetition rate
and the CEP) we will now describe the apparatus to transform these measurements into error signals by
lsquo
102
the PDH technique (see Fig 66) Side-bands to the laser optical harmonics are produced by an electro-
optic modulator (EOM) modulated by a generator coming from the digital system Side-bands and laser
harmonics are reflected by the FPC the first ones without acquiring any phase the second ones shifted
in relation with the discrepancy between the laser and the FPC frequencies All the harmonics and the
side-bands of the reflected laser are combined into the photodiodes and the front-end electronics giving
an error signal at the modulation frequency
Figure 66 Pound-Drever-Hall scheme with digital phase detection
The photodiode front-end (see Fig 67) amplifies these signals at a level compatible with the digital
feedback system (DFS)
Figure 67 Electronic scheme and picture of the PDH front-end
Inside the DFS the PDH signals are filtered by a band-pass filter to remove unwanted frequencies and
multiplied by the same modulating signal with a phase shift to allow for the phase detection A low-pass
filter at the end of the chain removes the image signal at twice the modulation frequency Figure 68
shows an example of the input and the output signals
lsquo
103
Figure 68 Modulated input (blue) error signal (yellow) FPC transmitted signal (pink)
614 Actuators
In the previous sections we described how the two parameters (the repetition rate and the CEP) of a
pulsed laser have to be controlled in order that all the laser harmonics match the FPC resonances and
we also detailed how to measure the variations of these two parameters We will now describe the
devices inside or around the laser on which we need to react to correct the variations of the laser
harmonics relative to the FPC resonant frequencies
Figure 69 Typical scheme of a laser with all its actuators
The actuators will produce an action on the two parameters to be controlled with different dynamic
ranges and different bandwidths (see chart in Fig 610)
Figure 610 Dynamic ranges and bandwidths of the laser actuators
lsquo
104
All of the actuators are located inside the laser except the AOM (see Fig 610) They are the key items
to achieve a good stabilization
All the actuators measurements have been made on the 1785MHz laser version used at KEK
- Mirror motor dynamic range precision and bandwidth
The motor is a step motor with a precision of 140 nm on the round trip variation (equivalent to 15 Hz on
Frep) Figure 611 shows the linearity and the maximum dynamic range available with the motor stage
while Fig 612 shows a step response which indicates the maximum bandwidth we can obtain
Figure 611 Mirror motor range Figure 612 Time response of Frep for
1k motor steps X-axis repetition rate
offset (Hz) Y-axis repetition rate
The repetition rate range is 178033 - 179244 MHz (gt 1MHz)
- Piezoelectric sensitivity dynamic range and bandwidth
The PZT sensitivity is approximately 3Hz V 28 nmV (length variation to be compared to the
length of a round trip of the FPC pulse)
The PZT can be operated up to 150V providing a total dynamic range of 42 microm (round trip length
change) Its first peak resonance has been measured and found above 40 kHz
lsquo
105
Figure 613 Repetition rate change produced by a Figure 614 PZT frequency response X-
PZT voltage variation from 0 to 28 V frequency range 01-5 kHz Y-axis
Attenuation (dB) ndash arbitrary offset
Figures 613 and 614 show the PZT sensitivity and its transfer function
- EOM sensitivity measurement
Measuring the sensitivity of the EOM is not direct thus we chose to apply a 10 V amplitude ndash 10 Hz
frequency modulation ndash to the EOM and to measure the power of the harmonics (see Fig 616)
Assuming a pure frequency modulation the power of the harmonics fis a Bessel series that depends on
the sensitivity Thus taking the ratio of two consecutive harmonics gives directly the sensitivity
Figure 615 EOM modulation spectrum used for sensitivity measurement X-axis frequency centered
on the repetition rate Y-axis relative optical power around the repetition rate due to the modulation
The table on the left gives the frequency and the power of each harmonic
0 5 10 15 20 25 300
10
20
30
40
50
60
70
80
90
100
PZT voltage (V)
Fre
p v
ariation (
Hz)
Frep variation vs PZT voltage
AJ0(z)
AJ1(z)
AJ2(z)
lsquo
106
We now check that the sensitivity calculated from the ratio of the first harmonic to the fundamental and
the one obtained from the ratio of the second harmonic to the fundamental are the same
∆
∆
~1
2
∆
10~ minus 2909 rarr ∆~07
∆
∆
~1
8
∆
10~ minus 6398 rarr ∆~07
Since the ratio is 07 Hz for an applied sinusoidal signal of 10 V the sensitivity is around 007 HzV
- Isochronic wedge
It is made of two glass prisms with different refraction indexes chosen to maintain a constant group
delay in order to avoid repetition rate variation
We measured that the maximum variation of the repetition rate for a full CEP range (several times 2π)
is about 300 Hz for a repetition rate of 1785 MHz (see Fig 616 which shows the measured repetition
rate shift plotted along the X-axis versus the date of the measurement plotted along the Y-axis) Thus
the influence of this actuator on the repetition rate is about 210-6
Figure 616 Measurement of the influence of wedge displacement on Frep
- Acousto-Optic Modulator (AOM)
Such an actuator has already been used with a laser which is not the same as the one used for the other
actuators namely a 76 MHz laser locked on a two-mirror FPC
In Fig 617 we present the closed loop measurement The lowest frequency part (lt10 kHz) is
attenuated by the loop gain If the loop were open this part of the transfer function would be quite flat
To estimate the AOM bandwidth limits we can use the frequencies where the gain starts to drop
significantly this bandwidth is about 300 kHz
300 Hz
lsquo
107
Figure 67 Frequency response of the AOM in closed loop
615 The digital feedback system
The feedback system will deal with several coupled inputs and outputs To ensure flexibility we have
chosen a digital system Since we also want to work with a large bandwidth in closed loop (gt 1MHz) a
low latency system (lt 100 ns) is needed A digital system featuring such performance does not exist on
the shelf Thus we decided to design and build such a system at LAL (see Fig 618)
Figure 618 Scheme of the DFS and picture of the DAC daughterboard
This system will perform several functions (see Fig 619)
103
104
105
106
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
Frequency (Hz)
Magnitude (
dB
)
Bode plot
lsquo
108
Figure 619 Organization of the digital feedback functions
- The first important function is to produce the error signal from the modulated signal coming from the
photodiodes It is a phase detection block (Digital PDH DPDH) the main problem being to reduce the
latency as much as possible This will be achieved by optimizing the low-pass filter and the modulation
frequency (see Fig 620 for a digital implementation)
Figure 620 Digital phase detection (DPDH) to produce the PDH error signals
- The second important function is to provide Infinite Impulse Response (IIR) filters to build
Proportional-Integrator (PI) filters in order to cancel the system drifts
lsquo
109
Figure 621 Digital IIR filter implementation with adders and multipliers
These filters can be programmed by the user to change the filter coefficients and it can be cascaded to
realize complex filters They have been cut in Second Order Sections (SOS) to minimize the
propagation of fixed precision errors due to quantized data and coefficients (see Fig 621)
- The last important function integrated in the digital system is performed by an identification block
which works like a network analyzer by (i) sending a small harmonic signal (ii) looking at the system
response in closed loop at this frequency and (iii) sweeping this small harmonic signal frequency to
cover all the range to be measured (see Fog 622)
Figure 622 Identification system
lsquo
110
This kind of identification system can provide information on (i) the feedback system itself (ii) the
system to be stabilized or (iii) the open and closed loop transfer functions by producing Bode or
BlackNichols diagrams (see Fig 623) These informations are very useful for system analysis and for
tuning or designing the feedback filters
Figure 623 Typical Bode (top-left magnitude bottom-left phase) and Black-Nichols (right) diagrams
produced by the identification system
62 High average power laser system and its coupling to a high gain FP cavity
The all-fibered laser system will draw from the system developed in the MightyLaser project It will
consist in an oscillator associated with three amplifying stages involving single and double clad Yb-
doped fibers (see Fig 625) At present the amplifiers are based on commercial components to facilitate
a potential replacement of damaged parts The architecture is designed to provide the mandatory
diffraction-limited beam This relatively simple and compact architecture will implement the lsquoChirp
Pulse Amplificationrsquo (CPA) scheme In order to limit non-linearity effects in the fiber the signal is
initially stretched in time thus reducing its intensity in the waveguide then amplified in the fiber and
finally recompressed to its minimal duration The difference between the MightyLaser and the ThomX
repetition frequencies (respectively 1785 MHz and ~357 MHz imposed by the different repetition rates
of the two accelerators) requires an adjustment of the laser design
Figure 625 Sketch of the all-fiber laser system
lsquo
111
621 The MightyLaser oscillator
This oscillator is an all-normal dispersion mode-locked fiber laser based on the nonlinear polarization
evolution technique equipped with a fibered output It uses an ytterbium doped fiber which allows an
emission centered around 1030 nm with a full width at half maximum of 10 nm (see Fig 626) It
delivers short pulses at a repetition rate of 1785 MHz with a low average output power (~10 mW) in
order to minimize the noise level (see Fig 628) The main advantage of the all-normal dispersion
regime resides in the significant peak power reduction provided by the pulse stretching during the
propagation along the cavity Non-linear effects are thus reduced allowing a high-energy extraction
The output pulses are naturally chirped with an estimated duration of 2 ps
Figure 626 Emission spectrum of the oscillator
The oscillator is passively mode-locked by taking advantage of the non-linear evolution of the
polarization This technique is based on the optical Kerr effect The polarization of a pulse propagating
in a fiber evolves non-linearly as a function of intensity Thus by combining polarization controllers
and a polarizer one may generate losses that depend on the intensity (see Fig 627) Beyond the
polarizer all the pulse components are linearly polarized The wave plates transform the linear
polarization into an elliptic one The latter polarization state then evolves in a non-linear way while it
propagates along the fiber This evolution results from the combined effect of the self-phase and the
cross-phase modulations induced on the two polarization components The polarization rotation of the
more intense central part of the pulse is larger than the polarization rotation of the pulse wings Since
the pulse center crosses the analyzer with a reduced loss compared to the wingsrsquo loss the pulse duration
is shortened
Figure 627 Principle of mode locking based on the non-linear polarization evolution
lsquo
112
Figure 628 Spectrum of the oscillator phase noise noise level (dBcHz) versus offset frequency (Hz)
622 Amplification stages
The amplification chain is divided in three stages in order to distribute the gain and to saturate the
second and the third amplifying stages The first and the second stages are based on the same
architecture It is an all-fibered amplifier whose central component is a polarization maintaining single
mode using an Ytterbium doped fiber with a core diameter of 6 microm and a clad diameter of 125 microm (see
Fig 629) A wavelength division multiplexer (WDM) is spliced on the doped fiber to transfer the pump
power into the gain medium and to couple the signal of the picosecond oscillator to the amplifier The
first stage is pumped by a single mode diode centered at 976 nm and delivering 200 mW The second
one is pumped at a higher average power namely600 mW The two amplifying stages are separated by
an optical isolator to avoid any parasitic feedback from one amplifier to the other
Figure 629 Schematic of the first and the second amplification stages
The average power level at the input of the first amplifying stage is about 1 mW A limitation is
imposed on the output of this stage which should remain below the damage threshold of the isolator
fixed at 100 mW Consequently we adjust the pump level in order to obtain 80 mW at the output of the
ytterbium-doped fiber The 3 dB losses of the isolator allow one to inject 40 mW into the second
amplifying stage The evolution of the average output power versus the pump level is shown in Fig
630 For the maximum pump level of 600 mW the second amplifying stage delivers 330 mW
lsquo
113
Figure 630 Second stage output power plotted versus the pump power
Beyond the second isolator the third amplifying stage (see Fig 632) is built by using as its central
component a polarization maintaining double-clad Ytterbium-doped fiber with a core diameter of 25
microm and a clad diameter of 250 microm The amplifier is clad-pumped through a special combiner 6+11
(see Fig 631) In practice this component has an input signal fiber 6125 microm directly spliced to the
previous amplification stages an output signal fiber 25250 microm spliced to the double-clad Ytterbium
doped fiber and six multimode fibers of 105 microm to couple the pump diodes Here only three pump
ports are spliced to multimode diodes emitting up to 55 W at 976 nm corresponding to a pump power of
at most 165 W The development of a mode field adapter was necessary to splice the pump combiner to
the double-clad Ytterbium-doped fiber because their numerical apertures are different Losses entailed
by this component are compensated by the last amplification stage
Figure 631 Pump combiner Figure 632 Schematic of the third amplification stage
Figure 633 is a plot of the average output power versus the pump level For a pump power level of
76W the last amplification stage delivers 52W This was the first objective The fiber laser power
stability was measured during 1 12 hour at this power level (see Fig 634) The residual pump power
the pump and signal feedbacks into the pump diodes are also plotted in this figure
lsquo
114
Figure 633 Third stage output power Figure 634 Laser stability at 52 W
versus the pump power
New measurements were made after optimizing the central wavelength of the multi-mode pump diodes
The efficiency was increased to reach 787 for an output power of 24 W (see Fig 635) The fiber
laser power has been tested at this level during two hours (see Fig 636) The observed power variation
is less than 01
Figure 635 Output power versus pump power Figure 636 Laser stability at 24 W
after wavelength optimization
The all-fibered amplifier setup has been integrated in a 19rsquorsquo rack as shown in Fig 637 An FCPC
connector placed in the front panel allows the connection of the fibered oscillator to the master
amplifier module Several 955 optical fiber couplers have been inserted in our setup at strategic
locations to monitor the power level in real time by using photodiodes they may also signal potential
problems The output fiber comes out of the module through its front panel The back-side is dedicated
to all the electronic controls (diode pump photodiodes and modulators for noise management described
elsewhere)
lsquo
115
Figure 637 Pictures of the 52 W all-fibered setup integrated in a 19rsquorsquo rack
623 Stretcher and compressor
A chirp volume Bragg grating (CVBG) is used in our setup to stretch and compress the pulse with a
very good efficiency of 94 The CVBG transmission bandwidth is around 2 nm (see Fig 638) leading
to a pulse duration ranging from 200 to 250 ps depending on the grating dispersion It is possible to
recompress the pulse after amplification by using a second CVBG Then the pulse length ranges
between 20 and 87 ps depending on the second CVBG dispersion (see Fig 639)
Figure 638 Transmission bandwidth of the CVBG
1‐2 couple 3‐2 couple 3‐1 couple
Figure 639 Time profile of the recompressed pulses for different grating pairs
624 ThomX development
Some changes must be made to the all-fibered lsquoMightylaserrsquo design in order to use it in the ThomX
project The oscillator repetition rate needs to be scaled down to match the frequency imposed by the
accelerator As far as the laser design is concerned the stretchercompressor couple must be modified to
limit non-linear effects The increase of the output power from 50 to 150 W imposes an upgrade of the
second amplification stage and eventually of the third one
lsquo
116
6241 Oscillator
The oscillator for ThomX will be based on the same principle as the one used for Mightylaser The only
difference is the repetition frequency imposed by the accelerator (357 MHz for ThomX) The low
average output power around 5 mW permits to limit the phase noise A particular attention will be paid
to the noise level of such a long cavity as no commercial products have been tested at this repetition
rate
6242 Amplification stages
For the ThomX project the lower repetition rate and the higher average output power require a change
of the second amplification stage architecture in order to limit non-linear effects and to saturate the last
amplification stage Consequently the second pre-amplifier will be based on a polarization-maintaining
double-clad Ytterbium doped fiber with a core diameter of 15 microm and a clad diameter of 130 microm (see
Fig 640) The amplifier will be clad-pumped through a special combiner 2+11 with an input signal
fiber 6125 microm directly spliced to the previous amplification stages an output signal fiber of 15130
microm spliced to the double-clad Ytterbium doped fiber and two multimode fibers of 105 microm to couple
the 9 W pump diodes
Figure 640 Schematic of the second amplification stage of the ThomX laser
Concerning the final amplification stage various fibers are available (see below) Micro-structured
fibers can be engineered and tailored but suffer from the lack of splicing techniques Hence their use in
an all-fibered setup will require additional developments dedicated to splicing (see Fig 641) and
integration If splicing is not possible we still have the possibility of proceeding with air propagation
and injection However this approach requires precise mechanics control and advanced engineering
The lsquocoherent beam combining techniquersquo will also be considered at this point In such a configuration
amplification would be supported by two parallel amplification stages dividing by two the energy level
in the gain medium Yet a first version will be developed with commercially spliceable flexible fibers
and preliminary tests of cavity locking with high power from these fibers will be performed to validate
or invalidate operating with a high level of non-linear effects
lsquo
117
Fig641 A micro-structured fiber
6243 Stretcher and compressor
Because of the high average output power required by ThomX we must modify the stretcher size Two
techniques can be considered First we may continue to use a CVBG which can stretch the pulse up to
450 ps with a maximum size of 25 cm (see Fig 642) It would be possible to stretch the pulse up to
900 ps with an efficiency of 88 through a double pass in a CVBG or a single pass in cascaded CVBG
The second way involves a chirp fiber brag grating (CFBG) as depicted in Fig 643 When associated
with an optical circulator it allows a pulse stretching with up to 800 ps in an all-fibered setup The
efficiency is around 70 (92 for the CFBG and 75 for the circulator) in this configuration This
solution is less efficient than the CVBG but the all-fibered configuration ensures the set-up robustness
and stability and the losses can be easily recovered by the amplifier chain However dispersion issues
have been reported for these devices and a detailed investigation is needed prior to a definitive
integration
Fig 642 Examples of CVBG Fig643 Example of CFBG
The high output average power forces us to use a CVBG to compress the pulse one or two units
depending on the stretcher configuration The final pulse duration can be varied between 1 ps and 50 ps
thus leaving the possibility to optimize the X-ray production by adjusting the pulse duration
We also performed simulations to evaluate the influence of the couple stretchingfiber core diameter on
the non-linearity with 3 different fiber cores 25 microm 30 microm and 40 microm Our results are summarized in
Table 61
lsquo
118
Fiber
dimensions
mode field
diameter stretching B-Integral
25250 20 microm 250 ps 47 rad
25250 20 microm 500 ps 263 rad
25250 20 microm 1 ns 118 rad
30150 22 microm 1 ns 09 rad
40200 30 microm 1 ns 0525 rad
Table 61 Pulse stretching as a function of the fiber core diameter (simulations)
A first configuration will be developed with a stretching of 500 ps If the non-linearity effects remain
too high to allow for an efficient locking on the Fabry-Perot cavity we will modify the setup to induce
a larger stretching factor or insert a fiber with a larger core diameter in the last amplification stage or
both
63 Mechanical design
Collisions between the laser pulse and the stored electrons will take place between two dipoles (see Fig
643) The constraints on the optical cavity mechanical design are as follows
- High stability level (vibrations)
- Good accessibility to the cavity mirrors
- Weak impact on the electron beam dynamics
- Ultra high vacuum (UHV) system with baking capability
- Motorized granite table for micrometric scan oof the electron beam
lsquo
119
Figure 643 Implementation of the cavity in the electron ring
The cavity mirror system is mounted on a massive granite table which can be driven to perform a
micrometric electron beam scan (see Fig 643)
lsquo
120
Figure 644 Scheme of the Fabry-Perot cavity mounted on the optical table
The Fabry-Perot cavity is composed of two cylindrical UHV vessels linked by a dipole chamber
Bellows are placed between each element to absorb the dilatation due to the backing (see Fig 644)
e‐
e‐
X‐Ray
1rsquorsquoMirrors
IP
Bellows
1rsquorsquoMirrors
Vessel
Vessel
Dipole
chamber
lsquo
121
Figure 645 On the left detail of the standard mirror assembly and on the right detail of the piezo
electric mirror assembly
The Fabry-Perot cavity is composed of two spherical and one flat mirror mounted in a standard
assembly and one flat mirror mounted on a special assembly with a piezo electric actuator for very
small longitudinal displacements (see Fig 645)
Figure 646 Detail of a truck system
Mirrors
Piezo
electric
actuator
Mirror
Gimbal
Actuators
lsquo
122
Figure 647 Cut of an UHV actuator
All the mirrors are mounted on a truck system that porvides three degrees of freedom (two rotations and
one translation) driven by three micrometric actuators These actuators are specially designed to be
compatible with UHV and backing constraints (see Fig 646 and Fig 647)
Figure 648 Cut of a vacuum vessel
Trucks (two trucks per vacuum vessel) and plugs with special cables composed of ceramic connectors
oxygen- free copper cables and ceramic pearls and spacers for electrical isolation are placed in a UHV
vacuum vessel One of the vacuum vessels has a beryllium window to extract the X-ray beam (see Fig 648)
Micrometric Actuator
Capsule
Trucks
Beryllium
window
lsquo
123
631 Optical cavity vacuum
A simulation was carried out in the most unfavorable case when the whole optical vacuum chamber is not
baked The area of one half of the optical cavity is about 2 m2 the conductance of the rectangular chamber
connecting the two cavity halves is C =70810-1
m3s
-1 (value for one half of that rectangular chamber) The
conductances of the slits connecting the optical cavity to the ring are respectively C = 45210-2
m3s
-1 (9 x 48)
and C = 16010-2
m3s
-1 (6 x 30) The outgassing rate of one optical cavity half without any baking (σ = 210
-8
Pams-1
) is Q = 410-8
Pam3s
-1 The outgassing rate of one rectangular chamber half (σ=10
-9 Pams
-1) is Q =
7610-10
Pam3s
-1 The outgassing rates of the two slits (σ=10
-9 Pams
-1) are respectively Q = 22810
-13
Pam3s
-1 (9x48) and Q=14410
-13 Pam
3s
-1 (6x30) The diode ion pump used for the optical cavity has a
pumping speed of 05 m3s
-1 DN 150 S = 0320 m
3s
-1 at ~10
-10 mbar The conductance of the pumping port
being C = 136 m3s
-1 the effective pumping speed is Seff= 0278 m
3s
-1 The diode ion pump used for one half
of the rectangular chamber has a pumping speed of 015 m3s
-1 DN 100 S = 0090 m
3s
-1 at ~10
-10 mbar The
conductance of the pumping port being C = 0046 m3s
-1 the effective pumping speed is Seff= 0075 m
3s
-1
The outgassing rate of the optical cavity without baking is very high 2 x 410-8
Pam3s
-1 (which is
approximately 40 times the outgassing rate of the whole ring) With such a rate one would need a pumping
speed of 8 m3s
-1 to reach the required pressure level of 10
-8 Pa (the present pumping speed is 07 m
3s
-1) The
conductance of the rectangular chamber that connects the two halves of the optical cavity (710-1
m3s
-1) and
the conductance of the two slits between the optical cavity and the ring (610-2
m3s
-1) are high These figures
do not allow a differential pumping between the cavity and the ring Simulations of the ring vacuum level
show that it is mandatory to bake the whole optical cavity
Figure 649 Location of the cavity pumps
The vacuum equipment needed for the optical cavity is as follows
‐ 2 ion pumps 150 l s-1
with their baking equipment and their power supplies
‐ 2 ion pumps 500 l s-1
with their baking equipment and their power supplies
‐ 2 gate valves manually actuated (pipe diameter DN100) and their baking equipment
‐ 4 Pirani gauges
‐ 4 cold cathode gauges
‐ 2 power supplies for the gauges
‐ 4 residual gas analyzers (RGA)
‐ 1 baking equipment for the vacuum chamber
‐ 4 angle valves manually actuated for pre-pumping (pipe diameter DN63)
lsquo
124
Chapter 7 X-RAY BEAM LINE AND EXPERIMENTS
Introduction
The equipment of the X-ray beam line should firstly ensure a continuous detection and monitoring of
the beam (mainly in the first optical hutch) and secondly allow a complete characterization of the
beam (in the experimental hutch) This characterization consists in measuring the beam absolute
intensity its total energy range its spectral and angular distributions and its degree of transverse
coherence Time analyses of the bunched X-ray beam will also be performed This line also aims at
confirming the ThomX assets in such fields as cultural heritage biomedical and materials sciences by
performing demonstrative experiments
71 Available extracted X-ray beam
The X-ray beam is extracted through a 50 mm diameter beryllium window as schematized in Fig 71
Because of limitations due to the dimensions and the location of the Fabry-Peacuterot cavity mirrors the X-
ray beam available for characterization and experimentation is a cone with a half-aperture of 16 mrad
Figure 71 The X-ray beam extracted from ThomX through a beryllium window The beam fills a cone
whose total aperture is 2 x16 mrad
In the case of a lsquoperfectrsquo beam (ie without taking into account the electron beam emittance) the X-ray
spectral and energy distributions are shown in Fig 72 as a function of the scattering angle for two
electron beam energies 50 and 70 MeV and a laser wavelength of 106 microm In Fig 73 the energy
bandwidth and the flux per unit solid angle are shown for on-axis Compton photons produced by either
a 50 MeV or a 70 MeV electron beam
72 Global view of the X-ray lne
As can be seen in Fig 74 the positioning of the machine in the Igloo imposes the splitting of the X-ray
line into two sections a first optical table (3x2 m2) located just after the beryllium exit window and a
second table (4x2 m2) located outside the Igloo in hall D1 Along its path and until the optical table 2
the beam is transported in the primary vacuum inside stainless steel pipes with the exception of a short
upstream section of a few tens of cm between the beryllium window and the optical table this
insulating section will prevent the transmission of harmful vibrations to the Fabry-Peacuterot cavity This
primary zone is kept oxygen free by a nitrogen flow that prevents ozone production
lsquo
125
The beam diameter at the entry of the first optical table is 45 cm After this first optical table the beam
reaches the second table via a 13 cm diameter beam pipe inserted in a hole which was drilled long ago
in the Igloo wall About 8 mrad beam angular divergence are preserved at the entry of the second
optical table where the beam diameter is 13 cm
Figure 72 X-ray spectral distributions (left panel) and X-ray energy (right panel) versus the scattering
angle for an e- beam of 50 MeV (black) and 70 MeV (red) The laser wavelength is 106 microm Grey
areas correspond to emission angles larger than 16 mrad
Figure 73 X-ray flux (left panel) and X-ray energy bandwidth (right panel) of on-axis photons per unit
solid angle versus the emission angle for an e- beam of 50 MeV (black) and 70 MeV (red) The laser
wavelength is 106 microm
lsquo
126
Figure 74 Global view of the X-ray line in ThomX
73 Chraracterization of the X-ray beam
731 Optical table 1
A global view of the optical table 1 design is presented in Fig 75 Just upstream of this optical table a
60 mm diameter beam shutter activated when one needs to take background data will allow stopping the
beam A few millimeter thick tungsten is sufficient to stop the full beam (tungsten attenuation mass
factor microen ~ 4043 cm-1
for 100 KeV photons) This shutter does not have to be cooled since the ThomX
radiated power will not exceed 100 mW A drawing of the shutter mechanical design is shown in Figure
76 (a)
Figure 75 Global view of the optical table 1 showing the location of the main devices placed on it
lsquo
127
Beam monitoring
The optical table 1 equipment (see Fig 75) is mainly devoted to the detection and the continuous
monitoring of the X-ray beam The first device monitoring the beam presence is a fluorescent screen
mounted on a two-position motor (up or down) as shown in Fig 76 (b) It is inserted in the beam path
and viewed by a CCD camera Its screen absorbs almost the totality of the beam and thus has to be put
in the up position when not in use Downstream of the fluorescent screen a device composed of two Si
PIN diodes measures the beam scattered by a thin Kapton foil inserted in the beam path This setup
shown in Fig 76 (c) will monitor the beam flux variations and beam position displacements The
Kapton foil is fixed at an inclination angle of about 60o with respect to the beam path The sum and the
difference of the two diode currents will provide a relative measurement of the beam flux and of the
beam position respectively This set-up which is nearly transparent to 20-90 keV radiations is a non-
intrusive monitor and can be used continuously Then a high precision double slit [71] intercepts the
X-ray beam and allows one to let through only the desired part of the Compton cone (see Fig 76 (d))
The minimum dimensions of the slit aperture are 10 microm in the horizontal and the vertical directions
Beyond the double slit a wire monitor [72] shown in Fig 76 (e) is inserted in the beam This device
consists in two tungsten wires stretched horizontally and spaced by a few tens of a millimeter This
monitor will be driven vertically through the beam The difference of the two wire signals allows the
measurement of the absolute beam position
Figure 76 Drawings of the X-ray line devices located on the optical table 1 (a) Beam shutter
(b) Motorized two-position fluorescent screen (c) Kapton foil and Si diodes (d) Double slit (e) Wire
profiler
lsquo
128
Focusing device
Downstream of the beam monitoring instruments a focalizing device will be installed (see Fig 75) to
make the X-ray beam parallel andor to focus it on a sample It can be either a device based on
compound refractive lenses [73] (a so-called lsquotransfocatorrsquo) or two bendable mirrors in the Kirkpatrick-
Baez configuration (KB Optics) or a capillary optical device These three techniques will be
investigated to evaluate the advantages and disadvantages (aperture efficiency focusing strength etc)
of their eventual implementation in the ThomX facility Such a focusing apparatus has to be located as
close as possible to the X-ray source because of the rapid growth of the beam transverse dimension
Finally at the end of the optical table 1 some space is kept free for any specific study which might
concern optics detectors or samples
732 Beam transport
As shown schematically in Fig 77 beyond the optical table 1 the X-ray beam crosses the Igloo wall
inside a beam pipe that runs through a 15 cm diameter hole and reaches the D1 hall where the second
optical table is installed in the lsquoexperimental hutchrsquo At the entrance of the wall hole a second beam
shutter in lead around 15 cm in diameter and 25cm thickness is installed This shutter is connected to
an interlock system that ensures radioprotection It absorbs radiations coming from the electron
machine which propagate through the Igloo cylindrical hole and allows X-ray users to work in the
experimental hutch without stopping the electron machine
Figure 77 Schematic of the X-ray beam transport across the Igloo wall from the optical table 1 located
in the Igloo until the optical table 2 located in the D1 hall
733 Optical table 2
Several kinds of instruments will be placed on the optical table 2 The goal is to study in detail the X-
ray beam produced by ThomX firstly to determine how the beam characteristics depend upon the
machine operation parameters and secondly to prepare the demonstration experiments A global view
of the optical and detection instruments placed on that table is provided by Fig 78
lsquo
129
Figure 78 Global view of the optical table 2 indicating the main instruments that will be used to
characterize the beam
Absolute and spectral flux measurement
The measurement of the X-ray beam absolute flux and of its energy spectrum will be performed by
using two detectors a calibrated Si diode [74] and a cooled high-purity Ge solid-state detector (SSD)
[75] The Si diode will be bought already calibrated [76] A lsquogoodrsquo Ge SSD has an energy resolution
(ΔEE) around 1 at 20 keV 06 at 45 keV and 045 at 90 keV [77] The energy calibration of the
Ge SSD will be carried out in the X-ray beam line by using the Kα fluorescence lines of reference
materials (see Table 71)
Element Ge Mo Sn Cs Ce
EKα1 (keV) 9886 17479 25271 30973 34719
EKα2 (keV) 9855 17374 25044 30625 34279
Element Eu Er W Pt Pb
EKα1 (keV) 41542 49128 59318 66832 74969
EKα2 (keV) 40902 48221 75981 65112 72804
Table 71 Kα fluorescence lines of chemical elements used to calibrate the Ge SSD in the 10-90 keV
energy range
The two detectors will be mounted alternately on a (X Y) translation stand to measure a large number
of points of the emission spectrum
- the mean emission energy E (Ge SSD)
- the energy spread ΔE (Ge SSD)
- the absolute flux (photon numbers) (Si diode)
These beam characteristics will be measured as a function of the size of the selected X-ray sample the
selection being carried out with a double slit device (see Fig 76 (d)) Our goal is to check
lsquo
130
experimentally the theoretical angularspectral distributions shown in Fig 12 of the Introduction by
measuring the number of Compton photons per second per mrad2 and per 01 of energy bandwidth
Time analysis of the bunch structure
The X-ray source has a time structure which involves two main scales a 50 ns period which is the
interval between two successive collisions between the laser pulse and the electron bunch and the 20
ms e- storage cycle To perform a pulse by pulse analysis we will use a high resolution time detector
(~1-5 ns) with a high repetition frequency (~10-100 MHz) eg a micro-channel plate (MPC) detector
[78] or an avalanche photo-detector (APD) Such time measurements have already been performed at a
similar facility by Sakaue et al [79] MCP Hamamatsu devices can reach a 07 ns (FWHM) time
resolution which is good enough not only to resolve each one of the collisions between the laser pulse
and the electron bunch but also to measure their relative intensity accurately from the beginning of the
e- storage cycle to the end These measurements will be performed on the X-ray beam axis They will
have to be synchronized with the ThomX clock
Transverse coherence of the beam
Some simple diffraction and interference measurements will be performed in order to analyze the
source spatial coherence The diffraction of the X-ray beam by a slit of a known aperture or the
interference effect produced by the insertion of a phase-shifting object in the beam allows one to
quantify the transverse coherence of the beam [710] In these experiments the on-axis photon intensity
distribution will be recorded with a 2D high resolution detector (such as for instance an XPAD CdTe
camera with 130 microm pixels) This measurement will be repeated at several positions along the beam
The pattern analysis will inform us about the degree of coherence of the source A monochromator will
be inserted upstream of the diffractive slit or the phase-shifting object to select a well defined energy
Later on Young double-slit experiments could also be carried out [711] to measure the beam
coherence length beam
74 Demonstration experiments
In parallel with the complete characterization of the ThomX beam described in the previous
sections some simple experiments will be performed in order to demonstrate the potentialities
of the ThomX source In this phase we will focus on medical and materials science
applications An important aim of the ThomX project is to perform medical and materials
imaging exploiting the high and tunable energy of its beam A second aim of the ThomX
project is to show its capability in the structural investigation of materials using X-ray
diffractionscatteringspectroscopy techniques
741 Experiments using the full X-ry beam
Three experiments issued from the expertise of the ESRF-INSERM teams in medical applications of
synchrotron radiation have been chosen to be carried out as demonstrators on the ThomX prototype
The goal of setting up such experiments is to demonstrate (i) the suitability of ThomX of performing
both imaging and therapy experiments (ii) the tenability of the source adjustable at energies
corresponding to the K edge of high Z elements routinely used in medicine (Gd I) or of those of new
experimental radiotherapy adjuvants (Au Gd nanoparticles CisPt) (iii) suitability of the source for
phase contrast imaging (iv) exploitation of the advantage of using a 2D beam with respect to the typical
laminar synchrotron X-ray beam Some imaging and therapy experiments in the biomedical field
require a wide 2D beam Using the natural divergence of the ThomX beam a sample of a few
lsquo
131
centimeters in diameter located at 10-20 meters from the ThomX interaction region can be fully
illuminated by the a conic beam with no need of optical elements Calculations have been performed in
order to show the potential of using the ThomX source in comparison with a hospital X-ray generator
(fluoroscopy unit type) as well as in comparison with the beam from the biomedical line ID17 at ESRF
Results of these comparisons are given below for each medical experiment we intend to perform with
ThomX
An imaging experiment
For imaging experiments the energy range of the beam has to around 60-90 keV According to Fig 73
the corresponding half-aperture will be 5 mrad leading to a photon flux of 61012
phs When the beam
reaches the sample stand placed on Table 2 at 20 m from the ring interaction region it will have a
diameter of 20 cm This conical beam will then have a cross section of ~300 cm2 and permit
tomographic acquisitions of a full human head using a 2D detector This wide 2D beam is a major
advantage of the ThomX source for biomedical imaging with respect to the laminar synchrotron
radiation beam The dose rate (DR) in Gys can be expressed as
(71) DR = 16 10-13
Ф[phscm2] E[keV] (microenρ)[cm
2g]
where Ф is the photon flux per cm2 E is the mean energy of the beam and (microenρ) is the mass
attenuation coefficient For an X-ray beam of 75 keV (ie the average of our 60-90 keV energy range)
the mass attenuation coefficient microenρ for soft tissues is 0028 cm2g leading to a dose rate of 67 mGys
delivered to a 300 cm2 area The corresponding expected lsquodose area productrsquo (DAP) of the ThomX
source for such imaging experiments is DAP = 20 Gyscm2 For comparison at the ESRF ID17 line a
dose rate of 800 mGys is delivered to an area of 150x2 mm2 corresponding to a DAP of 24 Gyscm
2
As far as conventional sources are concerned typical laboratory devices used in 2D fluoroscopy deliver
a DAP of 06 Gyscm2 [712]
A feasibility experiment will be performed by using a standard human head phantom (see Fig 79)
which will be mounted on a rotating stand that allows a 360deg rotation It will be possible to translate this
stand in three directions (along the X-ray beam axis and in the two other perpendicular directions) to
optimize the sample position in the beam Acquisition will be performed using the 60-90 keV pink
beam before and after the insertion of a high-Z element solution (typically iodine or gadolinium) in a
slot available in the phantom head Several images will be acquired using different concentrations of
high-Z element chosen in the range of concentrations that can be typically reached into a tumor (in
humans the high Z element is injected intravenously) The study will also aim at finding for each high-
Z element the lowest concentration that can be measured with the ThomX imaging system
Figure 79 Typical phantom featuring a human skull from X-ray absorption point of view
lsquo
132
The detector will be placed downstream of the sample A typical detector that may be used is a large
field of view 2D detector equipped with Si or CdTe sensors with ~100-200 microm pixels [713] Finally a
standard computed tomography (CT) algorithm will be used to reconstruct the tomography images and
to calculate the concentrations (in mgml) of the given high Z element in the skull The results will be
compared to previous work done at the ESRF [714]
A radiotherapy experiment
For this experiment the targets will be rats implanted in their right brain hemisphere with rodent brain
tumors (F98 9L) at an appropriate tumor stage (typically at D10 or D14 after implantation) For the
irradiation we need a pink beam with a 10 keV energy bandwidth and a median energy above the K-
edge of the high-Z element to be injected as a radiosensitizer (ie in the 45-80 keV range for both
iodine and gadolinium) The beam size will be a few centimeters in diameter Figure 710 shows the X-
ray beam energy and beam flux generated by a 69 MeV electron beam versus the photon back-
scattering angle X-rays of energy 80 keV within a 10 keV bandwidth are contained in a cone of half-
aperture 27 mrad and their flux is ~31012
phs At the sample location on optical Table 2 at 9 m from
the ring interaction region the beam has a diameter of 5 cm ie a cross section of ~20 cm2
The center of the ratrsquos right hemisphere will be placed on the rotation axis of the irradiation system and
slits will limit the beam transverse extension in order to define a precise field of irradiation Assuming
an X-ray flux (Ф) of 31012
20 phscm2 with an average energy (E) of 80 keV a mass attenuation
coefficient of 0025 cm2g (soft tissue 80 keV X-rays) Eq (71) leads to a dose rate DRe of 48 mGys
close to the tissue region where the X-ray beam penetrates Before reaching the tumor the radiation is
attenuated by the bone (~ 02 cm thickness (microρ)att = 022 cm2g ρ=192 gcm
3) and by the skull healthy
tissues (~ 15 cm thickness (microρ)att = 018 cm2g ρ = 10 gcm
3) Thus the dose rate DRt delivered at
the tumor is DRt = TDRe (where T = exp (-02219202) exp (-0181015) = 070) and is
expected to be 337 mGys The dose will be delivered while the rat undergoes a 360deg rotation The total
dose to be delivered to a tumor ranges between 10 to 20 Gy Therefore according to the expected rate
of 337 mGys the irradiation will take typically from 5 to 10 minutes with the big advantage of not
having to move the rat A dosimetry protocol (such as the one used at ID17 [715]) will be set to check
the homogeneity of the irradiation and to measure the absolute dose with a 2 to 5 uncertainty
Figure 710 X-ray energy (left panel) and X-ray flux (right panel) versus the photon back-scattering
angle for stored electrons of 69 MeV The laser wavelength is 106 microm
For humans and a tumor placed at 8 cm into the skull the transmission coefficient becomes T= 019
Therefore the expected dose rate DRt delivered by the ThomX source at the tumor is 91 mGys One
may compare with the ESRF ID17 beam-line where 800 mGys can be delivered for irradiation on a
lsquo
133
surface of 2x50 mm2 ie with T=019 a dose rate DRt of 015 Gys on 2 mm in height at the tumor
which is equivalent to a rate of 6 mGys to cover 5 cm of height with a scanning rate of 50 mms
ThomX could therefore deliver doses for human skull irradiation larger than those available at
synchrotron radiation facilities and also perform the irradiation without having to move the irradiated
sample Concerning the conventional hospital devices they deliver low doses deposited by X-rays
whose energy spectrum is broad and moreover they are not made to operate continuously for such long
time irradiation
Also the tunability of the ThomX beam will allow testing several high Z drugs containing iodine
gadolinium platinum or gold proven to be promising radiosensitizers [716]
Conformal radiotherapy
A final step in this sequence is performing conformal radiotherapy experiments ie doing both imaging
and therapy at the same time In fact one major issue of radiotherapy is to spare as much as possible
healthy tissues located around the tumor and at its border In this regard it is a key issue to adapt the
field of irradiation to tumor geometry size and positioning by performing first a tomography
Phase contrast imaging
The absorption medical imaging technique is based on the fact that different parts of a sample absorb X-
rays differently But the fact that X-ray absorption by biological material is weak constitutes also the
limitation of this technique To enhance or replace absorption-based imaging refraction diffraction and
interference of X-rays can also be used A better visibility at the edge of different regions of a specimen
can therefore be obtained by using the information contained in the phase of an X-ray wave For this a
spatially coherent X-ray source is required and was available only since the development of modern
synchrotron sources Experimentally the simplest way to visualize phase-contrast imaging is provided
by the propagation-based imaging (PBI) method In this method a spatially quasi-coherent radiation
illuminates an object which gives rise to a spatially varying phase shift in areas with spatial variations
of the refractive index As the radiation propagates beyond the sample parts of the wavefront which
have experienced different deflections on both sides of an edge of the sample produce a serie of fringes
due to the classical Fresnel-edge diffraction phenomena [717]) This gives rise to a characteristic phase
contrast pattern which is then recorded by a 2D detector placed at a proper distance downstream of the
object (see Fig 711 left panel) The actual phase-contrast signal is convolved with the detector point
spread function which has to be sufficiently narrow to avoid the smearing of the interference pattern
Additionally the phase contrast signal can be visualized if and only if the transverse coherence is
sufficient ie if Lcoh = λRs is larger than the details to be imaged (here R is the source-sample distance
and s is the source size) In contrast the requirements in terms of longitudinal coherence
(monochromaticity) are rather weak even a beam with a bandwidth of several keV does not lead to a
significant smearing of the phase contrast signal [717]
Assuming s = 70 microm R = 15 m a sample-detector distance of 5 m and a detector point spread function
of 50 microm the phase contrast signal expected when the ThomX beam illuminates a nylon wire of 300
microm diameter ranges from 10 to 3 when the X-ray energy increases from 20 to 80 keV (see Fig
711 right panel)
lsquo
134
Figure 711 Left panel The set-up for propagation-based phase contrast imaging Right panel The
simulated signal produced by a nylon wire of 300 microm diameter illuminated by the ThomX source
(simulation)
The experiment with ThomX will comprise two parts a planar radiography and a computed
tomography For the radiography three kinds of samples will be used (i) a custom-made phantom
composed of nylon wires of different diameters (ii) the ACR (American College of Radiology)
accredited phantom used in mammography quality (iii) a 4 cm thick formalin-fixed human breast tissue
obtained from a hospital bank The X-ray energy will be chosen in the 20-80 keV range For each
sample and for several energy values images will be acquired at two different sample-detector
distances in the absorption regime for comparison purposes (the detector will be in contact with the
sample) and at 5 m A silicium detector with 50 microm pixels will be used Images will be acquired for
different doses (measured with a standard PTW dosimeter) in order to obtain the signal-to-noise ratio as
a function of the dose These measurements will allow one to qualify the phase contrast imaging set-up
at ThomX
Computed tomography will be applied to (i) a meacutethyl polymeacutethacrylate (PMMA) custom-made
cylindrical phantom containing insets of different compositions and densities (water ethanol paraffin
etc) (ii) a formalin-fixed human breast tissue (5 cm in diameter) Images will be acquired using the
same parameter values as the ones chosen for the radiography Samples will be installed on a rotating
stand positioned on a double cradle that will permit an alignment with the beam and with the detector
Images will be taken during the sample rotation over 360 degrees
The final step will consist in applying standard algorithms to the images in order to calculate the phase
and to extract quantitative information about the samples This data will be compared with equivalent
images that will be acquired at the ESRF
742 Experiments using the central part of the beam
Most specimens from cultural heritage and functional materials are heterogeneous these lsquorealrsquo
materials often exhibit complex hierarchical structures involving various levels of order and
heterogeneity To understand the properties of such materials better one needs at least a description of
their structure at the atomic scale and at the macroscopicmesoscopic scale which takes into account
weak structural signals (relating to disorder minor constituents microstructure hierarchical orderhellip)
lsquo
135
This requires much probing as already demonstrated with synchrotron radiation sources A major goal
of materials science is to link global properties to local atomic features (orientation strain structural
distortion grain size) of some components in relation with neighboring domains and to deduce from
that the macroscopic properties As the functionalities of these materials often stem from the association
of different phases or constituents or of specific inter-relation between them (microstructure) highly
selective structural probes need to be used to characterize them Also in the particular domain of
cultural heritage many specimens require non-destructive methods and probes In order to understand
how they were made and to preserve them from degradation over time their physical and chemical
properties must be determined There is an increasing demand for material probing with complementary
techniques Fluorescence emission techniques excel in revealing the elemental composition of the top-
most layers of samples Diffraction is suited for studying quantitatively the phase composition the
structural details of the constituent phases and the characteristic distribution of sizes and orientations of
crystallites Tomography techniques excel in revealing the 3D mapping of the chemical andor the
density heterogeneities within a sample This is why the cultural heritage community is today strongly
involved in the development of new instruments that may characterize patrimonial artifacts by mapping
not only the chemical composition of a variety of samples but also their physical phases
Experiments described in section 741 require a large 2D beam and exploit the natural divergence of
the ThomX beam In this section diffraction scattering and spectroscopy experiments require a
focalized or a parallel beam and it should also be monochromatic or lsquopinkrsquo The optics will therefore
select the central part of the beam ie its higher energy part Let us give an order of magnitude
assuming a perfect 50 MeV electron beam a collimator of 2 mm diameter located at 2 meter from the
IP will cut the X-ray conic beam at 05 mrad leading to a flux of ~8 1010
phs with an energy of 45 keV
and an energy bandwidth ΔEE on the order of 10-3
(see Fig 73) In practice this bandwidth will be
broadened and will reach a few percent because of the actual e- beam divergence and energy spread For
experiments requiring a narrower energy bandwidth an additional selection will be performed with a
monochromator For instance selecting an energy bandwidth of 10-4
would lead to a flux on the order
of 109 phs
Since the goal of the first series of demonstration experiments is to qualify ThomX as an X-ray source
these experiments will be performed with samples previously characterized with other sources such as
synchrotron radiation sources
Diffraction and scattering
Diffraction and scattering experiments are major ways of performing materials structural and micro-
structural analyses This concerns highly ordered samples as well as powders or ill-ordered samples
Complete scattering analyses are based not only on Bragg diffraction but also on diffuse scattering in
order to look beyond the average structure and to investigate the local structure or short-range order
The crystalline quality of powders depends on the preparation methods Grain sizes vary from about 10
nm to several hundreds of microm In the case of nano-materials Bragg peaks are strongly broadened and
X-ray diffraction performed with standard laboratory sources does not provide suitable data for
structural investigations As illustrated by Fig 712 laboratory X-ray sources do not allow the
identification of minor phases in a poorly crystallized powder Figure 713 shows an example of a
diffraction pattern obtained with an excellent resolution The sample used for this analysis is an
Egyptian cosmetic powder dated to 2000-1200 BC The patterns were obtained with the ESRF
synchrotron source (bottom panel) and with a classical laboratory source (top panel)
lsquo
136
Figure 712 Powder X-ray diffraction patterns obtained by using a laboratory X-ray source Top panel
Poorly crystallized Prussian blue sample Bottom panel Prussian blue crystals From [718]
Figure 713 Powder X-ray diffraction patterns obtained by using a laboratory X-ray source (top panel)
and the ESRF synchrotron source (bottom panel) Arrays indicate minor phases From [719]
We intend to use the ThomX source to record diffraction patterns and scattering plots in order to
characterize short-range ordering and identify minor phases in such heterogeneous samples
For the first demonstration diffractionscattering experiments we will study ill-ordered Prussian blue
and white lead samples Pigments used for paintings or frescos have various compositions Since their
usage followed a definite chronology such pigments can sign an era Pigment characterization can also
help the undertaking of a restoration while a thorough knowledge of pigment synthesis through
centuries can help to apprehend the degradation mechanisms better
lsquo
137
For ordered samples diffraction will be performed with a single crystal by using a monochromatic
beam but also by using a lsquopink beamrsquo as done in the Laue method For ill-ordered samples the whole
scattering pattern will be recorded and analyzed with the Pair Distribution Function (PDF) method This
analysis technique (often called the ldquototal scattering methodrdquo) can be used alone or associated with
diffraction For experiments using the lsquopink beamrsquo Laue method the beam will be focused directly on
the sample (by using convergent refractive lenses (CRLs) or a capillary optic system as mentioned in
Section 731) To determine powder crystalline parameters diffraction analyses require the
monochromatization of the beam in order to reach ΔEE ~ 10-4
In the optic hutch (optical Table 1) the
diverging beam will first be made parallel by the CRLs or the capillary optic method and then
monochromatized in the experimental hutch (optical Table 2) by a monochromator The set-up for these
measurements needs a goniometer to align the sample and a low noise 2D-detector sensitive to high
energy photons
Fluorescent spectroscopy
X-ray fluorescence spectroscopy experiments allow one to determine the chemical composition of
materials and the mass concentrations of elements Indeed depending on the atomic number of a given
chemical element energies of the fluorescence lines emitted after an X-ray excitation are a signature of
the element An illustration of this approach is presented in Figure 714 where the fluorescence
tomographic images of a Globorotalia inflata specimen are shown and compared with a classical
radiography image
Figure 714 Fluorescence tomographic imaging (a) Set-up (b) Radiography (c) Fluorescence
spectrum measured at a given angleposition of the sample (d) 3D tomography reconstruction From
[720]
We plan to carry out fluorescence experiments since the energy of the ThomX beam can reach the
excitation energy of most chemical elements usually involved in material science for instance the
critical excitation energy of lead regarded as a heavy element is 88 keV) For this experiment the
excellent energy resolution of cooled Ge detectors will be an asset to detect the fluorescence light (the
detectors will be oriented at 90o with respect to the incident beam)
lsquo
138
K-edge and XANES spectroscopy
In K-edge experiments two images of a given element present in some sample are performed one at an X-ray
energy just below the K edge of the chosen element and the other at an energy just above Then they are
subtracted (see Fig 715 left panel) The K-edge imaging technique is used in particular in the medical and
the cultural heritage domains In XANES (X-ray Absorption Near Edge Structure) the absorption by a
material is measured over an energy range of about 100 eV on both sides of the absorption edge This
technique provides information about the local structure prevailing close to a particular chemical element (see
Fig 715 right panel)
Figure 715 Left panel K-edge subtraction imaging Right panel Principle of the XANES method
The first XANES spectroscopy experiments with ThomX could exploit the K-edge of heavy elements
such as W Pt or Pb By first selecting the central part of the ThomX beam with an energy spread of a
few 10-2
a monochromator could then provide a beam with an energy spread of 10-4
Samples will be
aligned by using a goniometer and the transmitted intensity will be collected by a low noise 2D-
detector
Combined diffractionabsorptionfluorescence tomography
A major limitation of diffraction methods comes from the fact that only average properties can be
obtained while most lsquorealrsquo materials are heterogeneous and complex Thus structural studies of such
materials are difficult to carry out A new tool for structure mapping of materials by means of X-ray
diffractionabsorptionfluorescence combined with tomography allows one to perform non-destructive
3D structure selective mappings of heterogeneous samples This way one may identify minor phases
and determine crystallite orientation and grain size Furthermore the selectivity of the method allows
one to separate and locate the origin of the diffuse or diffracted patterns to identify amorphous phases
In archaeology such complementary micro-structural analyses could shed light on the mechanical
history of artifacts
A recent experiment carried out with a micrometer beam at synchrotron radiation sources have
established the validity of such a local probe by analyzing a mixture of chalcedony and iron pigments
containing haematite [721] This experiment illustrated in Fig 716 consists in (i) performing a
structure tomography mapping directly from the diffraction data (top panel) (ii) obtaining selective
structure patterns of unknown minor phases from these maps (bottom panel) This study was realized by
the Neacuteel institute and the C2RMF and was one of the first diffraction tomography experiments
performed with a micro-beam
With the ThomX source we will use poorly crystallized samples and combine in the same experiment
and simultaneously tomography diffraction with fluorescence and absorption measurements Diffraction
lsquo
139
tomography is beam-time consuming it will be performed to demonstrate its feasibility at ThomX The
experimental set-up will be similar to the one used for multimodal analyses (cf the top panel of Fig
716) The sample will rotate and this rotation will be combined with a translation to provide a 2Θ scan
of 2D diffraction patterns obtained with a 2D detector sensitive to high energy photons The diffracted
2D fluorescence patterns will be recorded with our Ge detector while the sample absorption will be
recorded with our calibrated Si diode
Figure 716 Top panel Experimental set-up for multimodal spectroscopydiffractiontomography
analysis Insets Images of the chalcedonyndashFe mixed-pigment sample are reconstructed by combining
the various signals X-ray absorption (bottom) Fe X-ray fluorescence (top left) and chalcedony
diffraction (top right) Bottom left panel Reconstructed diffraction image based on the whole
diffraction diagram (black) compared with the calculated pattern for pure chalcedony (red) Bottom
right panel Reconstructed image that only uses the iron pigment scattering contribution (black)
compared with the calculated pattern for hematite and siderite grains (red) From [721]
75 Summary
As shown in this chapter two types of beams will be available at the ThomX compact X-ray source
(i) wide beams well adapted for imaging (conventional or phase contrast imaging) and therapy in the
biomedical field and (ii) almost monochromatic beams that can be either focused or made parallel for
diffraction and spectroscopy experiments in the field of materials science
The X-ray line is designed
- to ensure a continuous monitoring of the X-ray beam
- to measure the beam brightness and its transverse coherence
- to demonstrate the potential of this new type of compact source and identify the benefits it can
bring to the currently available X-ray sources
lsquo
140
Chapter 8 CONTROL SYSTEM
Introduction
The ThomX control system combines those hardware and software elements involved in the
management of all the facility components But more than a system dedicated to carrying out operating
processes (processes launched from the X-ray beam control room in order to undertake some specific
task such as a performing an imaging experiment or a material chemical analysis) it must also be an
advanced tool for various studies (data processing new equipment integration operating mode
evolution) and scientific experiments which will be performed with the X-ray beam line Hence the
control system has to be a versatile tool easily scalable by various users (scientists operators IT
technicians)
In the following a lsquoSub-systemrsquo is a physically connected set of hardware items sharing a global logical
function The ThomX sub-systems are as follows the linac (LINAC) the transfer line (TL) the ring
cell 1 (RI-C1) the ring celle 2 (RI-C2) the optical cavity (OC) the extraction line (EL) the section
of the X-ray line located in the Igloo (XLI) and the section of the X-ray line located in the X-ray hall
(XLH) A lsquoDomainrsquo is a set of hardware items related by a common purpose The ThomX domains are
as follows alignmentmetrology (AM) control-command (CC) diagnostics (DG) infrastruture (IF)
magnetic elements (ME)n optics (OP) pulsed elements (PE) power supplies (PS) radio-frequency
(RF) safety (SA) synchronization (SY) and vacuum (VA)
81 General principles
Some general principles drive the design of such a control system In broad terms the following criteria
will serve as guidelines
- Since monitoring is a major role of a control system comprehensive monitoring interfaces must be
provided to all users Moreover the monitoring system must take into account sub-systems specificities
dedicated GUIs must be designed for every need for instance in synoptic form to give a global system
status overview or as charts to show the time evolution of some specific parameters of a particular
device (eg pressure readings on TPG300)
- Operating processes management a control system also aims at the prioritization and sequencing
of processes It has to provide high-level functions in order to remotely launch complex operations (eg
machine startstop interlockshellip) Computing and data analysis tools which could launch such particular
procedures will be integrated or developed to maximize the automation of the machine
- Default management the facility control system must be able to manage operation defaults It will
be necessary to design graphical tools to quickly identify defaults when they occur Color display of
warnings and errors according to a criticality-based convention will be implemented in addition to the
recording of running process logs
- Safety for obvious reasons the ThomX radioprotection and safety system must standalone It should
not use any data generated by the control system Nevertheless each end-user must be able to make
sure that the ThomX facility does not present any danger whenever the accelerator is running This
information must be available in every control room through a continuous display of all the safety
equipment status (radioprotection sensors door openingshellip) A full historical record of safety-related
lsquo
141
events will be made available for analysis and a procedure of detailed acknowledgement (operatorrsquos
name action to close an incident) will be defined in order to guarantee safe operations
- Ease of use The ThomX control system is intended for various usages therefore it will concern
various users including operators who are not familiar with software development Hence it is crucial
to make it modular through relevant to sub-systems GUIs with various visualization interfaces control
buttons graphs sub-system overviews in synopsis or detailed display of subset status The definition of
new interfaces will be possible through visual development frameworks to fit new needs
82 Technical considerations
In order to guarantee a proper accelerator operation to make its use as easy as possible and to limit
costs some technical points have to be taken into account
- Distributed architecture an accelerator is a complex structure which results from various designs in
different fields The hardware part of the control system relies on dedicated IT devices (industrial PCs
programmable logic controllers) whose role is to drive sub-systems Each of these nodes will be fully
and remotely controlled by the system software from the control rooms (or from any authorized user
terminal in the ThomX network) independently of possible other issues All of these components must
communicate with each other when required by the operation needs Moreover the supervision part of
the control system must be available in different places (main control room X-ray line control room
perhaps others) The chosen system must meet these requirements and must run on several terminal
types
- Standardization of the hardware items and of the software codes is necessary in order to simplify
their integration in the control system (eg same brandmodel for all the stepper motors) Then it will be
possible to mutualize high-level controlling tools whether pre-installed or homemade Additional
benefits of this approach concern the overall maintenance the deployment of new releases and the
replacement of hardware parts
- Reliability The service continuity of the control system must be guaranteed A spare strategy must be
set up to make sure that the replacement of defective parts will be done on short notice In particular
storage memory redundancy will guarantee that none of the machine parameters or of the collected data
will be lost Periodic backup of main software systems (device databaseshellip) will be performed right
from the start Uninterruptible power supplies are foreseen to protect major elements of the control
system from electrical failures
- Upgradability Special attention will be paid to communication features when choosing hardware
devices Ethernet is now one of the major protocols and must be preferred when possible For the PLC
driven devices the fieldbus Profibus [81] is recommended since one single master interface can
manage more than 100 slave devices without data losses This product is also valuable as far as cabling
cost is concerned
- Safety Since several users may wish to perform various machine-related tasks (integration
maintenance measurementshellip) a policy must be established to control the access to the ThomX
network and its control system The management of these access permissions will be provided It will
take into account the role and the field of expertise of each project member
lsquo
142
83 The TANGO control system software
Tango [82] is the software chosen to be the backbone of the ThomX control system It is an open-
source software initially developed at the ESRF It is now used in several Institutes to control huge
accelerator facilities like the SOLEIL synchrotron In fact the SOLEIL staff is playing a major role in
maintaining and developing new Tango features
831 Software bus
An accelerator equipment is usually highly heterogeneous Thus its control system must enable various
devices to communicate with each other even if they use different protocols or communication
standards like Ethernet RS232 USB PCI DAQ Cardshellip
The first asset of Tango is the communication standardization that it provides between any type of
devices Tango implements a CORBA-based communication protocol so that a common network is
virtually created for the whole hardware If needed the parameters of a given device are thus available
to every other element of the control system This is achieved by adding an extra Tango-compliant
software layer above the hardware low-level libraries This layer is specific to a device brand and model
and is a simple executable file which can be instantiated as many times as required by the number of
devices This key element of the Tango system is called a lsquoDevice serverrsquo
832 A control system for accelerators
For basic reasons the Tango core and many of the tools developed around this software bus are open
source tools It is therefore an accessible system fully customizable for specific ThomX needs and a
system which can be maintained Since Tango is Linux and Windows compliant issues originating in
driver availability for the Linux OS can be avoided while Tango enables the use of scientific
applications which are often developed for Linux
Furthermore Tango is developed and maintained by a community which involves several accelerator
Institutes Even if Tango is not an accelerator-dedicated tool a number of developments have been
carried out or are being carried out that deal with devices currently used at such facilities Thus a large
Tango library of device servers is presently available to any new user
Finally more than a simple tool which provides a network access to a set of devices Tango is also a
framework that provides high level operational services such as configuration tool logging archiving
services and alarms permission management For developers specific tools are provided for the
generation of codes that fit in the common structure required by all device servers Other tools allow
easy GUI development As a result the integration of new devices or the upgrade of control and
supervision windows do not need in-depth coding As far as the compatibility with commercial software
is concerned dedicated bindings can be freely used eg applications developped for Matlab or
Labview can be used to perform a variety of tasks involving the ThomX devices
lsquo
143
84 Control system architecture
841 Equipment to control
On a complex machine like ThomX a wide range of devices and control signals must be taken into
account motors cameras scopes diodes or valves will deal with analog and digital inputsoutputs
complex signals transmitted through GPIB USB Ethernet or profibus interfaces
The inventory of devices to be controlled is presented in the following table Safety equipment is not
mentioned as it is a fully standalone sub-system
LINAC TL Ring OC XL
Power Supplies
Coils 2
Steerers 2 4 24 2
Dipoles 5 4 8 3
Dipole chicane 1
Quadrupoles 7 24 3
Sextupoles 12
Pulsed elements
Septum 1
Kickers 2
Synchronization
3GHz synthesizer 1
500 MHz synthesizer 1
Phase shifter 5
Programmable delays 2
Counter 1
Vacuum
Ion pump 8 4 17 4
Gauge 8 8 22 8 1
Valve 4 4 4 2
Primary pump 1
Diagnostics
BPM 1 4 12
Low frequency Signal
generator 2
ICT 1 2
Stepper motors 1 3
Camera CCD 2 3 1
RF
Klystron coil 1
Modulator 1
Amplifier 1
Canon 1
Feedback 2
Source laser
Photodiode 1
Camera CCD 1
lsquo
144
Motor 1
Mirror 2
Laser ndash FP cavity
Camera 4
CCTV Camera 10
Motor 16
Thermal control unit 2
Laser equipment 3
Scopes 3
FPGA 1
X‐ray Line characterisation
Xpad camera 1
Si PIN diode 3
Ge diode 1
Motor 13
Stepper motor 25
Others
Translation table 1 1
Slot 1
Table 81 Devices to control (TL=Transfer line OC = Optical cavity and XL = X-ray line)
842 Inputsoutputs
All devices listed above communicate with each other by digital inputsoutputs (DI DO) ie one bit
signals analog inputsoutputs (AI AO) or more complex signals as n-bit digital signals (n = 8 12 or
16) Several communication protocols mentioned in sect832 above will be used
They are detailed in Table 82
PLC InputsOutputs Bus Ethernet
connections
DAQ (ADCampDAC)
Vacuum 26 DI 13 DO 26 profibus
Diagnostics 2 gpib 30 3 x ADC 12 bits 100 kHz
Linac 2 DI 2 DO 5 AI 3 ADC 8 bits 1 GHz
Magnets 35 profibus
Feedbacks RF (x2) 10 DI 10 DO 10 AI
10 AO
USB
Ampli RF 64 DI 8 DO 32 AI 16
AO
RS232 2 4 channels ADC 12 bits
1MHz
Modulator RF 23AI 4 AO 69 DI 5 DI 5 DO
8 channels ADC 8 bits
Pulsed Elements (x3) 12 DI 3DO profibus ADC 12 bits
Laser ndash FP cavity RS232 22
Timing 2 gpib 14 15 channels DAC 16 bits
X Line 26 DI 26 DO 7
Table 82 Equipment IO to control (not complete at the time of writing)
lsquo
145
843 Front-end machine architecture PLCs and industrial PCs
The front-end architecture will comprise equipments individually dedicated to sub-systems These
machines are controlling nodes which will be located in sub-system server racks as close as possible to
the equipment to be controlled (when required)
Two kinds of front-end machines will be used
- PLCs programmable logic controllers are automation OS-based machines able to manage digital and
analog IO and to control processes carried out by devices connected to a field bus (eg Profibus) or
specialized devices They are reliable devices designed to operate in an industrial environment A
proprietary operating system runs on these machines which are powerful tools (eg automats) to
implement logic appropriate to experimentation Interconnection with the other Tango network
equipment is made through an Ethernet adapter interface
- Compact PCIs (industrial PCs) these crates can embed complex operating systems like a Linux
distribution or some Microsoft Windows Operating Systems hard drives Ethernet connections and
USB adapters Such crates are based on a PCI bus backplane to interconnect specific cards (DAC
ADC counters optoelectronics convertershellip) cPCI is a standard widely used in an industrial
environment it is compatible with numerous card suppliers These front-end machines will also allow
the running of Tango processes or specific computing tasks locally installed in a sub-system
The ThomX distributed control system will be build on these controlling nodes This structure will
allow the bandwidth optimization of the network main links and will provide cabling simplifications
The control equipment may be placed in the accelerator hall as well as in the technical rooms where
most of the electronics that must be controlled will be clustered in 19rsquorsquo standard server racks
‐ Some devices need not be controlled through these front-end machines They will be directly
connected to the control network (eg ethernet cameras or electronics for motor control) Three
ways to interconnect such devices are shown in Fig 81
‐
Figure 81 Control architecture for one sub-system
lsquo
146
These acquisition and controlling units can be Profibus PLC or a cPCI crate Each driven element will
be controlled through one of these acquisition and controlling unit
The overall control system will be composed of more than 25 front-end machines as described in Table
83
PLC cPCI Crate
Linac (wout vacuum diags modulator
magnets) 2 1
Vacuum 2
Diagnostics 2
Feedbacks 2 2
RF amplifiercavity 1 1
RF modulator 1 1
Pulsed elements 3 3
Fabry-Peacuterot cavity 1 1
X-ray line 1
Synchronization 1
Magnets 2
Total 13 14
Table 83 List of front-end machines (not complete at the time of writing)
85 IT infrastructure
- Network The ThomX local network will be organized around an IP backbone It will be based on the
Ethernet TCPIP standard in order to link user terminals front-end machines specialized servers etc
Because of the large number of devices and the requirements bearing on the bandwidth (eg for image
storage) 1 Gbitss compliant equipment will be chosen (LAN adapters switcheshellip) Depending on the
actual number of cameras an image-dedicated network could be added On the cabling side shielded
copper cables will be used as interconnection wires to limit electromagnetic interferences as much as
possible
- Data storage During the ThomX commissioning or for the operation monitoring or for the X-ray
experiments it will be crucial to store a large amount of data It is foreseen to implement the three
storage processes provided by Tango historical (long term storage at a slow rate) temporary (short
term storage at a fast rate) and snapshot (storage of parameters on operator demand or event triggered)
It will also be possible to meet the X-ray line needs for the storage of large size images during short
periods (at most for several days) This specific storage will not be necessarily on local servers it could
be convenient to move these data and store them on servers close to the final users possibly outside the
ThomX network
Two storage-dedicated servers will be installed for analyses replay of accelerator operation scenarios
and for some specific needs of the X-ray line These storage units will be specific machines which
prevent data outages or losses by redundant power supplies stored data redundancy (RAID) hard disk
hot swap etc The first of these two storage units will be a 2 To server equipped with a disk array
dedicated to the accelerator data management The second one will have a 10 To capacity it will be
available for the large size images of the X-ray line
lsquo
147
- Equipment of the control rooms Users will work in control rooms on system administration
accelerator control development and integration (device servers GUIs high level computing software
safety analysis tasks) Administration and end-user terminals should be provided in sufficient number
(see below)
The main control room will be equipped with
- 5 PCs 1 for the Tango core system 2 for operations 1 for development 1 for safety (dedicated to
visualization storage and analysis of safety related events)
- 1 server to run high-level applications and device servers
- 2 screens one displaying the ThomX status with synopses and the other one for the safety
supervision
- camera monitors for some direct controls
- 1 UPS (uninterruptible power supply)
The X-ray line control room equipment will consist of
- 2 PCs for the operation equipment and for macro development
- 1 server to run high-level applications and device servers
- 2 screens dedicated to the monitoring of the machine status to safety synopses and to the display of
safety related events
- Software needs
- Both the Linux and Windows OS must be installed in the control room PCs in order to run Tango and
many other operating processes (see the Introduction above) for development purposes and to use
configuration software Together with the Tango core and specific tools high-level software will be
installed to meet usersrsquo needs Labview and Matlab for science applications SPEC to control the
motors and for the analysis of the X-ray line data
- Data bases will run on the MySQL open-source database managing system
- An open-source http server Apache will allow the implementation of WEB interfaces for the control
system authorized users this way these users will be able to remotely visualize experiment data or
equipment status
- Finally all end-user stations will provide standard office tools
The following Figure gives an overview of the control room equipment
lsquo
148
Figure 82 Schematics of the equipment foreseen for the main control room and the X-ray line control
room
Overall the ThomX control system ndash designed to ensure harmonization and standardization and based
on open source tools ndash should be cost effective The investment cost is estimated around 200 keuro
including IT small equipment not described in this document cables a gateway to provide internet
access extra screens to display the ThomX status in the laboratory etc
lsquo
149
Chapter 9 SAFETY AND RADIOPROTECTION
91 Introduction
The ThomX facility will be sited in an existing building called lsquoIgloorsquo where a source of hard X-rays was
operated until 2003 by the Laboratoire pour lrsquoUtilisation du Rayonnement Synchrotron The ThomX linear
accelerator and storage ring will be built inside this building while the facility control room and the one of the
X-ray line will be located respectively in the D1 and D2 experimental halls The D4 hall will be devoted to
experimental tests
The ThomX operation will produce high-energy electrons and other particles such as X-rays and neutrons
which generate ionizing radiations through their interactions with matter An adequate protection against these
radiations must be ensured for people working at the facility The safety logic adopted is the so-called
lsquopositive safetyrsquo which means that any system dysfunction will immediately cause a complete stop of the
accelerator Furthermore redundant safety devices will be implemented allowing various actions that will
guarantee the staff safety such as
- Shining the beam into an area shall be submitted to prior people evacuation and to the interlocking of this
arearsquos doorways
- Access to a sensitive area shall be submitted to the prior stop of the upstream beam by using at least two
different means independent of each other
Complementary to these two procedures safety measures will be applied involving hard-wired logic safety
PLCs shielding beam dumps shutters personnel exclusion interlocks radiation monitoring of the
experimentation areas staff dosimeters and radiation hazard training of the personnel In case of dysfunction
radiological monitoring hard-wired logic or safety PLCs will act through interlocks that will break
connections to the mains voltage (Pre-Amplifier modulator or electron gun depending on the situation) in
order to stop the accelerator
92 PLC safety
The safety hard-wired logic works in association with an automaton PLC system via two completely separate
circuits A Safety Programmable Logic Controller (PLC) is a special purpose machine which is used to
provide a critical control and safety applications to automaton users This controller is an integral part of
lsquosafety instrumented systemsrsquo (SIS) which are used to detect situations involving potentially dangerous
processes When such a situation occurs the SIS is programmed to bring the process to a safe state
automatically
A safety PLC is specifically designed to accomplish two important tasks (ensured by a diversitary redundant
architrecture)
- lsquodo not fail but if this cannot be avoidedrsquo
- lsquofail only in a predictable safe wayrsquo
Hard-wired logic is a safety circuit fully independent of automatons It takes in charge in particular the
access door interlocking feedback contacts which provide a large number of checks bearing on the magnetic
field the high voltage the pre-amplifier status etc and the running control panel
The PLC safety level must be as high as the one of a positive safety hard-wired logic It manages access doors
emergency stop buttons safety rounds and reports issue authorizations to operate from a command-control
system
lsquo
150
93 Radiation monitoring
Concerning the radiation protection the ThomX facility design satisfies the following principles
- No one will receive a dose of radiation in excess of the regulatory dose limits as a result of normal
operation
- Professionnal occupation exposures will be kept As Low As Reasonably Achievable (ALARA)
- All practical steps will be taken to prevent accidents with radiological consequences and to minimize the
radiological consequences of any accident
Arrangements will be made for the monitoring of working areas such as the Igloo the bunker and the Optical
hutch For this purpose several monitoring modes will be available such as thermo-luminescent dosimeters
measurements made by the qualified person in charge of radiation protection (Competent Radiological
Protection Person) and a modular system for continuous monitoring A continuous monitoring of gamma rays
and neutrons will provide radiation dose rates and will manage alarms its measured values will be transmitted
to a central data acquisition unit Placed in the Compton room a special treatment unit will also be
implemented that will provide a local display of the dose rate signage and will manage alarms
94 Radiation protection zoning
The D1 D2 and D4 halls are not classified lsquosupervised arearsquo therefore their access is free to people who
satisfy access requirements In contrast the Igloo hall is classified lsquosupervised arearsquo
The D1 hall contains the X-ray line and technical equipment
- The X-ray line is not a classified area with the exception of the Optical hutch
- The Optical hutch is alternately classified lsquoprohibited arearsquo and lsquonot classifiedrsquo area depending on the
permission conditions issued by functional elements important to safety (shutter position permanent
radiological level)
The D2 hall contains the ThomX control room and a room which contains the laser equipment
The D4 hall is devoted to tests and mechanical assembly
The Igloo hall contains the electron accelerator located inside a bunker The Igloo is classified lsquosupervised
arearsquo therefore only workers subject to individual dosimetry are admitted However in exceptional
circumstances public access might be authorized as long as regulations and procedures are respected
The Igloo bunker on the one hand and on the other hand the basement area devoted to the facility are
alternately lsquoprohibited arearsquo during the accelerator operation and respectively lsquosupervised arearsquo and lsquonot
classified arearsquo when the facility operation has stopped
The accelerator energy is high enough to expect nuclear reactions of photons and neutrons with matter
leading to the activation of structural materials This is why it is particularly important to use appropriate low
activation structural materials or structural materials that are activated for short periods of time only
Furthermore one must ensure a nuclear waste management which will require a specific room to store and
monitor activated materials This room should be a lsquosupervised arearsquo located in the Igloo
95 Access control
951 General statement
It is a matter of absolute necessity to have appropriate procedures and devices in place to identify and screen
all persons entering the ThomX facility Consequently the LAL access control should be extended to the D1
D2 and D4 halls and to the Igloo
lsquo
151
952 Particular requirements concerning the lsquorestricted areasrsquo
The validation of door contacts as well as the validation of the position of protective shields and real-time
radiation measurement are prerequisites to access the restricted areas (ie Optical hutch and the accelerator
area) As it applies to all functional elements important to safety signage and door statements (openedclosed
lockedunlocked) from classified areas must be transmitted to the control room Local indications are
displayed alternately with the status messages The measured values of the dose rate and alarm symbols are
shown permanently
953 Access to the accelerator area and to the Igloo basement
The accelerator and the storage ring will be located in a concrete enclosure that will provide an efficient
shielding to protect people against ionizing radiations inside the igloo in line with regulations Prior to the
validation the floor load-bearing capacity must be checked by experts
The concrete containment structure will have a removable roof made of staggered concrete slabs Civil
engineering will be needed for the anchorage and supports to be used to adapt the bunker concrete blockds to
the igloo circular wall which will be part of the enclosure and to make sure that the roof of the whole
lsquoprohibited arearsquo provides a complete shielding Since doors will be made of standard steel the bunker access
will be indirect ie through (short) mazes to ensure protection against ionizing radiations
Access to the bunker and to the basement area which is part of the ThomX facility will be determined by the
equipment that guarantees peoplersquos safety in accordance with the facility operation As mentioned above
these devices ensure redundancy by using wired logic safety PLCs and also special safety door locks which
are simple but effective devices A mechanical system called lsquotrapped key lockrsquo will be implemented the key
is trapped in the lock when the door is open and it can be recovered only once the door is properly locked
Turning on the beam in lsquoprohibited areasrsquo is allowed only if the safety systems have been unlocked which
implies that the collection of access keys is complete as will be checked by the control board To meet the
safety requirements new access doors must be installed and some existing doors must be modified
954 Access to the X-ray line and to the Compton room
The D1 hall will not be a lsquoclassified arearsquo and will be open to authorized people The X-ray line sections will
be in open access during normal operation with the exception of the Optical hutch To meet the radiation
protection requirements the Optical hutch partition and its roof will be made of thick metal sheets A thick
lead shield will be installed at the end of the X-ray beam axis
The Optical hutch will be alternately classified lsquoprohibited arearsquo and lsquono classifiedrsquo area (under the worst-case
conditions it might become a lsquoregulated arearsquo) depending on permission conditions issued by safety functional
items External and internal light-signaling warning will indicate the zoning status Thus access to the Optical
hutch is firstly subordinated to the removable shutter position and secondly to the local permanent
radiological measurement values The shutter is dimensioned to protect people from high radiation rates
effectively A fail-safe closer device triggers the shutter closing automatically in case of electrical power
failure too (compressed air power supply) When a door is open the shutter command is prohibited
A local radiation detector located in the Compton room will check he radiation exposure and the dose levels
down to as low as reasonably achievable values Measured high dose rates will automatically shut down the
accelerator and activate the evacuation alarm The validation of these safety devices is mandatory to grant
access permission
lsquo
152
96 Beam dumps
The ThomX facility will comprise tow beam dumps made of concrete that will contain high energy particles
They are very important to ensure an adequate protection of people from the ionizing radiations that would be
produced if they were not used One will be aligned with the linac axis and the other one will equip the
extraction area These shields will be installed as close as possible to the radiation source to absorb ndash or at
least to moderate ndash as much as possible of the produced radiations Their size and thickness will be
sufficiently large to reduce efficiently all secondary radiation (X-rays and neutrons) produced by the stopping
of the e- beam They will be made of a lined lead shield designed to moderate back-scattered radiation placed
in a cavity made of concrete blocks or high-density polyethylene Ne peut-on pas ecirctre plus preacutecis donner des
nombres sur les dimensions et preacuteciser les mateacuteriaux utiliseacutes avec une figure
97 Interlocks
Depending on the dysfunction origin the safety system will operate and break command circuits such as the
pre-amplifier trigger the modulator high voltage the electron gun trigger or its high voltage Feedback
information from the control systems may be gradually requested according to the anomaly
98 Accelerator emergency shutdown
Accelerator Emergency Shutdown (AES) are signal and alarm devices located in specific areas A sufficient
number of AES should be installed in prohibited zones namely the accelerator area the bunker basement and
the Optical hutch For the users to spot them easily AES are devices equipped with flashing lights which are
turned on whenever the local zone is closed
If people are still present in a dangerous zone when a machine run is about to start (ie when the access is
going to be forbidden after a short while) they should signal their presence by pushing an AES This action
prevents the accelerator operation The system reactivation must follow a specific procedure
99 Control console
Placed in the control room the control console collects all access keys The accelerator may be turned on if
and only if all keys are inserted in that console
The safety circuit validation (PLC safety hard wired logic) allows one to launch the accelerator running
procedure and triggers audible warning devices for 30 seconds before operation becomes possible
910 Evacuation of dangerous areas
A specific procedure must be followed for the evacuation of the bunker and its basement Operators should be
trained in methodical evacuations with the validation of all checkpoints (roundsman function) Non
compliance with the procedure will lead to the non validation of the facility operation
A sufficient number of roundsman functions should be located in the accelerator area Also an exit path
should be marked leading to those areas that provide a possibility to regress
911 Audio system and horns
Loud-speakers and horns will be installed in the Igloo basement (in the so-called lsquorotundarsquo) and pits and
inside the bunker The sound system must be of good quality speeches and broadcast messages must be
intelligible Horns must be loud enough 80 dBA at least
lsquo
153
912 Safety notices
Local notices Posters should be put up in lsquosupervisedrsquo and lsquocontrolled areasrsquo to remind people of the
regulation requirements Such posts maintain the workerrsquos awareness whenever they are in a radiological
environment Access doors to zones with multiple classifications should be equipped with highly visible
luminous warning panels Also measured radiation levels and messages could be flashed regularly Regarding
the radiation measured level in the Optical hutch an alarm symbol must be permanently displayed that
informs of this level
Signal boards at zone entrances and within the zones will specify the real-time safety conditions display
warnings (such as presence of live voltage ongoing operation mode etc) and the access permissions (such as
open access prohibited zone etc)
913 Radiological monitoring
The control of the radiological environment will comply with the ALARA principle It will be performed by
- using bimonthly exposure dosimeters based on radio-photo-luminescence (RPL) and neutron sensitive
dosimeters
- PCR controls with live measurements
- a centralized modular system which will continuously monitor exposures feeding a data acquisition system
that can handle up to 10 sensors
This centralized system will perform dose rate and integrated dose measurements manage alarms test
measurement channels check settings and calibrations It will allow a special device placed in the Optical
hutch to display in real time local safety information
The ALARA principle is a commitment to keep radiation exposures and doses at a level as low as reasonably
achievable (social and economic factors being taken into account)
lsquo
154
Chapter 10 MANAGEMENT OF THE THOMX PROJECT
Introduction
The management of the ThomX project will have a simple structure that takes into account the three
project phases 1) The construction 2) The exploitation 3) The technology transfer Three boards will
govern the ThomX activity covering all project aspects the machine design the purchasing and the
assembly the commissioning the usersrsquo access the communication and the relationships with the
industries This management board will comprise
1) A steering committee that will supervise the project activity and define the collaboration scientific
policy
2) A scientific committee composed of the scientific coordinators of the collaboration partners It will
assess the usersrsquo projects and prioritize the beam-time allocation This committee must also define the
scientific policy of the machine exploitation and RampD of the ThomX project
3) A partnership and valorisation office This body will evaluate the cultural and the economic
potential of the ThomX innovations It will stimulate transfer initiatives and check their outcome It will
work in close contact with the steering and the scientific committees
The composition of these three bodies is given in Fig 101
Figure 101 The ThomX project committees
In parallel the communication group (see Table 101) will be in charge of all activity concerning
communication and reach-out in the ThomX project It will organise events realise the publications
relating to the project status and progressrsquo and coordinate the official announcements of the
collaboration
LAL Orsay
Achille Stocchi
CELIA Bordeaux
Philippe Balcou
UMPC
Philippe Walter
Institut Neel
Alain Schuhl
INSERM Grenoble
Franccedilois Esteve
Synchrotron SOLEIL
Amor Nadji
ESRF
Jean Susini
THALES
Yves Trinquet
Alessandro Variola
LAL Orsay
Alberto Bravin
ESRF
Geraldine Le Duc
ESRF
Jean-Louis Hazemann
Institut Neel
Philippe Walter
UMPC
Alexandre Loulergue
Synchrotron SOLEIL
Francois Esteve
INSERM Grenoble
Eric Cormier
CELIA Bordeaux
Alessandro Variola
LAL Orsay
Pierre Bordet
Institut Neel
Philippe Deblay
Synchrotron SOLEIL
Yves Trinquet
THALES
Annabelle Alves
CNRS DR4
Edward Mitchell
ESRF
Fabian Zomer
LAL Orsay
lsquo
155
The project management is organized according to a matrix structure The ThomX facility has been
divided in five sub-systems Linac Ring and transfer lines X-ray line Laser and Fabry-Peacuterot cavity
There is a coordinator responsible for the design installation and operation of each sub-system All
these sub-systems involve various technologies which have been grouped in expertise domains
Mechanics and integration RF systems Magnets Diagnostics Power supplies and Pulsed elements
Vacuum Synchronisation Controls Beam Physics Alignment Cabling Nomenclature and
Infrastructures There is a coordinator for each one of these domains These coordinators will be in
charge of the activity harmonization within each technical field and of the maintenance during the
facility exploitation (or ldquoscientific phaserdquo)
The project leader his deputy and a project engineer are responsible for the project overall
coordination Fig 102 describes the project structure
Figure 102 The project structure
Project and Quality Engineer Technical
coordinator
AVermegraves Francois Wicek
LAL Orsay
Deputy Project leader
A Loulergue
Synchrotron SOLEIL
Project Leader
A Variola
LAL Orsay
Systems
LINAC RRouxLAL Orsay
Ring and Transfer Lines A Loulergue Synchrotron SOLEIL
X line M Jacquet LAL Orsay
Laser E Cormier CELIA Bordeaux
Fabry-Perot Cavity F Zomer LAL Orsay
Groups
Mechanics and Integration MLacroix LAL Orsay
Beam Physics ALoulergue Synchrotron SOLEIL
Alignment ALestrade Synchrotron SOLEIL
RF systems P Marchand Synchrotron SOLEIL
Magnets ME Couprie Synchrotron SOLEIL
Diagnostics L Cassinari Synchrotron SOLEIL
Infrastructures BPieyre MLanglet LAL Orsay
Control PGauron LAL Orsay
Vacuum C Preacutevost LAL Orsay
Power Supplies - Pulsed Magnets P Lebasque Synchrotron SOLEIL
Synchronisation N Delerue LAL Orsay
Cabling JN Cayla LAL Orsay
Nomenclature PCornebise LAL Orsay
lsquo
156
In the following Table the support structures are presented
Support Structures
Administration Communication Quality
Administration Brigitte Renard LAL
Orsay
Procurements Anngaeumllle Mardam
Beck LAL Orsay
EDMS Support Agnes Vermes LAL
Orsay
Secretary Chloe SevestreLAL Orsay
DR04 Support Edouard Dorion
Dominique Bony LAL Orsay
Marie-Pauline Gacoin
Synchrotron SOLEIL
Claus Habfast ESRF
Sandrine Agnes Vermes
LAL Orsay
Table 101
The ThomX collaboration needs an important support from all administrative services For this an
administrative cell has been set up which is headed by the person responsible for the administration the
purchasing office the project database management the secretariat and the contact with the regional
CNRS delegation support Last but not least a quality engineer plays a major role in the project
101 Partnersrsquo complementarity
The ThomX project requires a multidisciplinary environment given the complexity of the facility and of
the set-up operation Each partner contributes its competences to reach the final goal The ThomX
collaboration was organized so as to meet the requirements of a multicultural scientific environment
where different communities are called upon to share their know-how and their knowledge to develop a
common scientific program This was already the case ndash and still is the case ndash for the MightyLaser
project In this project LAL and CELIA teams have developed a fully integrated set-up composed of a
fibre laser and a high gain optical resonator which is presently installed at ATF (KEK Japan) There the
complementarity between the two teams was crucial LAL has a solid experience in developing optical
resonators in an accelerator environment while CELIA is among the leading laboratories in fibre
amplification technology This program aims at a progressive increase of the light beam power stored in
an optical cavity in order to maximize the Compton back-scattering rate The final goal of this RampD is
to demonstrate the feasibility of a γ factory based on Compton backscattering It started as a
demonstrator for Compton-based polarized positron sources needed for the next generation linear
colliders The interest shown by other scientific communities in this technology made it clear that a low
energy Compton source would be very useful in various research fields This conclusion led to the
launch of the ThomX collaboration
The ThomX CDR has already been published and the present TDR is about to be widely circulated For
the TDR writing it was mandatory to develop close connections between the various communities
lsquo
157
participating in the ThomX project in order to merge technological developments and usersrsquo
requirements in a single document All partners have contributed to the scientific case elaboration the
study of the facility feasibility and the definition of its main components
- LAL and SOLEIL contributed the description of the Compton backscattering characteristics and
carried out the study of the accelerators and the Fabry-Peacuterot resonator
- CELIA Bordeaux Lyon provided their expertise for the laser dimensioning
- THALES provided its experience in industrial components and sub-systems
- UMPC and ESRF enriched the scientific case
This task sharing results from the multidisciplinary approach to the ThomX project where half a dozen
institutes play an important role
The writing of this TDR provided a unique opportunity of collaborating for laboratories that belong to
different fields LAL specialized in fundamental high energy physics SOLEIL and CELIA in advanced
technology in laser and accelerators LC2RMF in social sciences and compact detectors ESRF in
medical sciences In the future the diversity of this set of teams will still increase with the participation
of the Neel Institute and the GIN-INSERM in Grenoble
After a prototyping phase ThomX will operate as a mini-synchrotron whose beam line can be accessed
not only by the facility partners but also by other communities Moreover the aim of the
industrialization phase is to provide the high technology scientific and industrial with compact and
reliable sources that can be operated with little or no accelerator and laser expertise This should open
the way to a large community of CBS source users that would benefit from the complementarity
between techniques developed at synchrotron sources and Compton back-scattering facilities Imaging
techniques biological tests on cells and animals metallurgy fast chemistry lithography are examples
of future possible applications of these unique instruments
102 ThomX larger framework
ThomX is one of the EQUIPEX projects submitted for approval to the French government by the P2IO
Consortium (Physique des deux Infinis et des Origines) of the Paris-Saclay Campus This Consortium is
composed of eight laboratories or institutes (CSNSM IAS IMNC IPNO IRFU LAL LLR LPT)
some of which are among the largest French laboratories All together their staffs comprise 2000
scientists engineers and technicians working in particle physics nuclear physics accelerator science
astroparticle physics astrophysics and their interfaces P2IO represents between 20 and 90 of the
national potential in the various sub-domains Although obviously not all of the eight laboratories
participate in each EQUIPEX project each project ndash including ThomX ndash is fully supported by P2IO and
will contribute to the very ambitious common technological platform that P2IO wants to create in the
coming years in the framework of the Paris-Saclay Campus initiative Once equipped with these
EQUIPEX facilities this campus will represent a unique opportunity for French researchers and
students
lsquo
158
References
11 Geoffrey A Krafft Gerd Priebe Compton Sources of Electromagnetic RadiationlowastReviews of Accelerator
Science and Technology Vol 3 (2010) 147ndash163
12 VBesson et al NIMA 608 (2009) S75-S77
13 T Akagi et al 2012 JINST 7 P01021
14 CDR ThomX LAL RT 0928 SOLEILSOU-RA-2678 editors AVariola FZomer ALoulergue
15 PRussbueldt et al Optics Express Vol 17 Issue 15 pp12230-12245 (2009)
16 IPupeza et al OPTICS LETTERS Vol 35 No 12 June 15 2010
17 M Y Shverdin et al 2009 OSAIPNRANLOSL 2009
18 CVaccarezza et al IPAC Conference proceedings New Orleans USA2012 TUOBB01
19 AVariola IPAC Conference proceedings San Sebastian Spain 2011 WEOAA01 1903-1905
21 S Doumlbert ldquoIntegration of the PHIN RF Gun into the CLIC Test Facilityrdquo Proceedings of EPAC 2006
Edinburgh Scotland 2006 MOPLS129
22 R Roux S Cavalier M Bernard G Bienvenu MJoreacuteBLeblondBMercier BMoutonCPrevost A
Variola ldquoPHIL a Test Beamline at LALrdquo Proceedings of EPAC08 Genoa Italy 2008 WEPP078
31 J Payet Beta Code CEA SACLAY
32 JB Murphy et al Longitudinal Wakefield for an Electron Moving on a Circular Orbit Vol 57
Part Acc 1997
33 K Bane M Sand The Short-Range Resistive Wall Wakefields SLAC-PUB 95-7074
34 E Forest Phys Tech Part Photon Beam
35 J Zang et al lsquoDipole fringe field effects in the Thom-X ringrsquo Proc IPAC13
36 R Nagaoka et al Recent Studies of Geometric impedance At SOLEIL PAC2006
37 Z Huang RD Ruth Phys Rev Lett 80976 (1998)
38 S K Mtingwa African Physical Review 2 (2008) 001
39 P Chen G Horton-Smith T Ohgaki AW Weidemann K Yokoya
NuclInstrumMethA355107-1101995
310 K Ohmi Phys Rev E 55 7550 (1997)
311 S Haifets D Teytelman Phys Rev Spec Top Acc and beams 8 064402 (2005)
312 E Weihreter Compact synchrotron light sources Series On Synchrotron Radiation Techniques
And Applications - Vol 3 World Scientific
313 P Lebasque et al Synchrotron SOLEIL lsquoEddy current septum magnets for booster injection and
extraction and storage ring injection at Synchrotron SOLEILrsquo EPAC 2006 Edinburgh Scotland
314 P Lebasque et al Synchrotron SOLEIL lsquoOptimisation of the coating thickness on the ceramic vacuum
chambers of SOLEIL storage ringrsquo EPAC 2006 Edinburgh Scotland
315 P Lebasque et al Synchrotron SOLEIL lsquoFour matched kicker systems for the SOLEIL storage ring
injection a full solid-state solution of pulsed power supplies working at high currentrsquo EPAC 2006
Edinburgh Scotland
316 F Marhauser E Weihreter (BESSY) ldquoImpedance Measurements of the HOM-Damped 500 MHz
Metrology Light Source Cavityrdquo BESSY Gmbh Germany 08 January 2007
317 M Langlois P Sanchez M Cornelis F Perez (ALBA) ldquoMeasurements on the RF cavity for the ALBA
storage ringrdquo Proceedings of EPAC08 Genoa Italy
318 R A Rimmer (Lawrence Bekeley Lab) et al ldquoDevelopment of a high-power RF cavity for the PEP-II B
factoryrdquo DE-AC03-76SF00098 (LBL)
lsquo
159
319 P Fernandes R Massarino R Parodi ATarditi (INFN Italy) A Massarotti (Sincrotrone Trieste Italy)
rdquoThe Design of the RF Cavities for ELETTRArdquo IEEE 1989
320 M Svandrlik A Fabris C Pasotti (Sincrotrone Trieste) rdquoImprovements in curing coupled bunch
instabilities at ELETTRA by mode shifting after the installation of the adjustable higher order mode frequency
shifter (HOMFS)rdquo IEEE 1998
321 W Bruns web site httpwwwgdfidlde
322 P Marchand et al Development of high RF power solid state amplifiers at SOLEIL Proceedings of the
IPAC2011 San Sebastian Spain p 376
323 C Pardine et al Status of the 476 MHz 50 kW solid state amplifier Proceedings of the IPAC2010
Kyoto Japan p 3972
41 YAG(CE) data sheet Saint Gobain Crystals httpwwwdetectorssaint-gobaincomYAG28Ce29aspx
42 SPI Suppliesreg YAGYAP Comparison Table
httpwww2spicomcatalogscintillsintillator_properties_comparison_tablehtml
43 K Nakamura et al (Particle Data Group) J Phys G 37 075021 (2010)
44 Wartski et al Detection of optical transition radiation and its application to beam diagnostics IEEE
TransNuclSci20544-5481973
45]Saturation of YAG Beam Profile Monitors Measured at ATF A Murokh J Rosenzweig V Yakimenko X
J Wang
46 Radiation Detection by Cerenkov Emission in Optical Fibers at TTF E Janata M Koumlrfer Tesla-Report
2000-27
47 CERN-2009-005 p273
48 CERN-2009-005 p272
51 J Bonis et al 2012 JINST 7 P01017 doi1010881748-0221701P01017
61 J Bonis et al Non-planar four-mirror optical cavity for high intensity g ray flux production by pulsed laser
beam Compton scattering off GeV-electrons JINST SISSA Vol 7 January 2012
62 R V Pound lsquolsquoElectronic Frequency Stabilization of Microwave Oscillatorsrsquorsquo Rev Sci Instrum 17 490ndash
505 (1946)
71 httpwwwjjxraydkhome
72 RW Alkire et al Nucl Inst and Meth A 350 (1994) 13-16
73 B Lengeler et al J Synchrotron Rad 6 (1999) 1153-1167
74 RL Owen et al J Synchrotron Rad 16 (2009) 143-151
75 MS Nogueira et al Radiation Protection Dosimetry 111 (2004) 105-110
76 M Krumrey et al AIP Conf Proc 705 (2004) 861-865
M Gerlach et al Metrologia 45 (2008) 577-585
77 httpwwwcanberracompdfProductsUltra-LEGe-SS-M3843pdf
78 K Sakaue et al Rev Sci Instrum 80 (2009) 123304
79 httpsaleshamamatsucomassetspdfparts_FF9892-31_-32_TMCP1039E03pdf
710 O Chubar et al Proc DIPAC (2001) CT09
711 D Paterson et al Opt Com 195 (2001) 79-84
712 M Mahesh RadioGraphics 21 (2001) 1033-1045
713 K Medjoubi et al JINST 6 C01080 (2011)
httpwwwvariancomusxrayproductsdigital_radiographypaxscan_productshtml
714 H Elleaume et al Physics in Medicine and Biology 47 (2002) 3369-3385
lsquo
160
715 Y Prezado et al Medical Physics 38 (2011) 1709
716 I Martiacutenez-Rovira and Y Prezadoa Medical Physics 38 (2011) 4430
717 L Samain Thesis Liege university (ULG) 2012
718 P Martinetto Thesis Joseph Fourier university (UJF) 2000
719 P Bleuet et al Analytical Chemistry 29 (2010) 518-527
720 P Bleuet et al Nature Materials 7 (2008) 468-472
81 httpwwwprofibuscomncdownloadspecifications-standardsdownloadsprofibus-standard-dp-
specificationdisplay
82 httpwwwtango-controlsorgDocumentstango-kernel
lsquo
161
Table of content
INTRODUCTION 2
1 The ThomX project 2
2 Scheme and options 3
3 General layout of the machine 4
4 Machine parameters 6
5 Summary of the TDR main technical points 8
Chapter 1 THE THOMX INTERACTION REGION 9
11 Electron beam and laser pulse characteristics in the interaction region 9
12 Expected flux and its characteristics 10
13 Position stability 11
14 Timing stability 12
15 Energy stability 13
Chapter 2 INJECTOR 14
21 Injector specifications 14
22 Technical description of the linac components 16
221 Photo‐injector 16
222 Solenoids 17
223 Laser 18
23 Acceleration section 20
231 The RF source 21
232 RF network 22
24 Beam dynamics 23
241 Nominal operation 23
242 Alternative operation modes 24
Chapter 3 TRANSFER LINE and RING 28
31 Transfer line 28
32 Extraction line 31
33 Storage ring 31
331 Optics and lattice 31
332 Non‐linear dynamics 35
333 Collective effects feedbacks 37
334 CBS and IBS effects 42
335 Scattering on the residual gas 46
336 Ion effects 47
337 Electron beam injection and extraction 48
338 Orbit corrections 50
341 Dipole magnetic design 53
342 Quadrupole magnetic design 54
343 Sextupole magnetic design 56
344 Corrector magnetic design 58
345 Injection dipole 59
lsquo
162
35 Power supplies 60
351 Pulsed magnets 60
36 Storage ring RF system 65
361 HOM impedance thresholds as regard to beam instabilities 66
362 Cures to the HOM induced instabilities 66
363 RF power source 70
364 Low level RF system and longitudinal feedback 71
365 RF equipment status 73
366 RF system conclusion 73
37 Transverse feedback 74
371 Specifications 74
372 Electronics 75
373 Kicker 76
38 Vacuum 77
381 Transfer line vacuum 77
382 Ring vacuum 78
Chapter 4 INJECTOR TRANSFER LINE AND RING DIAGNOSTICS 80
41 Charge measurement 80
411 Current transformers 80
412 Faraday cups 81
42 Transverse profile measurements 81
421 Diagnostics stations 81
43 Emittance measurement 84
44 Beam position measurement 85
441 Beam Position Monitors (BPM) 86
442 Orbit feedback system 90
443 Tune measurement and feedback 90
444 Energy measurement 90
45 Bunch length measurement 90
451 Cerenkov radiation 90
452 Synchrotron radiation 92
453 Transport of the Cerenkov and synchrotron radiations to the diagnostics hutch 93
454 Streak camera for the bunch length measurement 93
46 Bunch imaging using synchrotron radiation 94
47 Beam loss measurement 94
48 Beam dumps 94
Chapter 5 SYNCHRONIZATION 95
51 Overall scheme 95
52 Specifications 96
Chapter 6 OPTICAL SYSTEM DESIGN 97
61 Feedback system and results on a Fabry‐Perot cavity locking 97
611 Electron and FPC pulse synchronization 98
612 Laser parameters to be stabilized 99
lsquo
163
613 PDH Measurement technique 101
614 Actuators 103
615 The digital feedback system 107
62 High average power laser system and its coupling to a high gain FP cavity 110
621 The MightyLaser oscillator 111
623 Stretcher and compressor 115
624 ThomX development 115
63 Mechanical design 118
631 Optical cavity vacuum 123
Chapter 7 X‐RAY BEAM LINE AND EXPERIMENTS 124
Introduction 124
71 Available extracted X‐ray beam 124
72 Global view of the X‐ray lne 124
73 Chraracterization of the X‐ray beaugrave 126
731 Optical table 1 126
732 Beam transport 128
733 Optical table 2 128
74 Demonstration experiments 130
741 Experiments using the full X‐ry beam 130
742 Experiments using the central part of the beam 134
75 Summary 139
Chapter 8 CONTROL SYSTEM 140
Introduction 140
81 General principles 140
82 Technical considerations 141
83 The TANGO control system software 142
831 Software bus 142
832 A control system for accelerators 142
84 Control system architecture 143
841 Equipment to control 143
842 Inputsoutputs 144
843 Front‐end machine architecture PLCs and industrial PCs 145
85 IT infrastructure 146
Chapter 9 SAFETY AND RADIOPROTECTION 149
91 Introduction 149
92 PLC safety 149
93 Radiation monitoring 150
94 Radiation protection zoning 150
95 Access control 150
951 General statement 150
952 Particular requirements concerning the lsquorestricted areasrsquo 151
953 Access to the accelerator area and to the Igloo basement 151
954 Access to the X‐ray line and to the Compton room 151
lsquo
164
96 Beam dumps 152
97 Interlocks 152
98 Accelerator emergency shutdown 152
99 Control console 152
910 Evacuation of dangerous areas 152
911 Audio system and horns 152
912 Safety notices 153
913 Radiological monitoring 153
Chapter 10 MANAGEMENT OF THE THOMX PROJECT 154
Introduction 154
101 Partnersrsquo complementarity 156
102 ThomX larger framework 157
lsquo
6
Fig 3 Layout of the interaction region and of the optical cavity integration
4 Machine parameters
The ThomX machine is composed of four main systems the injector the storage ring the laser and the
Fabry-Peacuterot resonator As mentioned previously we foresee some flexibility in the machine operation
so that the X-ray energy can be varied For this all four systems are dimensioned to withstand an
operating energy up to 70 MeV Nevertheless to characterize the machine a baseline configuration has
been chosen at 50 MeV In Tables 1 2 3 and 4 the parameter values which are given below correspond
to this baseline configuration
Table 1
Injector
Charge 1 nC
Laser wavelength and pulse energy 266 nm 100 microJ
Gun Q and Rs 14400 49 MWm
Gun accelerating gradient 80 MVm 5 MW
Normalized rms emittance 44 π mm mrad
Energy spread 04
Bunch length 43
lsquo
7
Table 2
Table 3
Laser and FP cavity
Laser wavelength 1030 nm
Laser and FP cavity Frep 3568 MHz
Laser power 50 ndash 100 W
Laser pulse energy 14 ndash 28 microJ
Fabry-Perot pulse energy 28 mJ
Fabry-Perot pulse length (rms) 5 ps
FP cavity finesseGain 3000-30000 1000- 10000
FP waist 70 microm
Power circulating in the FP cavity ~ 007 ndash 07 MW
Ring
Energy 50 MeV (70 MeV possible)
Circumference 168 m
Crossing-Angle (full) 2 degrees
βxy IP 01 m
εxy just after injection 5 10-8
m
Bunch length just after injection (rms) 4 ps
Bunch length at the end of a 20 ms storage
cycle
50 ps (rms)
Beam current 1784 mA
RF frequency 500 MHz
Transverselongitudinal damping time 1 s 05 s
RF Voltage 300 kV
Revolution frequency 1784 MHz
σx IP (just after injection) 70 microm
Tune xy 317 174
Momentum compaction factor αc 00136
InitialFinal relative energy spread (with IBS
and Compton back-scattering)
0406
lsquo
8
Table 4
Source
Photon energy cut-off 46 keV (50 MeV) 90 keV ( 70 MeV)
Total Flux 1011
-1013
photons
Bandwidth (with diaphragm) 1 - 10
Divergence 10 mrad (1γ) without diaphragm 50 MeV
5 Summary of the TDR main technical points
After the publication of the CDR much work has been put in simulations design and technical choices
The goal of this TDR is to provide a reference document in view of the machine construction as well as
a more precise planning and a more precise costing
The first chapter deals with the machine performances and the tolerances to be met so that the collisions
between the electron bunch and the laser pulse do take place in the conditions specified in the CDR
Then the main systems are described and our technical choices are presented This last section is
subdivided as follows
1 Linac
2 Beam transfer and injection section ring
3 Laser and Fabry-Peacuterot cavity
4 X-ray user line
A special attention is given to the integration of each system and to the global integration of the ThomX
machine The lsquoOptical systemrsquo chapter includes a short report on recent results obtained in the laser and
Fabry-Peacuterot cavity RampD program both in the laboratory and on a beam line at KEK-ATF (Japan)
Special chapters are dedicated to diagnostics the control of the main systems and their synchronization
safety and radioprotection The last chapter presents the project management
lsquo
9
Chapter 1 THE THOMX INTERACTION REGION
11 Electron beam and laser pulse characteristics in the interaction region
The ThomX injection frequency has been fixed at 50 Hz (20 ms period) by taking into account the
Compton scattering recoil effect which continuously deteriorates the beam longitudinal emittance This
effect is all the more important as synchrotron damping is strongly suppressed since the beam energy is
low Because of this lack of damping the Compton scattering recoil effect and eventual beam
instabilities must be controlled by longitudinal and transverse feedback systems
We stress the fact that in ThomX the beam does not reach a permanent state during a storage cycle its
properties are continuously varying To describe the Compton interaction dynamics we define a set of
nominal beam characteristics which are those prevailing just after the injection of a fresh e- bunch
Simulations have been carried out to characterize the X-rays emitted during a single crossing of the
electron bunch and the photon pulse (stored in the optical cavity) as a function of the collision
geometric parameters
These nominal collision parameters are as follows
Parameter value unit
Crossing angle 2 degree
Electrons energy 50 MeV
Electrons energy spread 03
Number of electrons 625 109
Fabry Perot pulse energy 28 mJ
Repetition rate 1784 MHz
Laser pulse transverse dimension (rms) 35 microm rms
Electron bunch transverse dimension (rms) 70 microm rms
Laser pulse duration (rms) 5 ps rms
Electron bunch length at injection 4 ps rms
Table 11 Nominal collision parameters
To extend these results to other geometric configurations one has to take into account the parametric
dependence of the Compton collision rate The X-ray flux is the product of the Compton cross-section
and the luminosity It is proportional to the electron bunch intensity and to the laser pulse power The
geometry dependence is given by the luminosity formula which involves the geometric 6D size and the
crossing angle Assuming Gaussian bunches and neglecting the hourglass effect the ring luminosity is
given by the following expression
Eq 11
L = NeNγ f cosφ
2π
1
σ ye
2 +σ yγ2 σ xγ
2 +σ xe
2( ) cos2φ + σ ze
2 +σ zγ2( ) sin2φ
lsquo
10
where σeγxyz represent respectively the two transverse dimensions and the longitudinal one of the
electron bunch and of the laser pulse f is the collision frequency Ne and Nγ are the number of
electrons per bunch and photons per pulse Φ is half the crossing angle
The X-ray energy cut-off is higher for head-on collisions and for photons which are exactly back-
scattered In this case the photon energy gain is 4 γ2
This calculation provides a useful reference to assess the efficiency of other configurations but it
remains an approximation since exact results must take into account the beam evolution during a
storage cycle and should be obtained from a full revolution by revolution simulation
12 Expected flux and its characteristics
In nominal conditions the expected flux is 78 1012
X-rayss whether one uses Eq 11 or the monte-
carlo simulation code CAIN Figure 11a shows the energy spectrum of the scattered photons and its
cut-off at 45 keV As shown in Fig 12b most photons are emitted very close to the interaction region
axis within a small cone with a 2 mrad opening angle They may be selected by using a small
diaphragm The energy spectrum of the X-rays emitted in a cone of 225 mrad opening angle is shown
in Fig 12a with a FWHM energy spread of 25 keV
Figure 11 Results of simulations performed with CAIN using the parameters listed above a)
Energy spectrum b) Transverse size of the source c) Angular emission distribution d) Horizontal (x-
xrsquo) phase space of the scattered X-rays
lsquo
11
The spot size at the source is dominated by the electron bunch of 7 10-5
m as illustrated in Fig 11b
The dimansions are assumed to be the same in the horizontal and vertical plane The total divergence
of the source is around 15 mrad given by the electron beam energy (ie the 1γ parameter) with an rms
size of 4 mrad (Fig 11c) The emittance of the photon source shown in Fig 11d is 03 mm mrad The
energy-angle correlation and the collimation effect are illustrated in Fig 12 As expected the high-
energy X-ray flux is peaked with a 1γ opening angle The diaphragm which is assumed for this
simulation selects emission angles and thus photon energies resulting in a ~ 10 monochromatic
photon beam
Figure 12 Results of simulations performed with CAIN using the parameters listed above
Left panel Energy spectrum of photons emitted in a cone with an opening angle of 225 mrad
Right panel Number of photonsmradsup2s versus energy and angle (the color bar indicates the photon
number)
13 Position stability
To optimize the X-ray flux a precise transverse alignment must be acieved in the interaction region It
can be performed either by steering the electron beam or by adjusting the optical table position These
procedures will be monitored by means of a double BPM which is integrated in the Fabry-Peacuterot cavity
vacuum chamber exactly at the interaction point The resolution of this technique is a few microns A
simulation has been performed to calculate the flux reduction resulting from a possible mismatch
between the electron bunch orbit and the laser pulse trajectory Figure 13 shows the number of X-rays
produced per bunch crossing and per keV as a function of a transverse displacement of the optical
cavity axis with respect to the electron orbit from zero to 140 microns
lsquo
12
Fig13 Number of back-scattered photons and their energy as a function of a transverse alignment
error This figure clearly shows that the losses entailed by a displacement comparable to the BPM
sensitivity are negligible The color bar indicates the number of X-rays produced at each crossing
14 Timing stability
A precise synchronization between the electron bunch and the laser pulse at the interaction point is
another important requirement A timing mismatch introduces a longitudinal shift between the two
bunches at the IP This can be due to an error in the location of the Fabry-Perot cavity with respect to
the e- storage ring or to a jitter in the synchronization system Fig 14 shows the variation of the X-ray
flux as a function of a longitudinal displacement
The flux is not so much reduced as long as the jitter is less than 10 ps This number fixes the
synchronization tolerance and thus the performance of the longitudinal feedback
Fig 14 Flux and energy spectrum of back-scattered photons as a function of the relative
synchronization of the beams (longitudinal displacement)
lsquo
13
15 Energy stability
The nominal ThomX injection energy is 50 MeV Phase rotation in the RF source or phase mismatch
will generate a jitter in the injection energy Such a jitter entails synchrotron oscillations which will be
damped by the longitudinal feedback However before this feedback has stabilized the longitudinal
oscillations the collision conditions will not be nominal This situation has been simulated the results
are illustrated in Fig15 One can see that it is in the higher part of the X-ray energy spectrum that the
loss is more important
Fig 15 Flux and spectrum of the back-scattered photons as a function of the e- energy
lsquo
14
Chapter 2 INJECTOR
The most important components of the linac injector are the electron gun and the acceleration section
which brings the beam to the desired energy Other components are the laser used for the e- gun the
magnetic elements used to transport the beam the diagnostics the vacuum system the waveguides and
the radio-frequency (RF) source Each component is detailed in the following First of all the injector
specifications are given since they lead to the choice of some of the linac components
21 Injector specifications
The project goal namely to produce a high flux of X-rays of energy 50 keV leads to the following
specifications for the linac
Energy 50 MeV
Charge 1 nC
Number of bunches per RF pulse 1
Emittance (rms normalised) lt5 mm mrad
Energy spread rms lt1
Bunch length rms lt5 ps
Average current 50 nA
Repetition frequency 50 Hz
Table 21 Nominal linac parameters
First of all one notices that the low average current 50 nA does not justify the use of cold technology
(ie of a superconductor) for the acceleration structure But there is still a large choice of possibilities
for an RF structure operating at room temperature For economic reasons and because of the long
experience accumulated in the field we have chosen an RF structure operating at 3 GHz This
frequency is a European standard
Furthermore the above specifications led us to choose a photo-injector for the electron source since a
low emittance beam is required Again to minimize risks we chose to build the same gun as the one we
constructed for the CLIC lsquoTest Facility 3rsquo at CERN [21] This gun has been running successfully for 4
years To increase the current per bunch we will use a metallic magnesium photocathode which can
deliver more than 1 nC with a laser pulse energy of a few tens of a microJ at a wavelength of 260 nm The
gun is made of 2 12 copper cells magnetically coupled to the waveguide To get 1 nC with an
emittance lower than 5 mmmrad an accelerating field of 80 MVm is required which means an RF
power of 5 MW in a 3 micros pulse The electron beam energy output by the gun is around 5 MeV
We will use an industrial laser to extract the electrons from the cathode by photoelectric effect The
main specifications of this laser are the following ones Wavelength 260 nm energy per pulse 100 microJ
pulse duration around 5 ps It must be synchronized with the RF frequency and triggered at 50 Hz This
laser will be installed in an optical hutch located outside the concrete shielding which will enclose the
accelerator Therefore its beam must be transported with dielectric mirrors and lenses over a distance of
about 20 meters
Concerning the acceleration section SOLEIL a partner of the project will lend us a lsquoLILrsquo section This
section was made by LAL for the pre-injector of LEP at CERN It has been kept as a spare for the
SOLEIL linac The ThomX project will provide an other section in case SOLEIL needs one The
section length is about 46 m To reach a final energy of 50 MeV the energy gain in the section must be
lsquo
15
45 MeV The latter value implies an electrical field of 125 MVm and an RF power of about 10 MW
However the accelerator should reach higher energies in order to produce X-rays beyond 50 keV Since
the maximum targeted X-ray energy is 90 keV the linac design should allow a beam energy of 70
MeV corresponding to an RF power of 20 MW Thus taking into account the RF system efficiency the
specification for the total RF power is set at 35 MW
This specification lead us to choose a klystron producing 35 MW in a 45 micros pulse To feed the
klystron a modulator is needed that provides high voltage pulses of typically 240 kV through a high
voltage transformer soaking in an oil tank The two technologies which can be used for this component
are discussed in detail below
For the electron beam steering two coils and two magnetic correctors are required The coils are placed
around the gun Opposite sign currents circulate in these coils (-310 A 336 A) so that the magnetic field
vanishes at the cathode location Its amplitude is 023 T roughly 7 cm downstream from the cathode
Concerning the magnetic horizontalvertical correctors once a good mechanical alignment is achieved
experience has shown that beam trajectory errors are not larger than a few mm between the gun and the
acceleration section Hence the correctors must produce a beam deflection of up to 10 mrad For the
model operating at LAL [22] two bipolar power supplies of 10 A per corrector are adequate
For the diagnostics three different instruments will be installed between the RF gun and the
acceleration section An Integated Current Transformer (ICT) will measure the charge per bunch
Beyond this device a beam position monitor (BPM) will measure the transverse positions (x y) of the
electron beam We will use a BPM based on striplines which was designed for SOLEIL The third item
comprises a vacuum chamber equipped with an arm which can be translated by compressed air in order
to introduce a YAG screen to intercept the beam The YAG crystal fluoresces when ionized by the e-
beam The emitted light shows the beam transverse shape A short optical transport line and a CCD
camera allow the measurement of the transverse dimensions LAL designed and constructed the
diagnostics chamber It has already been successfully tested in the PHIL facility A drawing showing
the linac layout is shown in Fig 21
lsquo
16
Figure 21 Linac layout In the upper figure the different components of the LINAC are illustrated
starting form the gun and subsequently passing trough the stripline the ICT for current measurements
the laser introduction chamber the diagnostics station the steerers and the accelerating section
22 Technical description of the linac components
221 Photo-injector
It is a copy of a gun built by LAL for the CLIC Test Facility 3 of CERN the so-called lsquoprobe beam
gunrsquo[22] Simulations have shown that the performances of this gun fulfill the ThomX specifications
We are thus benefiting from all the technical drawings which already exist and all the fabrication
processes are mastered by LAL from the machining to the brazing This RF gun is comprised of 2 12
cells The output cell is magnetically coupled and the other end is short-circuited The main parameters
are given in Table 22
Frequency 299855 MHz
Rs 50 MΩm
Q 14500
β 1
E 80 MVm
PRF 5 MW
Table 22 RF characteristics of the photo-injector at 30degC in vacuum
The photo-cathode will be made of magnesium It has the big advantage of producing the specified
charge without requiring an in-situ photo-cathode preparation chamber which is a heavy and expensive
lsquo
17
installation Preliminary tests performed at LAL have demonstrated the possibility to obtain good
quantum efficiency with such cathodes
The main difficulty expected in operating this device is the laser cleaning of the photocathode surface
when polluted by an exposure to air To perform this cleaning a laser intensity of 160 microJmm2 is
required together with an RF electric field of at least 50 MVm The magnesium disk holder is not
designed yet it could be press fitted screwed or brazed
222 Solenoids
The gun is equipped with two solenoids one is the focusing coil while the other is the bucking coil
which cancels the magnetic field at the photo-cathode location They have been modeled by usning
POISSON A schematic is shown in Fig 22
Figure 22 Schematic of the gun with its coils
Dimensions
For both solenoids the inner diameter is 160 mm and the outer one is 500 mm The copper part is 30
mm long for the bucking coil and 56 mm for the focusing coil
The conductor used for these magnets is a standard one with a 6x6 mm2 square shape It is cooled by
water circulating in a 4 mm diameter channel One can easily calculate the current the resistance the
dissipated power and the temperature rise assuming that the cooling water is supplied at 20degC and 8
bars Values are provided for each lsquopancakersquo since the conductors are wound by columns of two The
bucking coil Bbc has two pancakes and the focusing coil Bfoc four
Bbc Bfoc units
Number of coils 84 168 MHz
Number of pancakes 2 4
Current -274 326 A
P 23 33 kW
R 306 31 mΩ
Flow 164 164 lmn
ΔT +20 +28 degC
Table 23 Solenoid electric and cooling parameters lsquoNumber of coilsrsquo refers to one pancake
Two power supplies are required one for Bbc (400A 30 V) and one for Bfoc (400A 60 V)
lsquo
18
223 Laser
The laser extracts a bunch of electrons from the photo-cathode surface by photo-electric effect Table
24 specifies its characteristics
Parameter value unit
l 260‐270 nm
Emax 100 plusmn 05 microJ
Beam transverse quality M2 M2 lt 11
Gaussian Pulse width 5 ps
Transverse profile square
Pointing stability 10 of beam diameter
Oscillator frequency 74963750 externally triggered with jitter lt 1 ps MHz
Amplifier repletion rate 50 externally triggered Hz
Table 24 Laser specifications for the RF gun photocathode
The laser architecture is shown in Figure 23
Figure 23 Architecture of the laser proposed by industry
An oscillator provides a signal at the specified frequency It can be either a fiber or a TiSaphire laser
Then the pulse is stretched amplified (with one or two amplifiers) recompressed and converted in the
UV range
The laser will be installed on an optical table in a hutch whose temperature and hygrometry will be
regulated
224 Laser injection chamber
The laser injection chamber is located at a distance of 8011 mm downstream of the photo-cathode It
contains a metallic aluminum mirror placed 30 mm away from the accelerator axis It provides an 80
reflectivity The angle of incidence of the laser beam on the photocathode is 214deg This mirror is fixed
To adjust the laser beam position on the photo-cathode and its incident angle we will use a motorized
mirror with a remote control placed on an optical table just beside the laser injection chamber
The size of this optical table will be 1x1 m2 On it there will be
Oscillator Stretcher Preamplifier Amplifier Compressor
nJ microJ
mJ
Frequency
shifter
Clock
Syncro
Line syncro
Shaper
lsquo
19
- 2 dielectric mirrors
- 2 beam splitters
- 1 UV photo-diode to monitor the laser intensity
- 1 CCD camera to observe the laser spot on a screen (in paper or lanex) facing the photo-cathode
- An optical beam expander to adjust the laser spot size on the photo-cathode
This is the minimum number of optical items that will be put on the table some changes may be made
depending on the very final geometry of the accelerator and laser transport lines The set-up design can
be finalized during the last stage of the accelerator construction since it involves standard components
only
225 Beam steering coils
The e- beam trajectory will be adjusted by sets of two steering coils which produce dipolar magnetic
fields over short distances along the X and the Y axes One such device will be located at the gun exit
and the other about 800 mm downstream Only very small position and angle errors must be corrected
Figure 24 shows the LAL design for such a coil Experience with the PHIL photo-injector at LAL [22]
showed that an integral of Bmiddotdl of 52 G m is sufficient The latter figure has been validated by
calculations made by a private company With a current of 5 A a Bmiddotdl of 58 G m was obtained that
results in a 100 mrad deviation of a 5 MeV beam Thus a 100 mm lateral displacement of the beam can
be produced at the entrance of the acceleration section
Figure 24 Schematic of the beam steering coils Dimensions are in mm
All together the steering coils need 4 bipolar DC power supplies Their maximum current is 10 A
lsquo
20
23 Acceleration section
The acceleration section is shown in Fig 25 Its characteristics are given in Table 25
Figure 25 Picture of the SOLEIL LIL acceleration section
Length 45 m
Number of cells mode 135 2π3
Frequency 299855 at 30degC MHz
Rs 63divide74 MΩ
Q 14800
Attenuation 74 dB
Energy gain 15 MW 60 MeV
τ 135 micros
Ez max 182 MVm
Table 25 Characteristics of the acceleration section
In a second stage of the ThomX project a call of tender will be issued for a new acceleration section
that should be shorter in order to make the machine more compact
lsquo
21
231 The RF source
Figure 26 shows the RF source schematic
Figure 26 Schematic of the RF source
The RF timing and its synchronization are described in chapter 5 A clock signal is sent to the linac
which uses it to trigger a 3 GHz synthesizer Its output is fed to a preamplifier which brings its power to
350 W The preamplifier is triggered at 50 Hz Then the 3 GHz signal is amplified by a klystron whose
gain can reach 53 dB to produce a 35 MW RF wave as required by ThomX This power is distributed
via a waveguide network which is described below
To drive the klystron one needs a modulator delivering a pulsed high voltage (HV) whose
specifications are given in Table 26 The klystron cathode must be heated by a power supply that
provides a current rising slowly from 0 to 25 A and a voltage rising from 0 to 30 A in 20 minutes A
precision of 1 on the current value is required The klystron is generally equipped with focusing coils
and their power supply whose standard parameters are 40 A 120 V
Repetition rate 50 Hz
Plateau length 99 gt 3 micros
Plateau ripples lt plusmn 05
Pulse duration at 50 6 micros
Rise time 10 divide90 lt 05 micros
Fall time 90 divide10 lt 1 micros
Pulse to pulse stability lt 05
Voltage range 0divide300 kV
Peak current 285 kV 300 Hz
Table 26 Modulator specifications
The classical and well-proven modulator technology relies on the high voltage (40 kV) loading of a
delay line made of inductances and capacitances The delay line is switched by a thyratron A new
technology is presently emerging based on solid-state amplifiers Each amplifier produces a 100 V
lsquo
22
pulse Thus to produce a 40 kV pulse roughly 400 amplifiers are put in a rack and their output pulses
are added The performances of both technologies are similar
232 RF network
The RF power is transported and distributed to the electron gun and the acceleration section by a
waveguide network shown in Fig 27
Figure 27 Schematic of the RF waveguide network
The WR 284 waveguide network is pressurized at two bars with SF6 between ceramic windows at the
klystron output and at the acceleration structure entrance A 5 dB bidirectional coupler splits the power
in two channels In the branch which runs toward the electron gun there is a 10 dB attenuator and a
circulator to protect the klystron from the RF power reflected by the electron gun In the branch running
toward the acceleration structure there is a 10 dB attenuator and a phase shifter to adjust the relative
phase between the gun and the acceleration section Various RF loads (five in total) ensure the system
safety by absorbing of RF power of the reflected waves
lsquo
23
The network also includes three couplers to measure the incident and the reflected RF power One is
placed at the klystron power exit one is placed at the gun entrance and the last one is placed at the
acceleration section entrance The RF gun is equipped with a loop that measures the stored RF energy
All of these loops used for measurements are weakly coupled at -70 dB The signals are measured with
crystals or C boxes
24 Beam dynamics
241 Nominal operation
The beam dynamics was simulated with the PAMELA tracking code for the ThomX nominal
parameters ie a bunch charge of 1 nC at 50 MeV The 50 MeV can be obtained with an electric field
of 80 MVm in the RF gun and 125 MVm in the acceleration section It is clear that a different set of
field values can lead to the total of 50 MeV In particular one may wish to operate the gun at a higher
field in order to improve the beam quality We will operate the linac with this possible improvement in
view
The beam will be accelerated on crest Thus the coil magnetic fields are the only parameters savailable
to minimize the transveres and the longitudinal emittances Simulations have shown that for precise
magnetic field values (see Table 27) the e- beam emittances are optimized as illustrated in Fig 28
20
15
10
05
00
σxy
(m
m)
6005004003002001000
z (cm)
20
15
10
5
0
em
it (
mm
mra
d)
6005004003002001000
z (cm)
20
15
10
05
00
ΔE
E (
)
6005004003002001000
z (cm)
46
44
42
40
38
36
34
32
30
σz (p
s)
6005004003002001000z (cm)
lsquo
24
Figure 28 Beam characteristics (rms values) as a function of the distance along the accelerator (z
coordinate) Upper left panel Transverse size upper right panel normalized emittance lower left
panel Energy spread lower right panel Bunch length
The beam parameter values at the linac end are given in Table 27
Beam parameters value unit
Energy 50 MeV
Beam size 17 mm
Normalised Emittance 44 π mm mrad
Energy spread 04
Bunch length 43 ps
Table 27 Beam parameter rms values at the linac end
The accelerator parameters which should ensure the above beam characteristics are given in Table 28
RF structure parameters value unit
Ez in the gun 80 MVm
PRF in the gun 5 MW
Ez in the section 142 MVm
PRF in the section 14 MW
Solenoid parameters value unit
Bz max 2500 G
Ibc -274 A
Ifoc 326 A
Laser parameters value unit
Energy impinging on a Mg photocathode 10 microJ
Square transverse profile diameter 2 mm
Variation of the intensity from axis to border 14
Table 28 Summary of the linac parameters
242 Alternative operation modes
Concerning the injector other operation modes can be envisaged One at least will occur namely an
operation at 70 MeV in order to extend the X-ray energy range up to 90 keV Such an option has
attractive applications In this mode the electron beam performances should remain exactly the same as
the ones prevailing at 50 MeV (cf Table 27) For the accelerator parameters the only change is the RF
power required for the acceleration section it must reach 195 MW instead of 14 MW at 50 MeV
The operator can also be tempted to change the RF phase The one used in the simulations gives the
minimum emittance A phase shift of -10deg of the RF gun with respect to the laser could lead to an
energy spread reduction by a factor 2 at the expense of an emittance degradation Whatever the goals
the linac operator must be aware of the high sensitivity of the beam parameters to the RF phase This is
illustrated in Fig 29 An error of 5deg in the acceleration section phase results in almost a doubling of the
energy spread
lsquo
25
Figure 29 Energy spread at the linac end as a function of the RF phase in the acceleration section all
other parameters keeping their nominal values
The RF electrical field in the gun also plays an important role in the beam performances The results of
simulations of the emittance variation as a function of the gun electric field are presented in Fig 210 If
this field reaches 120 MVm the emittance is reduced by 20 with respect to the 80 MVm case
However the possibility to operate the gun at such a high gradient can only be assessed experimentally
Figure 210 Injector beam emittance as a function of the electric field in the electron gun (Gaussian
laser beam 1 nC e- bunches)
The injector performances as a function of the bunch charge are summarized in Table 29 Three modes
will be available in order to reduce the effect of collective instabilities at injection
Parameters Mode 1 Mode 2 Mode 3 Unit
Q 1 05 01 nC
σxy 165 15 08 mm
εxy 47 33 11 π mmmrad
σz 44 4 33 ps
σEE 04 03 02
Table 29 Linac beam performances (rms values) as a function of the bunch electric charge
As one can see in the above Table reducing the bunch charge by a factor 10 leads to an emittance
reduction by a factor 4 Although the X-ray flux is reduced the source brightness ie the ratio of the
flux to the divergence is improved by a factor 16 As a consequence a higher number of X-rays can be
20
18
16
14
12
10
08
06
04
02
δΕ
Ε (
)
151050-5-10
φ (deg)
10
9
8
7
6
5
ε (πmmmrad)
1401201008060
E0 (MVm)
lsquo
26
transported to a sample (number per unit area) Whatever the eventual benefits to the X-ray production
the advantage for the accelerator to operate at a lower charge per bunch is clear the reliability of the
accelerator is much enhanced the laser energy requirement is reduced and it becomes also possible to
use a copper photo-cathode instead of a magnesium one easier to handle But the main advantage lies
in the ring operation Indeed a 1 nC bunch whose length is only 4 ps induce a strong wakefield
stemming from the interaction between the beam and the vacuum pipe Such a wakefield perturbs the
beam and may cause oscillations of the bunch centroid possibly instabilities and thus a reduction of the
X-ray production rate
25 Photo-injector and acceleration section vacuum
Simulations of the photo-injector vacuum have been performed assuming a pressure of 3 10-8
Pa at the
cathode location and requiring a vacuum time constant (VS) that ensures an easy gun conditioning This
constraint must be respected to maintain a 10-7
Pa pressure at the acceleration section entrance where the
system will be baked to obtain an outgassing rate τ = 5x10-10 Pa m s-1
(N2)
Pressure in the line of injection
The use of a semi-conductor cathode (Cs2Te) puts severe constraints on the vacuum and requires a preparation
and a transfer chamber To avoid these complications some tests have been made on a metallic cathode (Mg)
The first attempt ran into difficulties (it took 12 h to reach a gradient of ~ 60 MV m-1
) we observed pressure
rises and sparks In presence of only the RF power we observed hydrogen CO and CH4 outgassing (as the
main components) A slight reduction in the quantity of water and oxygen was noticed due to chemical re-
composition When the laser beam was introduced to extract electrons a pressure increase by a factor of ten
was observed The 24th
25th
and the 26th
peaks also significantly increased for the HxCy compounds but no
Mg was measured
lsquo
27
Gas analyzes during the conditioning of cathode Mg
On the basis of simulations and experimental results we identified the need for the following items
‐ 2 ion pumps 40 l s-1
(upstream of the cathode amp injector) with their baking systems and power
supplies
‐ 2 ion pumps 55 l s-1
(for the RF network) with their baking systems and power supplies
‐ 1 ion pump 150 l s-1
(for the laser introduction chamber) with its baking system and power supply
‐ 1 ion pump 300 l s-1
(for the acceleration section) with its baking systems and power supply
‐ 1 gate valve manually actuated DN 100 (injector- upstream) and a baking valve
‐ 1 gate valve electro-pneumatically actuated DN 63 (injector - downstream) and a baking valve
‐ 1 gate valve electro-pneumatically actuated DN 40 (injector- downstream) and a baking valve
‐ 3 Pirani gauges (before the injector injector laser introduction acceleration section)
‐ 3 cold cathode gauges (after the injector injector laser introduction acceleration section)
‐ 2 power supply gauges
‐ 1 residual gas analyzer (RGA)
‐ 1 turbo pump (DN 63 ~ 80 l s-1
)
‐ 1 rotary vane pump (~ 12 m3 h
-1)
‐ 4 angle valves manually actuated for pre-pumping DN 63
‐ 2 T with RF grids
lsquo
28
Chapter 3 TRANSFER LINE and RING
31 Transfer line
The transfer line (TL) has to transport the beam from the linac to the ring and to match the optics of
these two devices In addition a set of beam diagnostics needed for an efficient injection in the ring are
localized along the line as sketched in Fig 31 The transfer line main characteristics are summarized in
Table 31
Figure 31 Layout of the injection and extraction transfer lines
A particular attention is paid to ensure a versatile optics that allows the matching of different linac
beams to different ring optics Simulations have been carried out using the BETA code [31] The
transfer line optics is optimized in order to accommodate some emittance degradation which may arise
from collective effects
Screen station + dump
Linac emit + pulse length diags
Screen + Scraper
Linac energy diags
Screen station
TL emit diags
Extraction line dump
lsquo
29
Figure 32 Transfer line optics including the ring The vertical line indicates the ring entrance point
The nominal transfer line optics is shown in Fig 32 from the linac to the ring together with the ring The
nominal input bunch Twiss parameters are x = z = 34 x = z = 42 for Q = 1 nC bunch length = 4
ps (rms) and an optimized normalized emittance of about 5 πmmmrad The strong bend produced by the four
dipoles of the transfer line increases the risk of emittance degradation under collective effects but simulations
show that the emittance increase is only of about 20 including coherent synchrotron radiation (CSR [2]) and
3D space charge effects (see Fig 33)
Figure 33 Emittance increase due to chromatic (cont lines) CSR (dashed) and space charge (dot dashed)
effects for 1 nC and 4 ps (rms) bunches at 50 MeV along the TL and the first revolution in the ring
The magnetic elements are identical to those of the ring
lsquo
30
Parameter value unit
Length 14 m
Nominal energy 50 MeV
Beta max x z 70 40 m
Maximum dispersion 1 m
Dipole numberfamily 4 1
Quadrupole number 7
Biplane corrector number 5
Table 31 Transfer line parameters and components
Besides ensuring the matching between the linac and the ring the transfer line must provide the measurement
of the beam emittances the beam energy and the energy spread Care has been taken to locate the diagnostic
stations in a way that ensures accurate measurements of these important parameters
The emittance measurements will be done by means of the lsquothree-gradientrsquo scheme (The pepper-pots
alternative is also foreseen) For instance with an adequate refocusing that produces a double waist (vertical
and horizontal) at the screen it is possible to scan both planes at the same time by varying only one
quadrupole gradient (see Fig 34) Other scan variations are also possible The following figures show the
optics retuning as well as the beam spot size variation at the screens of both stations the one at the linac end
and the one downstream of the U-turn
Figure 34 Optics retuning and beam spot size scan as observed on the screen (simulation)
The electron energy and its spread are measured in between the dipoles by a dedicated optics Note that it is
also possible to retrieve the bunch length by measuring the energy spread as a function of the linac phase
(three-phase scheme)
lsquo
31
32 Extraction line
To dump the residual beam at the end of the storage time (typically from 20 to 50 ms) an extraction line
composed of one small dipole and two quadrupoles is also foreseen This line will allow the measurement of
the beam properties after it has circulated in the ring The injectionextraction devices are symmetrically
implemented in the ring injection section
33 Storage ring
331 Optics and lattice
Thom-X must produce a high X-ray flux of 1011
divide1013
phs in a stable way The flux depends on the
characteristics of the electron bunch and of the laser pulse Assuming a 1 nC electron bunch charge at
50 MeV and a 20 to 30 mJ laser pulse ( 124 microm) the optics constraints at the interaction point (IP)
are as follows
- an rms size of the electron beam less than or equal to 70 microm in the transverse plane requiring
low beta functions of 01 m with an rms geometric emittance of 50 nmrad injected from the
linac (5 mmmrad normalized)
- a vanishing dispersion at the interaction point in order to avoid a beam size increase resulting
from an e- energy spread increase as well as to avoid potential synchro-betatron coupling
resonances
- a bunch length less than 20-30 ps to ensure a good overlap with the laser pulse (the crossing
angle will be 2deg) and long enough to reduce collective effects stemming from the pipe
impedance CSR bunch internal scattering etc
Besides these specific IP constraints there are two general layout constraints namely
- a ring circumference as short as possible with enough straight sections to accommodate the
interaction region the RF the injection and possible other equipment as RF and transverse
feedback systems
- the need for some flexibility as far as the optics and the working point are concerned
The ring layout and its optics are based on a two-fold symmetric lattice with four achromatic arcs and
four straight sections two short ones (02 m) and two long ones (16 m) This layout preserves the ring
compactness and provides an adequate interaction region in one of the two straight sections between
two adjacent dipoles (cf Fig 35) The main ring characteristics are summarized in Tables 32 and 33
lsquo
32
Figure 35 Ring layout
The laser pulse frequency is the harmonic 2 of the e- bunch revolution frequency in the ring ie 357
MHz This leaves open the possibility of storing two bunches in the ring in order to increase the
Compton flux but at present this operation mode is not taken into account because of injection
constraints The optical cavity mirrors are located outside the ring vacuum chamber thus enabling a
mirror substitution without affecting the ring In addition the extraction of the X-rays can be performed
closer to their source than in the case where the optical cavity is inserted in a long straight section
The two long free straight sections accommodate the injectionextraction devices for one and the RF
and feedbacks for the other
The ring is composed of 8 dipoles 24 quadrupoles and 12 sextupoles Its circumference is 168 m long
the revolution frequency is 1785 MHz The RF frequency is set at 500 MHz ie at the 28th
harmonic
of the ring revolution frequency The nominal optical functions have been optimized with the BETA
code [31] They are plotted in Fig 36 The dispersion vanishes in the long and in the short straight
sections (RF injection and interaction regions) The beta functions decrease down to 01 m at the IP to
provide a 70 microm focus assuming that the injected beam has an emittance of 50 nmrad (5 mmmrad
normalized) in both the vertical and the horizontal planes
RF and feedback section
Injection extraction section
IP 1
Interaction Point (IP)
lsquo
33
Ring parameters value unit
Circumference 1680 m
Nominal energy 50 MeV
Betatron tunes nx nz 317 174
Beta max xz 11 11 m
Dispersion max 08 m
Beta xz dispersion IP 01 01 0 m
Momentum Compaction Factor 0014
RF frequency 500 MHz
RF harmonic 28
RF voltage 300 kV
Synchrotron frequency 360 kHz
Period Revolution frequency 56 1784 nsMHz
Natural chromaticities -32 -82
Dipole nb family 8 1
Quadrupole nb families 24 6
Sextupole nb families 12 3
Corrector nb families 12 12
Table 32 Ring parameters
Max field 70 MeV Bore length Gap Diameter
Dipole 07 T 2765 mm
(R=352 mm)
40 mm
Quadrupole 5 Tm 150 mm 41 mm
Sextupole 15 Tm2 60 mm 42 mm
Correctors 510-4
Tm
Table 33 Main magnet characteristics
Taking into account the constraints imposed by the optical functions the chromaticity and the dynamic
aperture the operating point was chosen with tunes νx=317 and νz=174 There is still enough
flexibility to vary these tunes by plusmn 02 without spoiling the main ring performances The momentum
compaction factor can also be changed over a large range in order to scan the best working point
regarding the beam dynamics and the X-ray flux
Figure 36 Nominal beta functions and dispersion over one super-period
lsquo
34
The RF acceptance for the nominal momentum compaction factor (MCF) and a larger one including the
non-linear MCF and the vacuum pipe limit are plotted in Fig 37
Figure 37 RF acceptance Left panel nominal lattice Right panel for a MCF twice as large as the
nominal one VRF= 300 kV
RF acceptance nominal r56 = - 02 m r56 = - 04 m
Linear (RF bucket) 10 10
Non linear -32 +18 - 68 +4
Pipe limit 25 25
Table 34 RF acceptance for two different values of the momentum compaction factor
The two following plots (Fig 38) show (i) the vacuum chamber occupation by the beam at injection
when the emittance is 50 nmrad and the energy dispersion is 03 and (ii) at the end of the storage
cycle when the emittance reaches 300 nmrad and the energy dispersion reaches 06 (see section
438) The orbit components (misalignment errors) are not included since they are negligible once
corrected The choice of the pipe aperture (40 mm in H by 28 mm in V full aperture) takes into account
several constraints the beam pipe occupation the magnet bore radius the vacuum and the coherent
synchrotron shielding [32]
b a
lsquo
35
Figure 38 Transverse occupation (3 rms) of the vacuum chamber at injection (left) and at the end of
the storage cycle (right)
332 Non-linear dynamics
The natural horizontal and vertical chromaticities are respectively -32 and -68 Chromaticity
corrections are achieved by means of 12 sextupoles (3 families) located in the dispersive sections
Without performing a specific optimization the dynamic aperture is large enough about 50 times the
rms transverse size of the injected beam (see Fig 39) the quadrupole fringe fields being taken into
account Furthermore this dynamic aperture remains about the same for off-momentum particles (up to
plusmn3) compared to particles having the nominal energy The fringe field of the dipole magnets
introduces high order non-linear components Since the Thom-X dipoles are short with a large gap
their magnetic design (see section 34) has been carefully worked out in order to maintain the dynamic
aperture
Pole number 6 8 10 12 18 20 28 30
Inner dipoles +10 0 +3
Outer dipoles -16 -1 -6
Quadrupoles +10 +5 -5
Sextupoles +10 -10
Table 34 List of multipole tolerances for each kind of magnets expressed in 10-4
relative field variation
at 18 mm radial position
Figures 39 and 310 show the change of the dynamic aperture with and without considering the
multipole (the fringe field of the quadrupoles is also taken into account in these plots) These
Dynamical Apertures (DA) have been calculated with two different codes BETA and TRACYIII They
lsquo
36
both give very similar results In addition the TRACYIII code dedicated to long symplectic tracking
performs Frequency Map Analysis (FMA) giving more detailed information concerning the particle
diffusion process
Figure 39 On-momentum DA and FMA for the nominal ThomX lattice Beta code left and TracyIII
code (right) sextupoles only (top) and including magnet multipole tolerances (bottom)
lsquo
37
Figure 310 Off-momentum DA and FMA for the nominal ThomX parameters Beta code left and
TracyIII code (right) sextupoles only (top) and including magnet multipole tolerances (bottom)
333 Collective effects feedbacks
At low energy and thus low beam rigidity the electron bunch is sensitive to electromagnetic self-
perturbations that act revolution after revolution in the ring Several instability sources can be
identified
‐ Interaction with surrounding metallic items such as the resistive walls (RW) the RF cavity and
various discontinuities such as bellows BPMs and flanges HOMs etc
‐ Space charge forces
‐ Coherent synchrotron radiation (CSR)
These interactions affect the bunch energy and the bunch longitudinal time-profile leading to
distortions emittance growth bunch oscillations or even above some threshold instabilities and beam
losses Similar interactions may also affect the transverse beam dynamics but to a lower extent The
two relevant parameters for the instability threshold are the bunch charge and its length The strength of
the interaction is proportional to the bunch charge and varies inversely with the bunch length
The two plots in Fig 311 show the longitudinal wakefield over one revolution at 50 MeV generated by
a 1 nC Gaussian bunch with an rms length of 12 and 6 mm (4 and 20 ps) Five different contributions
are compared in this figure
‐ the shielded CSR following the Murphy and Skrinsky [32] formulation with a beam pipe full height
of 40 mm a dipole field of 05 T and a curvature of 0352 m
‐ the longitudinal space charge (LSC)
‐ the resistive wall (RW) following the K Banes and M Sands formulation [33] with a full beam
pipe height of 40 mm and an electrical conductivity of 14 106
‐ the vacuum vessel broadband component including 12 BPM 12 flanges and 12 bellows based on
the synchrotron SOLEIL type This potential has been calculated by means of the HFSS finite
elements code [34]
‐ the 500 MHz ELETTRA type cavity broadband component This potential has been calculated by
using the HFSS finite element code
They exhibit a very strong CSR wake field (up to few per mil) with the shorter bunch case
Figu
re
311
Lon
gitu
dina
l
wak
e
field
over
one
lsquo
38
turn generated by a 1 nC Gaussian bunch at 50 MeV for a bunch length of 4 ps (left panel) and 20 ps
(right panel)
The ThomX low energy operation (50 MeV) practically suppresses the natural damping effect of the
synchrotron radiation and complicates the bunch stabilization Each kind of perturbation grows
irreversibly implying that feedback systems in the three planes are essential to cope with the induced
oscillations of the bunch center-of-mass In addition to these collective effects the unavoidable
injection jitters have also to be taken into account To calculate these effects 6D tracking simulations
that include the longitudinal impedance as well as the large mismatch between the linac and the ring
optics in the longitudinal phase space have been performed (see Fig 312)
Large longitudinal oscillations
without feedback at injection
induced by collective effects
Longitudinal oscillations damped by the
RF phase feedback system operating at
F~360 kHz
Damping lt 50 micros
Figure 312 Tracking results (50 MeV
1 nC bunch) over the first 3000
revolutions just after injection without
and with the RF phase feedback system (VRF = 300 kV)
Large oscillations start very quickly after injection when the bunch is still short (4 ps) and are of the
same order of magnitude as the ones induced by the energy-phase jitter that affect the linac injection
This effect is the one to address in the design of the longitudinal feedback As far as the transverse plane
is concerned the growth times characterizing various instability sources are listed in Table 35 The
revolution-by-revolution feedback strength indicated for each instability source is the one needed to
suppress the oscillations
Source Type Growth time Revolution-by-
revolution kicker
strength
Beam pipe
geometry
TMCI
Head-Tail
-
160 micros
gt10 nrad
Resistive Wall
600 micros gt 2 nrad
Ions lt 100 micros gt20 nrad
Table 35 Transverse instability growth time and feedback kicker strength for a 1 nC bunch at 50 MeV
lsquo
39
In contrast with what prevails in the longitudinal phase space the main transverse oscillation source is
the injection position jitter Since the non-linear sextupole components are large this jitter may produce
a large emittance increase as a consequence of the phase space painting process (see Fig 313)
Figure 313 Sketch of the transverse phase space painting process
Simulations show that a very fast increase of the transverse emittance can result from steering errors at
injection in about 500 revolutions or 25 micros To overcome this effect a fast transverse feedback is
needed it has to damp the oscillation in about 100 revolutions or ~5 micros The strength of the revolution-
by-revolution feedback has to be on the order of 2 microrad This strength is far larger than the ones
estimated to cope with the transverse instabilities For this transverse feedback a dedicated strip-line
acting in both planes is foreseen in the RF section (see Fig 314)
Figure 314 Simulated transverse emittance increase induced by a 05 mm injection jitter with and
without feedback
After N
turns
lsquo
40
Phase space Specifications Device
Horizontal amp vertical Fr Frequency = 178 MHz (F0)
Strength = 2 microrad
Dedicated strip-line for
both planes
Longitudinal Frequency = 360 kHz (Fs)
Strength = 105 rads
Main RF phase loop
Table 36 Summary of the feedback specifications
Other collective effects such as the intra-beam scattering (IBS) which is important at low energy or the
Compton recoil effect play an important role during the 20 ms storage time Their dynamics differ from
an instability dynamics but they also increase the electron energy spread and the bunch emittance mdash
and thus reduce the X-ray flux
To summarize three different contributions to the e- bunch dynamics must be dealt with
‐ at injection jitters which may affect either the injected bunch energy or its trajectory or the precise
timing of its injection in the ring generate oscillations which may be large and which must be
quickly damped by feedbacks in all three dimensions (x y z)
‐ during a transition period corresponding to the first ~1000 revolutions strong collective effects
enhanced by a possible synchronization error between the linac and the ring may lead to a complex
turbulent regime in the longitudinal plane prior to ring matching (elliptical phase space density
profil)
‐ during the 20 ms storage period IBS and CBS produce a progressive increase of the bunch size in
all three directions This evolution cannot be stopped by a feedback system In contrast RF HOMs
and other perturbations may generate instabilities that should be controlled by feedbacks
To take into account collective effects a dedicated code has been developed including a collection of
wake field as well as non-linear dynamics It is based on a 6D multi-particle symplectic tracking
restricted to perfect hard edge drift quadrupoles and dipoles of finite length Other higher order
elements as sextupoles are treated in the thin length approximation The RF cavity is simply treated as
a longitudinal cosine kick The mapping through elements is given by the exact Hamiltonian solution
[34] for the drift sections and the dipoles (including their small curvature and dipole edge effects) The
quadrupole mapping is restricted to small radial angles It has been compared to various codes
(TRACYIII ELEGANT BETA and MADX) by performing simulations of the SOLEIL and the
ThomX rings with special care given to the dipole edge effects [35]
The transverse beam size at IP and the bunch length are plotted in Fig 315 over the first 10 000
revolutions or 056 ms with a 1 nC bunch charge This study shows that a small positive chromaticity in
the horizontal plane is necessary to stabilize the horizontal emittance During the very first revolutions
the tune dependent emittance behavior has not been fully investigated nor understood It appears that
this behavior follows from the bunch length mismatch between the linac and the ring The fact that the
linac produces very short bunches of 4-5 ps (rms) leads to a complex turbulent dynamics (see Fig
316) The effect of this mismatch disappears progressively over the first 1000 revolutions then the
bunch length reaches 20-25 ps (rms)
lsquo
41
Figure 315 Red RMS beam size and bunch length at IP Magenta bunch centroid position over the
first 10 000 revolutions (056 ms) for a 1 nC bunch The beam energy is 50 MeV
Figure 316 Turbulent dynamics in the longitudinal phase space that us taking place just at injection
These plots show the phase space density after 100 and 3000 turns
lsquo
42
334 CBS and IBS effects
The electron beam dynamics that prevails in a ring where Compton back-scattering is taking place is
similar to the one resulting from synchrotron radiation (SR) in a synchrotron light source CBS gives
rise to random excitation in the transverse and the longitudinal planes In the transverse plane the
emittance turns out to remain close to the one at injection [37] In contrast the CBS excitation strongly
affects the dynamics in the longitudinal phase space It leads to a substantial energy spread increase
over 50 ms storage time (see Fig 317a) resulting in a significant bunch lengthening
Based on the lattice parameters the calculation of the Touschek lifetime leads to a value on the order of
10 s this effect can thus be neglected
Figure 317 a) Energy spread increase and b) Radial (x xrsquo) relative emittance increase during a 100 ms
storage time Blue curves CBS effect only red curves IBS effect only black curves (almost on top of
the blue ones) CBS and IBS combined effect The IBS calculation is based on the Mtingwa formalism
[38]
In contrast the multiple Touschek effect (lsquoIBSrsquo) plays a significant role in the radial phase space (x xrsquo)
as shown in Fig 317b After 20 ms this horizontal emittance is increased by a factor ~ 4 from 50 to
200 nmrad and the relative energy spread by ~20 from 03 to 036 as can be seen in Fig 317a
Figure 317 shows that the Compton back-scattering process plays a role by far more important than the
IBS does It also shows that because of the large emittance increase produced by the CBS the bunch
density decreases by a large amount making the IBS much less effective (see Fig 317b where the
curve showing the combined effects lies below the curve illustrating the IBS effect by itself)
When the electron beam energy is increased up to 70 MeV the IBS effect on the horizontal emittance
becomes less important During a 20 ms storage cycle this effect is reduced by a factor 2 On the other
hand the energy spread degradation increases by more than a factor two compared to the 50 MeV case
This change is due to the higher energy of the back-scattered photons In contrast if the electron beam
energy is decreased down to 25 MeV the energy spread is reduced by a factor two while the horizontal
emittance increases by a factor seven compared to the 50 MeV case
Assuming an optimal overlap between the e- bunch and the laser pulse in the interaction region and
neglecting the e- bunch size variation across this region (the lsquohourglassrsquo effect) as well as the variation
of the laser beam envelope near its waist (characterized by the Rayleigh length) the X-ray flux is given
by the following formula (see Eq 11)
The nominal storage time is 20 ms only
lsquo
43
where σth is the Thomson cross section Nx is the number of photons per laser pulse f is the e- bunch
revolution frequency ϕ is half the collision angle σxyze σxyzγ are respectively the three dimensional
bunch rms sizes for the electron (e subscript) and for the laser pulse (γ subscript) In fact because of the
increase of the e- bunch size during the storage time the flux is reduced by roughly a factor two at the
end of a cycle as shown in Fig 318
Figure 318 X-ray flux as a function of the storage time These results take into account the energy
spread increase and the emittance degradation due to IBS only (blue curve) and to the combined CBS
and IBS effects (black curve)
To calculate more precisely the X-ray flux produced by Thom-X the code CAIN [39] (cf sect 12 above)
has been included in the previous 6D tracking code including the non linear effects of the lattice
magnets as well as collective effects The X-rays characteristics are computed at each ring revolution
The intra-beam scattering effect is calculated also at each e- bunch revolution based on the Bjorken and
Mtingwa formalism [38] To reduce the computing time the code extrapolates the IBS effect whenever
there is only a little change from one revolution to the next (this method reduces the computing time by
a factor 4) This code runs on linux OS Using the computer farm of the IN2P3 computing center in
Lyon it takes about 15 days to simulate a full Thom-X cycle (20 ms ie 400 000 revolutions) when the
e- bunch is modeled by 500 000 macro-particles
The way the total X-ray flux decreases during the first 10 000 revolutions is shown in Fig 319 This
decrease is due to the e-
beam quality degradation mainly due to the CBS effect Also shown in this
figure is the X-ray flux evolution as a function of the storage time for three different lower cut-offs in
the X-ray energy spectrum Figure 320 shows a typical angular distribution of the X-ray emission
Figure 321 plots the bunch longitudinal phase space for the first revolutions The macro-particles in
green are those which have been hit by photons at the last crossing while those in blue have been hit
before the others are in red
( ) ( ) φσ+σ+φσ+σσ+σ
φfNNσ=N
2
γzez
2
γxex
2
γyey
γeth
x22222
sincos
2πcos
lsquo
44
Cross comparison and benchmarking of this code against other codes developed for ThomX (6D
tracking without Compton effect or Compton + IBS without tracking) are in progress and indicate a
good agreement between the codes
Figure 319 Left panel Number of X-rays produced as a function of the X-ray energy at the end of the first
10 000 first revolutions (in red) and during the last 10 000 revolutions before extraction (in blue) Right
panel Number of X-rays produced summed over 2500 revolutions as a function of the revolution number
Figure 320 X-ray intensity as a function of the emission angle in mrad (horizontal axis) and their
energy in eV (vertical axis) The color bar indicates the number of X-rays
lsquo
45
Figure 321 Upper nine panels Evolution of the e- beam longitudinal phase-space after injection The macro-
particle positions are plotted along the horizontal axis while their energy deviations are plotted along the
vertical one Green electrons which have collided with photons at the last crossing blue electrons which
have collided before red electrons which have not yet undergone any collision These plots reveal a turbulent
dynamics Lower nine panels Time evolution of the beam longitudinal phase space as a result of a turbulent
regime from the initial lsquobanana shapersquo at injection to a Gaussian distribution after a few ms (colors indicate
the electron number-density red corresponds to the most populated region)
lsquo
46
Figure 322 Evolution of the X-ray energy spectrum when the e- beam energy is increased from 475 to 505
MeV Left panel The X-ray energy spectrum is plotted vertically for seven e- beam energies Colors
characterize the X-ray intensity Right panel Each curve shows the total number of X-rays above some lower
energy cut-off Green cut-off set at 20 KeV blue cut-off at 30 KeV red cut-off at 40 KeV
335 Scattering on the residual gas
Coulomb scattering of the electrons on the residual gas molecules induces an increase of the circulating
beam emittance The amplitude of this effect is proportional to the average of the beta functions and to
the gas partial pressure Moreover it varies as the square of the atomic number of the residual gas
atoms and is inversely proportional to the electron energy With a low beam energy of 50 MeV it is
therefore essential to keep a low pressure to minimize the time-averaged emittance Fig 323 shows the
growth of the transverse emittance during the 20 ms storage time in the nitrogen case and in the
hydrogen one Assuming that one deals with nitrogen at a pressure of 8 10-7
Pa the emittance increases
by a factor of two (see Fig 323) while the degradation is negligible in the dihydrogen case We
conclude that a pressure lower than 3 10-7
Pa must be maintained to keep the emittance growth at an
acceptable level
Figure 323 Emittance increase due to Coulomb scattering assuming that the residual gas is composed
of a) 100 nitrogen b) 100 dihydrogen
lsquo
47
336 Ion effects
The beam-ion instability [310 39 312] originates in the ionization of the residual gas molecules Ions
can subsequently be trapped by the electron beam electrostatic potential We recall that the on-axis
potential is inversely proportional to the beam size
When the trapped ion number density reaches some threshold beam instability is triggered Besides this
effect trapped ions damage the beam quality by shifting the tunes increassing the emittance and
lowering the beam lifetime since the local pressure is increased
For the Thom-X ring the critical mass above which ions are trapped is on the order of one in units of
the proton mass Thus all ions whatever their mass can be trapped in our case
Once trapped ions oscillate transversally in the potential well Their oscillation frequency depends on
the electron beam size and on the ion species (see Fig 324 1c) Because of the e- beam bunching
when the ion oscillation frequency becomes larger than the beam betatron frequency a lsquofast ion
instabilityrsquo may develop the e- bunch is submitted to the electric field of the oscillating ions eventually
resulting in an exponential growth of the oscillations of all particles (electrons and ions) This instability
arises above some value of the neutralization factor which depends on the lattice
The neutralization factor and the growth time of either the vertical or the horizontal oscillations are
shown in Fig 324 as a function of the horizontal beam size for several ion species For a horizontal
beam size averaged over the ring of 05 to 1 mm (which is about what is expected) the growth time is
very short (less than one micros) but for this would happen for a neutralization factor much larger than one
obviously this is excluded To keep under control the other ion harmful effects mentioned above one
may prevent ion accumulation over many storage cycles by introducing gaps between short series of
injections or by clearing the ions by an electrostatic potential that cancels out the potential well
produced by the e- beam A simple solution consists in polarizing the beam position monitors (BPMs)
thus avoiding the insertion of electrodes along the ring This scheme should be effective since ions will
drift longitudinally as the kicks they receive at each e- bunch revolution has a longitudinal component
The amplitude of these kicks along the z direction is shown in Fig 325 where the BPMs are
represented by circles They are placed near accumulation points where the longitudinal kick vanishes
ie where ions will converge A difficulty may arise at s =125 m This point is opposite to the IP in the
lattice at this point the e- beam has small transverse dimensions and a vacuum pump is placed Indeed
with a small beam size the neutralization factor threshold is very low (10-3
) and the growth time
constant is on the order of one micros This potential difficulty can be overcome by adding an electrode in
the short section between the two dipoles to clear the ions at this accumulation point
Ion clearing with polarized BPMs or gaps introduced in the injection sequence should keep other ion
effects at an acceptable level (more work is needed to quantify this statement)
Defined as the total number of ions divided by the total number of electrons
lsquo
48
Figure 324 a) Threshold neutralization factor b) growth time and c) ion oscillation frequency for the
following fractional becirctatron tunes 1) 064 and 2) 017 for different ion species The dashed lines show
the e- bunch betatron frequency σx is the mean beam size
Figure 325 Longitudinal kick given to on-axis ions at each e- bunch revolution
337 Electron beam injection and extraction
Originally the linac beam was injected in the ring by using only a septum followed by a fast kicker
Given the high repetition rate (50 Hz) radiation safety issues and a possible noise in the X-ray
experimentation area lead to set up an e- beam extraction line The symmetric septum magnet located in
lsquo
49
the middle of one of the long straight sections is used in a double pass mode for injection and
extraction It is flanked by two identical kickers one downstream for the injection and one upstream for
the extraction (Fig 326)
Figure 326 Sketch (top panel) implementation (middle panel) and simulation (lower panel) of the
injectionextraction process
The main characteristics of the septum and the kicker magnets are listed in Table 37 Since in both
processes the beam runs through quadrupoles some margins with respect to the nominal deflection
values must be provided to cope with the fact that quadrupole fields are not uniform
Kicker Septum
Deflection 15 mrad max (11 nominal) 170 mrad (40 mTm 70 MeV)
Pulse duration lt 56 ns half sine 130 micros full sine
Effective length 200 mm
250 mm
Overall length 450 mm 500 mm
Table 37 Main characteristics of the pulsed devices
Septum center
x =14 mm
xp=9 mrad + 170 mrad
Kicker center
x =0 mm
xp=0+11 mrad
2 fast kickers
Inj Ext
1 slow septum
lsquo
50
338 Orbit corrections
A set of 12 beam position monitors (BPMs) and 12 correctors in both planes are implemented along the
lattice in order to control and correct the orbit An additional BPM located at the IP is fixed on the
table that supports the laser cavity The main error sources are listed below Uncorrected and corrected
orbits are illustrated in Fig 327
rms value dBB dL dX dZ dS tilt
Dipoles 10-4 01 mm 02 mm 01 mm 02 mrad
Quadrupoles - - 02 mm - -
Dipoles 10-4 01 mm 02 mm 01 mm 02 mrad
Quadrupoles - - 02 mm - -
Table 38 Main magnet misalignment errors
Uncorrected orbit with typical magnet alignment errors
rms value xmax (mm) zmax (mm)
Max 7 3
IP 05 03
rms value xʹmax (mrad) zʹmax (mrad)
Max 12 4
IP 6 3
lsquo
51
Corrected orbit
rms value xmax (microm) zmax (microm)
Max 200 140
IP 200 30
rms value xʹmax (microrad) zʹmax (microrad)
Max 600 300
IP 350 60
Figure 327 Uncorrected and corrected closed orbits Red lines horizontal projection blue lines
vertical projection
Quadrupoles and sextupoles are fixed on girders with a maximum error of 50 microm The magnetic field
center error relative to the magnet yoke being of the same order the expected total alignment errors are
~100 microm in each plane The girders supporting all the elements will be aligned as solid blocks Since
lsquo
52
dipole misalignments have a large effect on the orbit dedicated additional support tools will allow the
readjusting of the dipole positions relative to the girder
One needs a maximum corrector strength of 05 mrad when this strength is computed by assuming just
one sigma alignment errors to cope with alignment errors which would be twice as large corrector
strengths of 1 mrad are thus necessary
Figure 328 Orbit bumps (red horizontal blue vertical) that can be produced at the IP
In addition to this standard orbit correction procedure a dedicated orbit scan at the IP will be very
useful to ensure an optimal overlap between the e- bunch and the laser pulse in the interaction region
(see Fig 328) To provide a scan range of up to 1 mm or 1 mrad the corrector strength must be further
increased by 1 mrad Thus the corrector maximum strength will be 2 mrad
The orbit correction will be performed by a slow feedback operating around 01 The correctors are in
the sextupoles magnets (combined magets) The orbit feedback correction will also control the RF
frequency in order to keep the orbit length at the ring design value A specific synchronization scheme
that ensures the proper injection timing and that maximizes the electron-photon collision rate is
described in chapter 5
Position amplitude scan at IP location up to 1 mm
Angle amplitude scan at the IP
location up to 1 mrad
lsquo
53
34 Magnets
Table 39 describes the magnets that will equip the transfer line the ring and the extraction line
Magnet type Dipoles Dipoles for inj Quad Sext Corr
Number 4+8+2 2 7+24+2 12 4+12+1
PS number 3 2 33 3 34
Strength 0180 T m 5 Tm 40 Tmsup2 05 mTm
Length 2765 mm 150 mm 60 mm 30 mm
Table 39 ThomX magnet parameters
341 Dipole magnetic design
The dipole bending angle is 45deg A magnetic field of 05 T is needed at 50 MeV Since their radius of
curvature is very small (0352 m) special attention has been paid to the field profile between the magnet poles
and to the pole edge shape where large multipole components are present Lateral shims and pole edge
specific profiles have been optimized with the TOSCA3D code to minimize the impact of the multipoles on
the beam dynamics
Characteristics unit value
Gap mm 40
Field at centre T 063
Overall dimension mm3 350 x 400 x 400
Current A 263
Voltage V 177
Pole width mm 100
Pole height mm 100
Yoke length mm 2765
NI Aturns 13000
Coil area mmsup2 67x40
Current density Ammsup2 116
Number of turns turns 48
Transverse size of the conductor mmsup2 6x6
Cooling channel mm 4
Water pressure drop bar 7
Temperature increase degC 47
Flow rate (per dipole) lmn 86
Electric power (per dipole) W 3100
Table 310 ThomX dipole parameters
lsquo
54
Figure 329 Drawing of a ThomX dipole
342 Quadrupole magnetic design
The quadrupole design (see Fig 330) was also performed with the TOSCA software The magnetic field
is produced by four coils carrying a DC current The conductor is wrapped around four poles which
concentrate and define the quadrupole magnetic field map
Figure 330 Quadrupole magnetic design
A quadrupole can be manufactured in four identical parts each one comprising one pole and one quarter
of the yoke or the yoke and the four poles can be manufactured separately The quadrupole will be
equipped with special shims which will be adjusted during the magnetic measurement to ensure a good
alignment of the magnetic axis and low harmonic components The ThomX quadrupoles are air-cooled
to reduce the infrastructure cost This is possible since the maximum current density is 15 Ammsup2
Figure 331 shows the magnetic field intensity versus the radial distance from the quadrupole axis The
maximum gradient is 5 Tm Figure 332 shows the magnetic field intensity at a distance of -20 mm
from the magnet axis versus the longitudinal coordinate z The maximum magnetic field is 01 T
lsquo
55
Figure 331 Magnetic field (in T) versus the distance (in mm to the magnet axis
Figure 332 Magnetic field intensity (in T) versus z (the longitudinal coordinate in mm)
lsquo
56
The main parameters of the ThomX quadrupoles are given in Table 311 Each quadrupole is powered
by a 10 A 32 V DC power supply thus the released heat power remains below 32 W A dedicated
magnetic bench equipped with a rotating coil will be used for the quadrupole magnetic measurements
This bench has been developed for the magnetic measurements of the SOLEIL storage ring
quadrupoles
Parameters unit value
Bore mm 41
Gradient Tm 5
Current density Ammsup2 15
Overall dimensions cm3 19x19x18
Current A 10
Voltage V 35
NI Aturns 810
Pole width mm 20
Pole height mm 50
Coil dimensions mmsup2 15x36
Number of turns turns 81
Wire dimensions mmsup2 5x125
Yoke length mm 150
Yoke thickness mm 20
Chamfer mm 5 (not optimised yet)
Table 311 Main parameters of the ThomX quadrupoles
343 Sextupole magnetic design
The ThomX sextupole design (see Fig 333) was also performed with the TOSCA software The magnetic
field is produced by six coils that carry a DC current The coils are wrapped around six iron poles which
concentrate the field and define the sextupole field map These magnets are combined function and are also
equipped with additional coils to produce the corrector dipole fields
Figure 333 Sextupole magnetic design
lsquo
57
Each sextupole yoke is manufactured as a single piece and the six poles are screwed on this yoke Then a coil
is wound around each pole During the design phase a chamfer was calculated with TOSCA to guaranty a low
harmonic content The sextupoles will be air-cooled to reduce the infrastructure cost The maximum current
density is 15 Ammsup2
The magnetic field intensity versus the distance to the magnet axis is shown in Fig 334 The maximum
sextupolar strength is 40 Tmsup2
Figure 334 The ThomX sextupole magnetic field versus the distance to the magnet axis
Fig 335 shows the variation of the magnetic field at a distance of -20 mm from the sextupole axis versus the
longitudinal coordinate z The maximum magnetic field is 0016 T
Figure 335 ThomX sextupole magnetic field versus z (the longitudinal coordinate)
Table 312 gives the main parameters of the ThomX sextupoles They will be powered by a 10 A 05 V DC
power supply
lsquo
58
Parameters unit value
Bore diameter mm 42
Sextupolar Strenght Tm2 40
Field 20 mm T 0016
Current density Amm2 25
Overall dimension cm3 19x19x8
Current A 10
Tension V 05
AmpereTurn At 100
Pole width mm 20
Pole height mm 40
Coil dimension mm2 6x40
Number of turns Tours 10
Conductor dimension mm2 2x2
Yoke length mm 60
Yoke thickness mm 20
Chamfer mm 15
Table 312 Main characteristics of the ThomX sextupoles
A dedicated bench equipped with a rotating coil will be used for the magnetic measurements of the sextupoles
This bench has been developed for the magnetic measurements of the SOLEIL storage ring sextupoles
344 Corrector magnetic design
Simple window frame correctors based on an already existing SOLEIL design will be used for the orbit
corrections of the two transfer line
Figure 336 Magnetic design of the window frame correctors
lsquo
59
345 Injection dipole
The two injectionextraction dipoles compensate the septum deflection with a 170 mrad deflection (cf sect 337
see Fig 326) They are also air-cooled Their design and their technical characteristics are illustrated in Fig
336 and Table 313
Figure 336 Magnetic design of the injection dipole
Parameters unit value
Gap mm 40
Field at the magnet centre T 0224
Current A
10
Voltage V 6
Pole width mm 60
Pole heigth mm 70
Yoke length mm 150
AI (per coil) At 3600
Coil cross section area mmsup2 60x60
Current density Ammsup2 09
Number of turns tours 360
Conductor size mmsup2 10 x 1
Pressure drop of the cooling water bar NC
Temperature increase degC NC
Power (per dipole) W 120
Table 313 Parameters of the injection and extraction dipoles
lsquo
60
35 Power supplies
In order to provide a high operation flexibility ThomX will be equipped with the following power supplies
ndash One power supply for the ring dipoles (the latter will be connected in series) one for the TL dipoles
and one for the DL
ndash Individual power supplies for all the other magnets (quadrupoles sextupoles and correctors)
All the power supplies will have a 16 bit-resolution and a plusmn20 ppm reproducibilitystability over 24 hours The
technology is a switched-mode one in order to achieve good efficiencies (gt 85)
Magnet Nb PS Nb mag current
(A)
voltage
(V)
type load
R
(mOhm)
L
(mH)
Dip ring 1 8 300 170 Unipol 565 48
Dip TL 1 4 300 85 Unipol 280 24
Dip DL 1 2 300 45 Unipol 150 12
Dip inj 2 1 12 18 Unipol 1330 180
Quad 33 1 12 6 Unipol 500 10
Sext 12 1 12 3 Unipol 250 08
Corr ring 24 1 plusmn12 plusmn3 Bipol 250 08
Corr TL DL 10 1 plusmn12 plusmn3 Bipol 250 08
Table 314 Overview of the ThomX power supply equipment
351 Pulsed magnets
3511 Layout
As discussed above the e- beam injection in the storage ring and its extraction will take place in the
same straight section (see Fig 337)
Figure 337 Injectionextraction section layout
Eddy current septum magnet
(under vacuum) Fast Kicker Fast Kicker
lsquo
61
The first injection step involves a thin septum magnet It bends the incoming bunch trajectory to bring it
inside the ring vacuum chamber close to the stored beam orbit Then a fast kicker compensates for the
residual trajectory angle and brings the injected bunch on-axis
Extraction is realized by another fast kicker followed by the same septum magnet as the one used for
injection This kicker deflects the bunch trajectory so that it enters into the septum channel in turn the
thin septum bends the bunch trajectory which enters then into the extraction channel that brings the
bunch into a beam dump
In this configuration the same thin septum magnet is used for both the bunch injection and the bunch
extraction This requires a symmetrical design with two flanges one dedicated to the injection and the
other to the extraction The magnetic gap is wide enough to accommodate the two bent trajectories
When operating at a 50 Hz injection rate the septum current pulse consists of a slow full sine whose
same positive half sine will be used successively for the lsquoold bunchrsquo extraction and the for the lsquonew
bunchrsquo injection with a suitable delay between the two steps
The two fast kicker magnets are identical their length has been reduced as much as possible because of
the very limited space available The current pulse in the kickers will have a half-sine shape One must
achieve a rise time (from zero current to peak current) and a decay time (from peak current to zero)
each less than a ring revolution period Thus the current pulse can have a maximum duration of two ring
revolution periods Consequently their charging voltage reaches high values even though most of the
deflection is performed by the septum magnet
The design of these three devices ensures that they can operate with a beam energy of 50 MeV
(nominal) or 70 MeV (maximum) and at a 50 Hz repetition rate
3512 Slow septum magnet and its pulsed power supply
The main constraints on the septum design are as follows This septum must produce a large trajectory
deflection (160 mrad nominal peak value 170 mrad maximum peak value) to bring the injected beam
very close to the ring straight section axis with no significant disturbance of the stored beam The
magnet mechanics should preserve the ring vacuum chamber geometry to keep its impedance as low as
possible Furthermore its stray field must remain very low within the vacuum chamber where the stored
beam circulates as the septum excitation is done by a full sine pulse whose width (130 micros) is very long
compared to the ring revolution time (56 ns)
An eddy current septum magnet installed in vacuum will fulfil these constraints The effective septum
blade thickness can be as small as 3 mm thus ensuring the closeness of the stored beam orbit and the
desired bunch trajectory at the output of the septum yoke An internal chamber connected to the magnet
yoke extends the ring vacuum chamber and preserves the impedance budget The injected bunch
deflected by the septum magnet field penetrates through a small hole in this internal chamber Such a
septum magnet is very asymmetric in the transverse plane it is driven by a copper loop that wraps the
steel yoke
In order to contain the magnetic flux the whole yoke is enclosed in a copper box which carries the eddy
current On the side facing the stored beam orbit the copper box thickness is reduced so that it
constitutes the thin septum blade As a result of the electric shielding provided by the eddy current
circulating in the copper septum such a structure is very effective to avoid a large magnetic stray field
outside the magnet gap The fact that the excitation pulse is a full sine reduces the duration and the
lsquo
62
amplitude of the stray field (the slow delayed stray field tends to be the sum of the two opposite half
current arcs this feature brings about a reduction of the stray field)
An additional magnetic shielding is required to strongly reduce the residual strayfield This shield is
provided by a 05 mm thick sheet of high permeability mumetal that will completely surround and
enclose the internal chamber (thus shielding the stored beam orbit from the strayfield)
Such a design that combines an eddy current shield a full sine pulse excitation and the use of a high
permeability magnetic material reduces the stray field to a very low level (a few 10 ppm of the main
pulsed field) [313]
Assuming a septum opening of H x V = 30 x 12 mm2 and an active length of 250 mm (the length was
chosen as short as possible because of constraints coming from the septum implementation in the
straight section) the septum magnet inductance is estimated to be 16 microH Connexions and cables
increase this inductance up to 23 microH
This design leads to the following parameters for the septum magnet and its power supply (values for a
70 MeV ring operation)
Septum deflection field integral field peak current charging voltage
170 mrad 40 mTm 160 mT 1500 A 226 V
Table 315 Septum main parameters
The pulsed power supply will be based on the lsquoclassicalrsquo resonant discharge circuit scheme composed
of a storage energy capacitor (190 microF) a fast high-current thyristor switch and a transmission line to
the magnet comprising a few low impedance coaxial cables
A 300V-20A power supply is foreseen It will be connected to the storage energy capacitor through a
current-limiting power resistor
3513 Fast kicker magnet and its pulsed power supply
The two fast kicker magnets must be identical Because of the very limited space available their length
has been reduced as much as possible The final numbers are 200 mm and 450 mm for the active length
and for the overall length respectively They will be located outside the vacuum vessel with their
lsquowindow framersquo rectangular ferrite yokes surrounding the ceramic vacuum chamber (see Fig 338)
Figure 338 Front view of the kickers
The two ceramic sections of the vacuum chamber will have a flange-to-flange length of 450 mm Their
aperture will be the same as the one in the ring standard sections but with a racetrack transverse shape
Both extremities of each ceramic chamber include a smooth shape transition and a short bellow that
lsquo
63
allows for RF finger contacts These bellows can accommodate thermal dilatations especially during
bake-out The ceramic chambers will be coated with a metallic deposit that will carry the bunch mirror
current and thus avoid an increase of the chamber impedance This coating needs to be very thin It will
consists of a titanium layer with a thickness le0050 microm The thickness has been calculated so as to
avoid the deterioration of the fast pulse shape and to avoid the temperature rise due to the mirror current
[314]
Figure 339 Time profile of the kicker pulse versus the titanium layer thickness
The kicker dimensions follow from
- the aperture needed by the stored beam H x V = 40 x 28 mm2
- the required deflection kick nominal 11 mrad maximum 15 mrad
- the beam energy nominal 50MeV maximum 70MeV
- the short revolution period of the stored e- bunch Trev = 56 ns
- the very short length of the injected bunch from 4 to 25 ps (rms)
With a ceramic thickness of 6 mm the effective magnet aperture must be H x V = 55 x 55 mm2 These
parameters determine the current and voltage pulse characteristics
The kicker magnets will be powered by a circuit delivering half-sine pulses [315] In fact because of
the e- bunch short revolution period (56 ns) the field pulse duration has to be less than twice the ring
revolution period (less than 112 ns) and thus must a have very fast rise time and decay time (both less
than a ring revolution period) On the other hand with such a very short bunch length (4 or 25 ps (rms)
whether one deals with the incoming bunch or the extracted bunch one may assume that the pulses do
not have to present a marked flat top
These considerations lead to the following kicker parameters
00000E
-2
17000E
-10
34000E
-10
51000E
-10
68000E
-10
85000E
-10
10200E
-9
11900E
-9
0000E+00
2000E-01
4000E-01
6000E-01
8000E-01
1000E+00
1200E+00
Thom-X injection kicker pulse field attenuation relating to Titanium thickness
Bext(t)
Bint(t) 002microm
Bint(t) 004microm
Bint(t) 005microm
Bint(t) 006microm
Bint(t) 008microm
Bint(t) 01microm
Bint(t) 02microm
Time
Fie
ld a
mp
litu
de
lsquo
64
Kicker beam energy deflection field integral
Operation at Enominal 50 MeV 15 mrad 25 mTm
Operation at Emax 70 MeV 15 mrad 35 mTm
field peak current estimated voltage
92 mT 406A 10 kV
175 mT 771 A 18 kV
A specific RampD program is necessary to guarantee that these constraints will be fulfilled by the power
supply and by the magnet Preliminary studies were based on the standard circuit used for such a pulsed
power supply (see Fig 340) involving a high voltage (HV) capacitor to store the energy in series with
a HV switch and a load whose inductance is close to the one of the kicker magnet In this circuit the
HV switch is a very fast MOSFET switch followed by a HV fast recovery diode module
Figure 340 Power supply circuit for the ThomX kickers
These first studies lead to unsatisfactory preliminary results
- The BEHLKE Ultra-Fast 17 kV 480 A MOSFET module can switch ON in a very short time less
than 8 ns Then the pulse shape is normally determined by the passive component parameters in the
circuit provided that the negative current is quickly interrupted either by HV diodes in series or by
a turn Off control of the HV switch
- But the different types of HV diodes used are not able to switch OFF the pulsed current quickly
enough Even the lsquofast recovery diodesrsquo have a reverse recovery time too much long to stop cleanly
the pulse at its zero crossing Thus it was necessary to work on other topologies and to find better
suited components
Further simulations were performed to see how the circuit could be improved It turned out that
controlling the switch both at the closing and at the opening can improves the shape of the current
pulse A prototype using the same Behlke MOSFET modules and including a coaxial transmission line
was built and tested A plot of the current pulse in the kicker coil that was obtained is shown below
lsquo
65
This plot was obtained with the following prototype circuit
As one can see more work is still needed to improve the shape of the current in the coil (namely one
should reduce the level of reflections improve the current pulse shape and work on the peak current to
meet the 70 MeV requirements) We are also looking for an other supplier who could provide controlled
HV switches ensuringr both ON and OFF fast operation
Mechanical and magnetic studies of the kicker magnets are ongoing
36 Storage ring RF system
The choice of 500 MHz for the RF frequency is a compromise between several constraints the cavity
fundamental and high order mode (HOM) impedances space requirements the availability of RF power
sources and other components The required accelerating voltage of 500 kV can be provided by only
one 500 MHz single-cell cavity with about 40 kW of RF power As shown in the following the choice
of the cavity design is mainly dictated by the need for a strong damping of the HOM
lsquo
66
361 HOM impedance thresholds as regard to beam instabilities
When a cavity mode is excited in resonance it generates beam oscillations with a rise time
approximately given by one of the two following expressions
(1τ)l = Ib α fm Rl (2 Qs Ee) in the longitudinal case and
(1τ)t = Ib βt fo Rt (2 Ee) in the transverse case
where fm is the HOM resonant frequency Rl [Ω] and Rt [Ωm] are respectively the longitudinal and the
transverse HOM impedances Qs = fs fo is the synchrotron tune (fs and fo are the synchrotron and the
revolution frequencies) βt is the beta function amplitude at the cavity location α is the momentum
compaction factor Ib is the average beam current and Ee is the beam energy in eV
Assuming typical values for the cavity HOM impedances (Rl fm = 01 to 1 MΩ GHz Rt = 1 to
10 MΩm) and the machine parameters listed in Table 315 the instability growth time is around 10 micros
in both the longitudinal and the transverse cases
Orbit length C 168 m
Electron energy E 50 MeV
RF frequency fRF 500 MHz
Revolution frequency fo 1786 MHz
Harmonic number h 28
Momentum compaction factor α 0015
β-function amplitude at the cavity location βxz 3 m 4 m
Relative energy spread σE E 40 10-3
Peak acceleration voltage VRF 300 (500) kV
RMS bunch length σs 27 (21) ps
Synchrotron frequency fs 358 (462) kHz
Bunch charge Qb 1 nC
Number of stored bunches Nb 1
Maximum current Ib 1786 mA
Table 315 Ring parameters used to estimate the rise time of the HOM induced instabilities
In a low energy ring like ThomX radiation damping is so weak (the damping time is ~ 1 s) that a
stationary state is not reached during the beam storage time (20 ms) On the other hand to ensure a
good ring operation it is sufficient to make all instability rise times much longer than this storage
period This can be achieved by attenuating the HOM impedances very strongly typically by a factor
104
362 Cures to the HOM induced instabilities
There are essentially two ways of coping with such HOM induced instabilities either by performing a
strong de-Qing of the HOM resonances or by tuning their frequencies away from the beam spectral
lines in order to prevent resonant excitation
In practice HOM de-Qing is achieved by extracting the cavity HOM power and transferring it towards
external dissipative loads But in this approach it is difficult to reach attenuation factors higher than
102 - 10
3 over a large frequency range and this technique requires heavy and cumbersome equipment
close to the cavity (coaxial lines with coupling antennas or ferrite loaded waveguides) The DAMPY
lsquo
67
cavity designed at BESSY [316] and which is used in ALBA [317] and the PEP II cavity developed
at LBNL [318] are two variants of this technique
The other method which consists in controlling the HOM frequencies is better suited to a small
circumference machine like ThomX where the beam spectral linesrsquo spacing (~ 18 MHz) is very large
compared to the HOM bandwidth (this bandwidth is of a few tens of kHz for Qo equal to a few 104) As
long as the HOM density is not too high and the modes can easily be tuned far enough from the beam
spectral lines (δf gtgt fm Qo) it should be possible to reduce their effective impedances (the ones lsquoseenrsquo
by the beam) down to tolerable levels ie Reff asymp Rs (2 Qo δf fm) 2 ltlt Rs
We are investigating the potential performance of this technique by applying it to an ELETTRA-type
cavity (see Fig 344) [319] which combines three frequency tuning means The HOM frequencies can
be precisely controlled by an appropriate choice of the temperature of the water cooling the cavity while
the fundamental frequency is maintained constant by a mechanism which deforms the cavity
longitudinally In addition a movable plunger tuner also called HOM frequency shifter (HFS) located
in the cavity equatorial plane provides an additional degree of freedom [320]
Tables 316 a and b list the characteristics of such a cavity fundamental and HOM resonances They
have been calculated with the 3D GdfidL code [321] One finds 18 significant HOM (9 monopoles and
9 dipoles) with resonant frequencies lower than the cavity cut-off frequency Figures 345 a and b
represent the corresponding HOM spectra fm Rl(f) and Rt(f) folded over a single base band of
18 MHz The horizontal dashed line in the graph represents the impedance threshold corresponding to
an instability rise time of 20 ms the beam storage time Although they remain somewhat critical these
results tend to confirm that it should be possible to find acceptable operating conditions as long as the
HOM frequencies can be shifted by a few MHz by combining temperature and HFS tuning
lsquo
68
Figure 344 Drawing and photo of the ELETTRA cavity
lsquo
69
Figure 345 (a) Longitudinal and (b) transverse HOM spectra folded over a single base band of
18 MHz for the ELETTRA-type cavity
τL = 20 ms
lb
os
I
eEQ
τα
2
ttob
o
fI
eE
τβ
2
τt = 20 ms
lsquo
70
a) Longitudinal modes fm [MHz] Q0 Rs Q0
[Ω]
Δfm (T)
[MHz]
Δfm (HFS)
[MHz]
L0 (fundamental) 499132 44164 765 000 000
L1 958618 48158 336 037 -100
L2 102781 57861 036 080 369
L3 14087 52562 220 174 021
L4 151528 69088 783 112 086
L5 155129 66375 850 240 721
L6 186912 55785 097 142 -092
L7 19495 81752 008 183 143
L8 203985 62729 086 400
L9 206896 87300 530 445 667
Table 316 Main characteristics of the monopole (a) and dipole (b) modes computed with GdfidL for
an ELETTRA-type cavity Δfm (T) and Δfm (HFS) are the effective temperature and HFS tuning ranges
assuming a constant fundamental frequency
It is worthwhile mentioning that in any event fast longitudinal and transverse feedback systems are
required to damp the beam oscillations resulting from injection transients (see section 333) They will
contribute to the HOM damping as well
363 RF power source
An RF power of about 40 kW is required to generate the desired voltage of 500 kV in a single 500 MHz
ELETTRA-type cavity For this frequency there exists a wide choice of power sources based either on
vacuum tubes (klystron IOT diacrode) or on solid-state technology The low operating voltage and
high modularity of the solid-state version offer significant advantages compared to vacuum tubes
Experience acquired at SOLEIL after few years of operation has definitely demonstrated all the benefits
of this technology [322] Figure 346 shows the 35 kW - 352 MHz solid-state amplifier used for the
SOLEIL booster synchrotron It consists of a combination of 147 modules Each one has a power of 330
W a gain of 11 dB and an efficiency of 62 This amplifier was commissioned about 7 years ago
During a running period of more than 35 000 hours it tripped only once and this trip was caused by a
human mistake
b) Transverse modes fm
MHz]
Qo Rsperp Qo
[Ω]
Rsperp[MΩm] Δfm(T)[MHz] Δfm(HFS)
[MHz]
T1 73701 50267 335 2599 04 075
T2 74606 52566 92 7557 155 285
T3 115202 47196 264 3006 049 -312
T4 125565 55386 51 7428 068 725
T5 130249 71674 36 7039 097
T6 169981 69325 103 2542 188
T7 173650 91039 165 5463 gt 2
T8 177337 59409 32 7061 gt 2
T9 183617 77697 55 1643 gt 2
T10 206142 88484 59 2254 gt 2
T11 222475 99092 11 5079 gt 2
T12 225933 56641 13 3484 gt 2
lsquo
71
This technology has already been extended to 476 MHz in the framework of a SOLEIL-LNLS
collaboration aimed at realizing two 50 kW amplifiers for the LNLS storage ring [323] More recently
the SOLEIL team has developed modules which can deliver up to 650 W at 500 MHz with a gain of
17 dB and an efficiency of 62 while still reducing the thermal stress [322] About a hundred of such
modules will be combined to obtain the 50 kW required by ThomX
Figure 346 35 kW - 352 MHz solid-state amplifier of the SOLEIL Booster (147 amplifier modules
and DCDC converters bolded on 8 water-cooled dissipaters)
364 Low level RF system and longitudinal feedback
The proposed low level RF (LLRF) system is conventional As shown by the synoptic diagram of Fig
347 it consists of three relatively slow loops (frequency amplitude and phase) aimed at controlling the
cavity voltage with an accuracy of about plusmn 02 in amplitude plusmn 02deg in phase and a response time
around 1 ms This scheme is suitable for compensating the pressure and temperature drifts as well as
the power supply ripples
Such a system is not fast enough to deal with the beam phase oscillations at the synchrotron frequency
fs on the order of 400 kHz that originate in injection transients These oscillations can produce an
undue emittance blow up and a phase jitter at the interaction point Beam dynamics simulations show
that preserving the beam quality requires a damping of these oscillations within a few tens of a micros (see
section 333) As the radiation damping is negligible during the storage time a longitudinal feedback
600 W 280 Vdc 28 Vdc converter
Power combination
330 W amplifier module
lsquo
72
(LFB) must be implemented It can be performed by a dedicated broad-band longitudinal kicker that
acts on the beam Since ThomX is operating with a single bunch we propose to use the main cavity as a
longitudinal kicker despite its narrow bandwidth of 25 kHz ltlt fs This difficulty can be overcome by
making the cavity effective bandwidth much larger than this value in fact larger than fs We will use a
high gain direct RF feedback (RFFB) that allows the operation of a fast phase feedback (ϕFB)
Assuming a cavity-to-amplifier distance of about 10 m the loop delay will be less than 150 ns Then
one expects an RFFB gain Go larger than 50 which leads to an effective bandwidth larger than 1 MHz
Simulations show that a LFB that combines such a RFFB with a ϕFB (see Fig 348) can damp injection
phase errors of up to 10 degrees within a few micros ie faster than the above specification
Figure 347 Basic LLRF system scheme with lsquoslowrsquo frequency amplitude and phase loops
lsquo
73
Figure 348 Schematics of the longitudinal feedback system combining a phase feedback with a direct
RF feedback
365 RF equipment status
For the ThomX commissioning we will use an ELETTRA cavity The design of all the components of
the 50 kW solid-state amplifier (600 W modules combiners power supplieshellip) was validated in 2011
The next two milestones are first the validation of a 9 kW unit (combining 16 modules) and secondly
the commissioning of the complete amplifier on a dummy load
Several versions of the LLRF system including LFB are under investigation either fully analogue (see
Fig 349) or mixed analogue for the fast feedback loops and digital for the slower ones (see Fig 350)
or even fully digital based on the new generation of fast ADC DAC and FPGA
366 RF system conclusion
For the RF system of the ThomX storage ring we plan to use a 500 MHz single-cell cavity of the
ELETTRA-type Maintaining at a tolerable level the impact of the RF cavity HOMs on the beam is a
challenge However this goal should be achievable with a proper control of the HOM frequencies It
can be done by combining three different means of tuning the cavity temperature control longitudinal
elastic deformation of the cavity and motion of a plunger The fast longitudinal and transverse feedback
systems - which in any event are required to damp the beam oscillations resulting from injection
transients - will contribute to the damping of the HOM excitations as well
In order to provide the required accelerating voltage of 500 kV the cavity will be powered by a solid-
state amplifier of the lsquoSOLEILrsquo type able to deliver a power of up to 50 kW The cavity voltage will be
controlled with an accuracy of plusmn 02 and plusmn 02 degree by means of conventional amplitude and phase
loops (either analogue or digital) with a few kHz bandwidths In addition a fast phase feedback system
combined with a high gain direct RF feedback will provide the longitudinal damping which is required
to preserve the beam quality
lsquo
74
Figure 349 Complete LLRF system longitudinal feedback included (analogue version with phase and
amplitude modulation ndash the option with IQ modulation is also considered)
Figure 350 Complete LLRF system longitudinal feedback included (mixed analogdigital version)
37 Transverse feedback
371 Specifications
Table 317 summarizes the instability growth time constants and the corresponding kicker strength
requirements for the main types of instability It shows that for Thom-X the betatron oscillations due to
the injection jitter in position and angle are the most critical ones They require a feedback damping
time constant of 5 micros or less (cf chapter 3)
lsquo
75
Source type growth time constant kicker strength
Beam pipe
geometry
TMCI
Head-Tail
-
160 micros
gt10 nrad
Resistive Wall 600 micros gt2 nrad
Ions lt 100 micros gt20 nrad
Injection Jitter 5 micros 2 microrad
Table 317 Overview of expected instabilities (1 nC 25 ps rms at 50 MeV)
372 Electronics
To cope with these instabilities we will use a SPring-8 based high performance digital transverse
feedback system which consists of a wideband detector (a BPM) an RF front-end an FPGA based
processor a power amplifier and a stripline kicker (see Fig 351) This system is capable of detecting a
coherent transverse motion and applying a counter kick to damp it bunch-by-bunch and turn-by-turn
with one or two bunches stored in the ring
Figure 351 Spring-8 based digital feedback system
Two BPMs (one for each plane) will provide the beam position information The RF front-end will
extract the betatron oscillation information which will be sampled by ADCs From the sampled data the
FPGA will compute a suitable kick to damp the oscillation The signal processor output will be
amplified by a 10 W wideband amplifiers before being fed to the kicker
In the RF front-end (see Fig 352) a baseband down-converter will be implemented in order to amplify
the beam signal and to minimize the jitter effect for the same reason the digital kick will also be
extended over four RF periods
lsquo
76
Figure 352 Front-end of the Thom-X RF electronics
For the commissioning phase a digital system (rather than an analog one) will be used to benefit from
the tuning flexibility provided by the digital parameters of the FIR filter Once the machine tune will be
fixed a more suitable low cost analog system will be installed
373 Kicker
The kicker is under development It consists of a stripline composed of four electrodes connected to
vacuum feedthroughs on each side (see Fig 353) The amplifiers will be connected to the downstream
ports whereas upstream ports will be used for signal detection and they will be equipped with 50 Ω
loads The same kicker will be used to act on the beam in both planes
Electrode line impedances must be adapted (50 Ω) Electromagnetic 3-D simulations are being
performed to optimize the bandwidth and shunt impedance (ie to maximize the stripline efficiency)
and also to minimize the kicker impedance to wakefields
Figure 353 Schematics of the Thom-X stripline kicker
lsquo
77
38 Vacuum
381 Transfer line vacuum
Concerning the design of the transfer line vacuum system three main constraints have been taken into
account
- Pressure of 10-7
Pa at the acceleration section output
- Ion pump operating at 10-6
Pa
- Ring pressure of 10-8
Pa
Simulations have been performed assuming that two 03 m3 s
-1 triode pumps will be installed one at the end of
the acceleration section and the other at the septum The vacuum chamber is a DN40 The septum output
conductance is 018 m3 s
-1 Without baking the outgassing rate would be σ τ = 2x10
-8 Pa m s
-1 and the
pressure at the ring entrance would be about 10-7
Pa Thus the septum and its environment must be baked to
bring τ down to 5x10-10
Pa m s-1
and obtain a pressure of 10-8
Pa at the ring entrance
Pressure in the line of transfer
The vacuum equipment envisaged for the transfer line is the following one
‐ 3 ion pumps 150 l s-1
with their baking systems and power supplies
‐ 1 ion pump 500 l s-1
with their baking system and power supply
‐ 4 Pirani gauges
‐ 4 cold cathode gauges
‐ 2 power supplies gauges
‐ 2 gate valves electro pneumatically actuated DN 40 (acceleration section output ring entrance beam
dump)
‐ 1 turbo pump (DN 63 ~ 80 l s-1
)
‐ 1 rotary vane pump (~ 12 m3 h
-1)
‐ 4 angle valves manually actuated for pre-pumping DN 63
lsquo
78
382 Ring vacuum
Concerning the ultra-vacuum to be maintained in the ThomX ring one must take into account first the
pipe relatively low specific conductance c = 452x10-3
m3 m s
-1 and secondly the pipe specific
outgassing rate θ τ = 108x10-10
Pa m2 s
-1 Also the pumpsrsquo effective pumping speed is reduced by the
T port with the grid Seff = 19x10-2
m3 s
-1 In a first stage the ionic pumps chosen for the simulations
are 0040 m3 s
-1 diode pumps This corresponds to S = 0030 m
3 s
-1 at 10
-8 Pa and with a grid pumping
port whose conductance is 00362 m3 s
-1 to an effective pumping speed Seff = 0019 m
3s
-1 The pressure
has been estimated by modeling a quarter of the ring including half of the non-baked optical cavity ie
in the most unfavorable case
Figure 354 Pressure distribution in the vacuum chamber
The calculated average pressure is 306x10-8
Pa It can be improved by increasing the grid pumping port
conductance by replacing the DN35 with a DN63 Monte-Carlo simulations show an improvement of ~
16 in efficiency
The entire ring must be baked Geometrical and temperature constraints (respectively a distance of only
07 mm between the magnet coils and the vacuum chamber a baking temperature of 150 degC for the
chamber while keeping the coilsrsquo temperature below 80deg C) are strict An ultra-thin baking system has
been constructed tested and validated
Figure 355 The vacuum chamber ultrathin baking set-up
lsquo
79
The following is the vacuum equipment needed for one quarter of the ring
- 3 ion pumps (40 l s-1
) with their baking equipment and their power supplies
- 1 residual gas analyzer (RGA)
- 2 turbo-pumps (DN 63 ~ 80 l s-1
)
- 2 rotary vane pumps (~ 12 m3 h
-1)
- 2 Pirani gauges
- 2 cold cathode gauges
- 1 gauge power supply
- 1 DN 63 gate valve electro-pneumatically actuated with an HF feed-through (one on each side of
the RF cavity and similarly for the septum) and the baking equipment
- the vacuum chamber baking equipment
- 3 angle valves manually actuated for pre-pumping DN63
- 3 T with the grid
lsquo
80
Chapter 4 INJECTOR TRANSFER LINE AND RING DIAGNOSTICS
Several diagnostic devices will be implemented in the ring to measure the electron beam parameters
listed in Table 41
Parameter unit value
Energy MeV 50
Average current mA 1784
Bunch charge nC 1
Energy dispersion 03 at injection to 06 at storage end
Emittance x mrad 5 e-8 at injection to 30 e-8 at storage end
Normalized emittance x mrad 5 e-6 at injection to 30 e-6 at storage end
Emittance y mrad 5 e-8
Normalized emittance y mrad 5 e-6
Bunch length ps-rms revolutions 1 ~ 4
revolutions 100-200 ~ 25
revolutions 200-300000 ~ 50
RF frequency MHz 500
Revolution frequency MHz 1784
Period ns 56
Injection repetition rate Hz 50
Ring circumference m 1680
(a) Ring parameters
Magnetic field T 05
Radius of curvature m 0352
R51 m 0 to -025
R52 m 0 to -045
R56 m 07 to -07
Beta x m 06
Beta y m 6
Eta x m 02 - 03
Transverse dimension x mm (rms) 06
Transverse divergence x mrad 2
Transverse dimension y mm (rms) 054
Transverse divergence y mrad 2
(b) Parameter values at the location of the bending magnets
Table 41 ThomX beam parameters
41 Charge measurement
411 Current transformers
The beam charge will be measured non-destructively with commercial in-flange high resolution
current transformers (ICT) Three devices will be installed
- one in the linac (LIDGICT010) located before the accelerating sections
- one at the transfer line entrance (TLDGICT010)
lsquo
81
- one at the transfer line exit (TLDGICT020)
In-flange ICTs are directly mounted in the beam line They are UHV compatible can be baked up to
150deg C (185deg C on request) and they are available for many pipe diameters (also with an elliptical
aperture or any other aperture shape) We will use the CF6rdquo model its internal diameter is 604 mm and
it is 60 mm long This device includes a ceramic gap a shield and a wall current bypass Bellows are
not required
The signal from each integrating current transformer will be processed by a BCM (beam charge
monitor) A built-in calibration generator provides an on-line check of the measurement reliability An
Integrate-Hold-Reset (BCM-IHR) device is suited for pulse repetition rates ranging from 10 kHz down
to single pulses and for bunch charges up to 2 nC The acquisition will be performed by using a 100
kHz 12 bit ADC
412 Faraday cups
Interceptive absolute beam charge measurements will be performed by Faraday cups There will be two
of them placed inside the two beam dumps (see Fig 2 of the Introduction) They should provide a
better resolution for small electric charges
42 Transverse profile measurements
421 Diagnostics stations
Five diagnostics stations will be installed to measure the transverse beam profile as shown in Fig 41
Diagnostics station 1 (DS1 ndash LIDGSST010) will be located after the RF gun at the beginning of the
linac Diagnostics station 2 (DS2 ndash TLDGSST010) will be located between the end of the linac and
the linac beam dump Diagnostics station 3 (DS3 ndash TLDGSST020) will be placed in the middle of the
transfer line Diagnostics station 4 (DS4 ndash TLDGSST030) will be located at the end of the transfer
line and Diagnostics station 5 (DS5 ndash ELDGSST010) will be located before the ring dump
lsquo
82
Figure 41 Location of the diagnostics stations (see text for the nomenclature)
The ThomX station design draws from the station built by LAL for PITZ (itself drawn from a SOLEIL
design) It is illustrated in Fig 42
Figure 42 3D view of a ThomX diagnostic station
Each station will have a motorized arm on which several screens will be mounted A stepper motor will
move this arm and insert the screens laterally The following screens will be available
- 1 YAG(Ce) screen in all stations
- 1 optical transition radiation (OTR) screen in all stations
lsquo
83
- 1 target to allow an online calibration of the camera in all stations
- one Cerenkov screen for bunch length measurements in station DS2 (described in more detail
below)
The YAG(Ce) and OTR screens and the calibration target will be tilted at 45deg with respect to the beam
axis so that the emitted or reflected light will travel downward and exit the vacuum through a DN60 CF
fused silica window A CCD camera will be located below the station to image the screens The
Cerenkov screen will be titled so that its light exits at the top of the diagnostic station Shutters will be
used to close either the Cerenkov window or the YAG(Ce)OTR window when not in use to prevent
measurement noise due to background photons Several LEDs located just after the bottom window will
be used to illuminate the inside of the chamber when needed
At the beginning of the Linac at station DS1 the beam size will be about 05-07mm (rms) in both
planes at the end of the Linac at DS2 the beam size will be 15 mm (rms) in both planes In the middle
of the transfer line at DS3 the beam size is expected to reach 25 mm (rms) in x and slightly less than
1mm in y At the end of the transfer line at DS4 it will be about 1 mm in both planes After the
extraction at DS5 the beam size will be about 05-07mm (rms) in both planes Given these beam sizes
we chose a screen diameter of 25 mm for all stations
Screen photon yield
The YAG(Ce) screen will be 100 microm thick Such screens have a light yield of 8 photons per deposited
keV [41] The YAG radiation length is 35 mm [42] assuming the usual model for energy deposition in
matter [43] each electron will deposit about 143 keV in the screen this corresponds to the emission of
~1100 photons Thus the total number of photons produced by an e- bunch crossing the screen will be
71 1012
We recall that YAG(Ce) emission occurs in all directions
The second screen will be of the OTR type The screen support design allows either the use of a 100 microm
aluminized silicon wafer or of an ultra-thin aluminized mylar film The silicon wafer has the advantage
of giving a better image quality while the mylar film allows a beam injection through the screen without
significantly affecting its quality The OTR yield within a cone of half-aperture θ for photons whose
wavelengths range between λ1 and λ2 is given by [44]
Thus for a standard CCD camera whose sensitivity ranges between 400 nm and 800 nm and assuming
an acceptance cone of 003 radian this expression gives ~0002 photon per electron at 50 MeV (station
DS2-5) A similar yield is reached at 5 MeV with an acceptance of 03 radians (station DS1)
To image the screens a lens will be located 500 mm away from them A small telescope made of two
lenses will be used to focus the light on the CCD camera At least one of the telescope lenses will be
movable to provide the needed dynamic range A Gigabit Ethernet camera with at least 600 x 400 pixels
will record the images The camerarsquos square pixels are about 74 x 74 microm2
With such a configuration one expects that a 50 MeV bunch passing through the YAG(Ce) screen will
emit 71x1012
photons and assuming that the CCD chip has an average efficiency of 50 1010
counts
will be recorded by the camera taking into account the solid angle covered by the lens
lsquo
84
The same bunch passing through the OTR screen will emit 82 107 photons within the lens acceptance
Thus 3 106 counts will be recorded by the camera
For a 5 MeV bunch (station DS1) the YAG(Ce) screen will emit 2 1012
photons and 109 counts will be
recorded by the camera The OTR screen will emit 82 107 photons within the lens acceptance and 3 10
6
counts will be recorded by the camera
Assuming a transverse beam size of 15 mm one expects 8 106 counts per pixels with a YAG(Ce)
screen and 25 105 counts per pixel with an OTR
If the 50 MeV beam is defocused (for example during a quadrupole scan) to produce a 5 mm spot size
these numbers become 8 105 for the YAG(Ce) screen and 2 10
4 for the OTR
Number of OTR
photons emitted
Number of
OTR
photons
recorded by
the camera
Number of
YAG
photon
emitted
Number of
YAG
photons
recorded
DS1 82 107 3 10
6 2 10
12 10
9
DS2-5 82 107 3 10
6 71x10
12 10
10
Assuming a YAG Cerium dopant density of 018 we get a Cerium density of 25 1019
cm-3
[45] and
a 15 scintillation efficiency It follows that the scintillating site density is 37 1018
cm-3
Therefore the
100 microm YAG(Ce) screen will saturate when the photon density reaches 36 1014
mm-2
This last figure
translates into an electron density of 32 1011
mm-2
for an e- beam energy of 50 MeV In these
conditions the YAG(Ce) screen will saturate if the beam is focussed on a spot smaller than 20 microm This
is five times less than the smallest beam foreseen at ThomX
Additional targets will be used to calibrate the cameras when necessary They will be made of stainless
steel plates with etched grids Each square of the grid will be 2 by 2 mm2 In some of these squares
smaller lines of known thickness will be etched Images of these targets will allow one to check the
whole optics
To minimize a possible radiation deterioration of the cameras lead-glass windows will be inserted
between the vacuum windows and the focusing lenses
43 Emittance measurement
The beam emittance will be measured with the quadrupole scan technique In this technique a triplet
(set of three quadrupoles) is used to find the minimum transverse beam size in each plane A fit of the
beam size variation versus the quadrupole strength enables one to retrieve the projected transverse
emittance
The sets of triplets and the diagnostic stations implemented along the ring allow an emittance
measurement at three different locations at the exit of the linac with DS2 in the transfer line with DS4
and in the ring with DS5 by using the extraction kicker The beam emittance measurement with DS5
lsquo
85
will provide a check of the turn-by-turn emittance growth thus bringing precious information on the
transverse beam dynamics
Pepper-pot measurements are also foreseen The advantage of this technique with respect to a
quadrupole scan is that it consists in a single shot measurement independent of machine operation
fluctuations
A pepper-pot set-up comprises a plate with evenly distributed holes Such a plate is placed in the beam
path together with a scintillating screen located downstream and an imaging system A pepper-pot
mounted on a movable arm could be located at the end of the linac between the last dipole and the
diagnostic station DS2 and another one could be located just before station DS5
At station DS2 the beam size will be about 15 mm (rms) and the geometric emittance about 5 10-8
m
rad Therefore the maximum beam divergence will be about 50 microrad By adjusting the quadrupoles one
may focus the beam down to a 20 microm spot and increase its divergence up to 250 microrad To measure the
transverse emittance one would have to position this focal point two meters before DS2 (or DS5) and
to position a pepper-pot mask with 100 microm holes or slits one meter upstream of DS2 (or DS5)
44 Beam position measurement
The beam position will be measured by a set of BPMs placed all along the ring Their locations are
shown in Fig 43
lsquo
86
Figure 43 Location of the BPMs along the ring (note the BPM located at the IP has been suppressed
since this drawing was made)
441 Beam Position Monitors (BPM)
Two types of BPMs will be used short circuited striplines in the linac and the transfer line and buttons
in the storage ring
Compared to button BPMs striplines generate a higher signal but need more space and contribute more
to the wake-field generating impedance For these reasons striplines are preferred for the linac and the
transfer line where we need single pass measurements and where wakefield is not an issue In the
storage ring button BPMs will be chosen for their small size and small contribution to the ring
impedance The lower signal produced by the small electrodes will be compensated by the high
repetition rate of the stored beam (averaging is possible)
BPM electronics detection frequency on the storage ring is usually equal to the RF frequency as it is the
only common frequency component present in the BPM data whatever the filling pattern is Even
though only one bunch will be stored in ThomX the same approach is chosen Therefore the storage
ring BPM electronics detection frequency is 500 MHz
Since it is more convenient to adopt the same electronics for all the BPMs (linac transfer line and
storage ring) stripline BPMs on the Linac and transfer line are optimized to give a signal with the
highest possible component at 500 MHz Their electrode length is thus adapted to this frequency which
means that it has to be equal to λ4 (150 mm 500 MHz) where λ is the RF wavelength
4411 BPM for the linac
The linac BPM equipment will be as follows
- Number of BPM 1
- Type short circuited stripline
- Electrode length 100 mm
- Required resolution 100 microm for a single pass measurement and a 1 nC bunch charge
Figure 44 Linac BPM
lsquo
87
Due to lack of space in the linac section where a stripline must be installed the electrode length cannot
be 150 mm but must be reduced to 100 mm Transfer impedance calculations show that at 500 MHz
the signal amplitude will be reduced by 13 compared to the optimal length of 150 mm (see Fig 45)
This amplitude reduction is acceptable
Feedthroughs will be soldered on CF16 flanges
Figure 45 Stripline transfer impedance versus frequency for various electrode lengths With an
electrode length of 100 mm the signal is reduced by 13 compared to its maximum value for a
frequency of 500 MHz
4412 BPMs for the transfer line
The transfer line will be equipped as follows
- Number of BPM 4
- Type short circuited stripline
- Electrode length 150 mm
- Required resolution 100 microm for a single pass measurement and a 1 nC bunch charge
lsquo
88
Figure 46 Drawing of the BPMs that will equip the electron transfer line
Feedthroughs will be soldered to CF16 flanges
4413 BPMs for the storage ring
The specifications relevant to the ring BPMs are as follows
- Number of BPM 12
- Type button
- Button diameter 107mm
- Required resolution ~1 microm 50 Hz and for a bunch charge of 1 nC
- Absolute precision plusmn 250 microm
In order to provide additional BPM signals for other applications (see below) 4 BPM blocks will be
equipped with a double set of electrodes In that case only 4 electrodes (among 8) will be used for
position monitoring The button size and the beam pipe size remain unchanged
Figure 47 Simple and double BPM blocks for the ThomX storage ring
The button positions and dimensions have been determined by simulations with Poisson Superfish (an
electrostatic simulation software) An evaluation of the impedance due to the button discontinuities has
shown that their effect will be negligible
Feedthroughs will be soldered directly to the BPM block They are bakable Connectors will be SMA
males to prevent any deformation during baking
lsquo
89
Alignment
BPM blocks will have an external mechanical reference This reference will be used for BPM alignment
during the installation phase and to position precisely the block in a calibration bench
Calibration
Storage ring BPMs will be calibrated by using a dedicated bench An antenna inside the block will be
fed with a 500 MHz wave of constant amplitude Calibration is done by measuring the signal output by
the 4 buttons A precise position of the antenna is not needed as position errors can be eliminated by
repeating the measurement with the BPM block turned by 180deg and averaging the results
The second halves of the blocks equipped with two sets of buttons will be dedicated to
‐ longitudinal (beam phase) feedback (one set of BPMs)
‐ transverse feedback (one set of BPMs)
‐ ion clearing To suppress ions which may generate beam instabilities BPM buttons will be
polarized with a high voltage (max plusmn200V) This electric field will deflect the ion trajectories
toward the vacuum chamber
Electronics
Electronics will be the same for all BPMs (whether equipping the linac the transfer line or the storage
ring)
Data rate
Revolution-by-revolution beam position measurements would be best But because of the high
revolution frequency (178 MHz) it might be difficult to get electronic devices that handle the
corresponding data rate Then one may work with averages over two turns This data will be provided
on demand and the acquisition will be triggered The buffer size should accommodate data collected
during at least 20 ms to cover full storage cycles Revolution-by-revolution data will be used for
machine physics studies and tune measurement The position measurement resolution must be better
than 100 microm
Slow acquisition
Slow data acquisition will be used for slow orbit correction and slow orbit feedback It will be obtained
by averaging the fast data flow with a sampling rate of ~10 Hz A data rate of 50 Hz would provide just
one measurement per storage cycle The resolution aimed at for this measurement is one micron or
better
Acquisition must be synchronous with the machine revolution frequency Moreover modules must be
synchronized together
Connections and cables
The four cables that carry the signals of each BPM should be put together in a strand Their electrical
lengths will be matched so that phase differences are kept below 13deg 500 MHz The cable attenuation
should not differ much from one BPM to another If the cable lengths cannot be the same for all BPMs
cable sections will be chosen in order to ensure the same attenuation for all cables To minimize both
lsquo
90
attenuation and cost cable lengths below will be kept below 30 m The LMR240 cable is an example
of what could be used its attenuation is ~6 dBm 500 MHz over a distance of 33 m
As radiation close to the machine can damage the cable insulator short (~1 m) pigtails (that can be
easily replaced if needed) will be inserted between the BPMs and the long cables
442 Orbit feedback system
A feedback system will keep the beam orbit stable within plusmn1 microm It will use the position measurements
provided by the 12 ring BPMs The correction rate will be comprised between 01 Hz and 10 Hz
Correction signals will be applied to the 12 correctors located along the ring The orbit feedback system
will be a matlab based application
443 Tune measurement and feedback
The tune will be obtained from the revolution-by-revolution data provided by the BPMs There is no
need for any kicker to excite betatron oscillations as the beam transverse position will naturally oscillate
after each injection (ie every 20 ms) If necessary a tune feedback will be implemented to keep the
tunes constant
444 Energy measurement
In the middle of the transfer line ie downstream of the dipole located at the end of the linac the
dispersive function is non-zero Therefore the beam energy and its spread can be deduced from the
electron horizontal distribution shown by the YAG screen in DS3 The same measurement will be
performed with DS4 using the dispersion induced by the extraction magnet
45 Bunch length measurement
The bunch length will be measured at two different places at the end of the linac and in the ring
Depending on the observation point two different physical processes are used to produce visible light
pulses which are then transported by dedicated optical lines through the ThomX shielding wall to a
double-sweep streak camera located in the experimental hall
451 Cerenkov radiation
Cerenkov radiation (CR) produced in a target placed at the end of the linac will be used to measure the
e- bunch length Cerenkov radiation is emitted when a charged particle passes through a medium at
velocity greater than the speed of light in that medium It is emitted at an angle θcr ndash with respect to the
particle momentum ndash determined by n(λ) the medium index of refraction and β the particle velocity as
follows
Above 10 MeV the emission angle is almost independent of the particle velocity Ncr the number of
radiated photons whose wavelengths range between λ1 and λ2 summed over a path of length d along the
charged particle trajectory is
( )
minus
minus
λnβλλπαd=N
cr 22
21
11
112
( )
minus
minus
λnβλλπαd=N
cr 22
21
11
112
( )( )λβn
=θcr
1cos
lsquo
91
where α=73 10-3
is the fine structure constant
Two types of radiator are usually used to produce Cerenkov radiation an aerogel or a solid target
Aerogels dramatically spoil the vacuum and therefore have to be mounted in a separate vacuum vessel
to maintain ultra-high vacuum conditions Solid targets are UHV compatible This is why they are
preferred
At the end of the linac when the bending magnet is off the beam goes straight into a beam dump
Before the dump several types of targets can be inserted in the beam trajectory for diagnostic purposes
For the production of Cerenkov radiation a sapphire target is adequate This materialrsquos index of
refraction is 177 With a 50 MeV beam the Cerenkov angle is θcr=5563deg Assuming that the window
thickness is 025 mm 16 photonse- are created with a wavelength ranging between λ1 = 300 nm and λ2
= 800 nm Their spectrum is given in Fig 48 For a 1 nC electron bunch 1011
photons are created The
energy deposited in such a target is negligible
Figure 48 Spectrum of the Cerenkov radiation emitted by a 50 MeV e- beam passing through of
sapphire window 025 mm thick
For simplicity the sapphire target is oriented so that the Cerenkov radiation exits in the vertical
direction For a Cerenkov angle θcr = 5563deg the window angle with respect to the beam axis must be θP
= 11475deg (see Fig 49)
8x10-8
6
4
2
0
In
ten
sity
[au
]
800700600500400300200
Wavelength [nm]
lsquo
92
Figure 49 Orientation of the sapphire target used to observe the Cerenkov light emitted by the e- beam
at the linac end
At the target location the beam dimensions will be σxy= 14 mm (rms) It will produce a light spot
whose dimensions will be σx = 14 mm (rms) and σy = 154 mm (rms) in the sapphire To intercept the
whole light beam and to leave some margin for the alignment we will use a target whose diameter will
be equal to 2 x 6 x σy ~ 20 mm
The Cerenkov radiation will exit the vacuum chamber via a UHV viewport made of fused silica (FuSi)
with a 90 transmission over the 300-800 nm wavelength range The viewport is mounted on a CF63
flange that provides a 68 mm clear aperture At the exit of the viewport a FuSi lens is used to focus the
light This collimated beam is then transported via a set of planar mirrors across the shielded area and
then through the ThomX shield to a dedicated hutch in the experimental hall (see sect 453)
452 Synchrotron radiation
Most of the synchrotron radiation (SR) emitted when the electrons circulate in a bending magnet (BM)
will be in the far infrared but visible light will also be present The cut-off wavelength of this radiation
corresponds to ~077 eV Thus the X-ray content is negligible This last feature makes the extraction of
the visible part of the spectrum easy The spectral flux per unit bandwidth per unit surface ΦBM
[Photons01bwmmsup2] is shown in Figure 410
Figure 410 On-axis spectral flux 1 m downstream of a bending magnet Photon energy in eV The
visible spectral range corresponds to [400 nm - 800 nm] or [155 eV - 31 eV]
The radiation is extracted at the exit of the BM ndeg5 through a porthole which provides a 63 mrad x 32
mrad aperture at 15deg with respect to the 0deg incident angle in the dipole The maximum power density is
lsquo
93
4 microWmmsup2 in the horizontal plane of this extraction porthole thus no dedicated absorber is required
(But for the porthole region the radiated power will be dissipated in the vacuum chamber wall) The
radiation will exit the vacuum chamber through a UHV viewport made of MgF2 (this material offers
the highest transmission coefficient over the 200-800 nm range) The synchrotron radiation distribution
at 5 eV one meter downstream of the bending magnet is around 6 mm (FWHM) So the spectral flux
integrated over a collecting area of 6 mm x 63 mm is about (around 5 eV)
Φe [Phs] = ΦBM [Phs01bwmmsup2] x 6 mm x 63 mm x 1000
Since photons of lower energy thus with a higher divergence will also contribute to the total spectral
flux this value gives a low limit estimate of the real total spectral flux Figure 411 shows the Φe
spectral flux
As was the case for the Cerenkov radiation a FuSi lens will be used at the exit of the viewport to focus
the beam The collimated radiation will then be transported via a set of planar mirrors across the
shielded area and then through the ThomX shield to a dedicated hutch in the experimental hall (see next
section)
Figure 411 Spectral flux Φe in a 63 x 32 mrad2 aperture 1 m downstream of a bending magnet
453 Transport of the Cerenkov and synchrotron radiations to the diagnostics hutch
Both the Cerenkov light and the synchrotron radiation have to be transported to the diagnostics hutch
located in the experimental hall Starting from different locations both radiations will be transported via
a set of planar mirrors to a lsquomeeting pointrsquo inside the shielded area A flip mirror (ie a double-
orientation mirror) will enable the selection of one of the radiations The one selected will be further
transported again with a set of planar mirrors through the shielding wall to the diagnostics hutch
Inside the diagnostics hutch the last lens will focus the radiation on the streak camera entrance The
lens aperture and its focal length will be adapted to the selected incoming radiation All the line
components and their parameters are chosen to minimize spatial aberrations and to maximize the
number of photons entering the streak camera
454 Streak camera for the bunch length measurement
We plan to use a double-sweep streak camera to measure the bunch length and to visualize its evolution
over short time scales Since the bunch length may vary from 4 to 20 ps (rms) a 2 ps (FWHM)
resolution streak camera is required
lsquo
94
The time resolution of the bunch length measurement depends on the radiation type (CR or SR) which
is used and involves several contributions the electron transverse beam size at the light emission point
chromatic effects etc All these contributions will be minimized as much as possible to maintain the
effective resolution as close as possible to the one of the streak camera
46 Bunch imaging using synchrotron radiation
The synchrotron radiation transported to perform the bunch length measurement (see section 45) can
also be used to image the bunch while it circulates For this purpose a beam splitter could be installed
at the radiation exit port the main part of the radiation would be sent to the streak camera while a small
fraction would be directed to a CCD camera mounted nearby A focusing lens placed between the
viewport and the CCD would enable the imaging of the stored beam
In the orbit section where the bunch emits the synchrotron radiation used for the above measurements
the e- beam transverse size is about 05 mm (rms) Since the size of a CCD chip is typically 5 mm the
lens gain should be about 10 Such a set-up would enable one to follow the evolution of the beam size
in real time (we will use a CCD whose acquisition rate is about 10 Hz) It would be a non-destructive
and cheap diagnostic Similar setups are currently used on storage rings to measure the electron beam
emittance assuming that the beta function is known
47 Beam loss measurement
Particle losses will be measured by using optical fibers [46] When a charged particle crosses a glass
fiber Cerenkov radiation is emitted within the fiber and propagates along the fiber It can be detected
by a photomultiplier that collects the light at the fiber end The measurement eg with an oscilloscope
of the pulse production (in the fiber) time enables one to recover the particle loss location
Two fibers will be installed One will be used to monitor e- losses along the linac and the transfer line
The second fiber will be used to monitor particle losses along the ring Each one of these fibers will be
connected to a fast photomultiplier (PMT) The PMT time resolution will determine the accuracy with
which the losses can be localized
A fused silica fiber has a yield of 227 photons per cm of a relativistic electron track [47 48] Therefore
a 5 mm thick fiber will have a yield of ~100 photons per relativistic electron which crosses it Such a
signal should be easily detected by a PMT
As it may be difficult to place the fiber close to the beam pipe in some parts of the ring the fiber may
have to be located at a distance d away from the beam axis Then the number of lost particles which will
cross the fiber will be reduced by a factor dsup2
48 Beam dumps
The ThomX set-up will include two particle dumps to stop the e- beam in such conditions that the
radioactivity then generated is minimal The first dump will be located at the linac exit The second
dump will be located downstream of the injectionextraction section The latter dump can be used to
stop the beam either at the exit of the transfer line or after any number of bunch revolutions in the ring
Both dumps are based on the SOLEIL design and can handle the 35 W power deposition (1 nC 70
MeV 50 Hz repetition rate)
lsquo
95
Chapter 5 SYNCHRONIZATION
51 Overall scheme
An ultra-low noise 500 MHz synthesizer will be used as the main clock for ThomX The 10 MHz
output of this synthesizer will be used to drive a second synthesizer (the so-called lsquolinac synthesizerrsquo)
operating at 3 GHz The RF output of the latter synthesizer will be used for the linac operation It will
be split in two lines The first one will be used to synchronize the gun laser oscillator (after appropriate
down conversion of the frequency) The second line will be used to drive the gun and the linac RF
The RF output of the main synthesizer (500 MHz) will be used for the ring RF it will be split in three
lines The first line will drive the ring RF (after appropriate phase shifting) The frequency of the second
line will be divided by 14 this line will synchronize the Compton cavity As in the MightyLaser
experiment [51] the laser used to inject power in the Compton cavity will be synchronized with that
cavity by using the PDH method The frequency of the third line will be divided by 28 and will
therefore be synchronous with the electron revolution frequency This line will be used to synchronize
programmable delays delivering turn-by-turn triggers and injectionextraction triggers To ensure that
injection is always taking place at a zero crossing of the mains the programmable delays will also be
synchronized with the mains A counter will reduce the mains frequency to the operations frequency
(when lower than 50Hz) The gun trigger will be generated when the mains will be at a zero crossing
and the ring RF at a preset phase that takes into account the time it will take for the electrons to travel
from the linac gun to the ring accelerating cavity
To ensure that the e- bunches will always be injected at a triple zero (50 Hz 500 MHz and 3 GHz) we
need to specify the tolerance on the 50 Hz zero crossing We may take 1 (ie 02 ms) for this
tolerance this means that the coincidence between the ring RF (~500 MHz) and the linac RF (~3 GHz)
must occur at a frequency greater than 5 kHz Therefore by choosing a heterodyne frequency (ie the
frequency difference between the linac RF divided by 6 and the ring RF) greater than 5 kHz we ensure
that there will always be a common zero crossing between the ring RF and the linac RF within 1 of
the mains zero crossing This sets a constraint on the ring radius of 10-5
ie 168 micrometres on a
168 m long orbit (see Table 2)
The RF bucket must be positioned with an accuracy of 3 ps in the ring RF Thus the phase noise
between the linac RF and the ring RF must be lower than 3 ps when integrated between 50 Hz and
500 MHz Most good-quality synthesizers achieve such a performance
The information collected by the ring diagnostics will be used to adjust the exact value of the frequency
(around 500 MHz) generated by the main synthesizer so that it matches the exact ring length This
information will also be used to adjust the phase between the injection trigger and the ring
To reach the correct signal level each RF line may have to be amplified An isolator will be added to
avoid reflections that may degrade the overall performance of the system
To ensure long-term low frequency synchronization a Network Time Protocol (NTP) server will be
available to allow all computers working at ThomX to synchronize their internal clock
lsquo
96
52 Specifications
Synthesizer
The main synthesizer will be used to generate a 500 MHz adjustable signal Its phase noise jitter
integrated between 50 Hz and 20 MHz must be less than a few picoseconds It must support a frequency
modulation of at least 100 kHz Typical mid-range synthesizers meet such specifications
Measurements done at LAL in collaboration with SOLEIL have shown that the addition of an external
frequency reference does not necessarily improve the phase noise of these synthesizers in the frequency
range of interest
The linac synthesizer specifications are the same ones except that it must be able to generate a
frequency of 3 GHz
Programmable delay
Two low jitters units with 8 channels each will be used
Phase shifter
The phase shifters must be voltage controlled and cover 360deg (two 180deg units)
Figure 51 Synchronization layout
lsquo
97
Chapter 6 OPTICAL SYSTEM DESIGN
Since the writing of the ThomX CDR three directions have been pursued improvement of the feedback
system improvement of the laser amplification and mechanical design of the optical resonator The
results are described in the following three sections
In the first section the ThomX feedback system is described During the past year a strong RampD
program has been developed on the locking of a TiSapph picosecond laser beam on a two-mirror high
finesse cavity at OrsayLAL Very good results in operating for the first time a 30 000 finesse cavity
in the picosecond regime were obtained The feedback techniques used for this task are directly usable
for ThomX and are reported in this section A 3000 finesse four-mirror cavity at KEK was also installed
and operated and we succeeded in producing γ rays off the ATF electron bunches (Mightylaser project
[61]) Hardware damages were however caused by the Japanese 2011 earthquake At the time of
writing this document we are recovering all our equipment to reach our final goal locking a 30000
finesse four-mirror cavity to produce a high γ ray flux at ATF Thanks to a contact with the SYRTE
laboratory we have acquired a new laser oscillator with a 1030 nm wavelength that we are testing and
that we plan to use for ThomX With this oscillator by embedding an intra-cavity EOM we are able to
increase our feedback bandwidth by more than one order of magnitude
The second section illustrates the laser amplification developments done at CELIA First a non-all-
connected fiber amplifier for the Mightylaser project was set up that allows us to extract an average
power of 50 W This amplifier was used to produce γ rays at ATF A new setup has been designed and
built to reach both the reliability and the power level required by ThomX An important activity is
devoted to the construction of a fully all-connected amplification chain As mentioned above the new
laser oscillator and the new laser amplification chain are being integrated in order to fill a 30000 finesse
high finesse cavity to conclude the ANR Mightylaser project
In the third section the details of the implementation of the cavity in the electron ring and the cavity
vacuum system are described with the opto-mechanical system that will be used for a precise alignment
of the cavity mirrors
61 Feedback system and results on a Fabry-Perot cavity locking
To produce a high X-ray flux the ThomX facility produces high intensity laser pulses and collides them
with electrons stored in a ring Hence the locking system (see figure below) has a double goal
- Stabilize the synchronization of the Fabry-Perot cavity (FPC) photon pulse with the electron bunch
- Stabilize the synchronization of the laser pulses with the FPC to obtain amplification
For this last point as will be seen in section 62 firstly the laser beam will be amplified with a CPA
technique using optically pumped fibers and then it will be amplified again by accumulation during
many round trips into a high finesse FPC (the target is a gain of 10000 which leads to a finesse of
30000)
This section describes the following topics related to the stabilization of the laser pulses relative to the
FPC
- Parameters to be stabilized repetition rate and carrier-envelop phase (CEP)
- Error signals the Pound-Drever-Hall (PDH) measurement technique
lsquo
98
- Actuators dynamic range and bandwidth
- Behavior of the digital feedback system (DFS)
General scheme for the locking system Detection and actuation elements are shown in red
The results on the locking of a TiSapph laser oscillator on a 30000 finesse cavity will also be
illustrated
Figure 61 Locking scheme for the synchronization of the electron bunch and the FPC pulse for the
ATF experiment at KEK
611 Electron and FPC pulse synchronization
This kind of stabilization has already been achieved for the MightyLaser experiment at KEK (ATF) a
1785MHz repetition rate A digital feedback system was used (i) to acquire two clocks one from the
laser and one from the master oscillator of the electron ring (ii) to perform a digital phase detection
(iii) to build a digital integrator filter to remove phase drifts and (iv) to react on a PZT inside the FPC
to change its length according to the measured phase drift (see Fig 61)
lsquo
99
612 Laser parameters to be stabilized
We will choose a laser with a repetition rate corresponding to twice the e- bunch revolution frequency
in the ring (two laser pulses per electron bunch revolution) around 500MHz14 = 357 MHz A phase
noise measurement on a similar product operating 1785 MHz was already carried out to characterize
the repetition rate phase noise and the drift (see Fig 62 and Fig 63)
Figure 62 Repetition rate phase noise spectrum Figure 63 Repetition rate drift measured
X-axis offset frequency from the carrier in Hz during about one hour X-axis repetition
Y-axis single side band phase noise power rate drift in Hz Y-axis time duration in
density in dBcHz seconds
The global shape of the phase noise in 1f will permit to use three feedback actuarurs a high bandwidth
low dynamic one (EOM for repetition rate and AOM or pump current for CEP) a middle
bandwidthmiddle dynamic one (PZT for repetition rate) and a high dynamiclow bandwidth one
(mirror motor and isochronic wedge motor)
For this kind of laser the harmonic content (index m) is well known and can be represented either in the
time domain or in the spectral domain (see Fig 63 and Fig 64)
( )2
CEL REP CE REPf m m f f m f
φ
π
Δ = + = +
lsquo
100
Figure 64 Time domain and spectral representation of the laser pulses
Since one is dealing with two parameters (the repetition rate and the CEP) one needs two different
sources of information to measure them The standard method to measure these two parameters consists
in using the Pound-Drever-Hall (PDH) technique [62] which gives the position of the laser harmonics
relative to the FPC resonances
( )FPC CAVf m m f=
One of the parameters is deduced from the whole beam signal using a central photodiode the second
one is derived from a part of the spectrum selected by a grating and a slit and measured by the grating
photodiode
If the repetition rate and the FPC round trip frequency are close enough the error signal coming from
the central photodiode (index m0) is proportional to the difference between the laser harmonics and the
corresponding resonances of the FPC
1 0 0
2
CEREP CAVm f m f
φε
π
Δ prop + minus
Frequency
domain
lsquo
101
As the locking system cancels the error the central laser harmonic m0 is perfectly aligned on the FPC
resonance but all other laser harmonics (m=m0+Δm) are slightly detuned from the corresponding FPC
resonances This introduces a correlation between the CEP and the repetition rate
( )( )
1 0 0
2 0
0 0
02
1 12
CEREP CAV
CEREP CAV REP CE
m f m f
m mm m f f f f
m m
φε
π
φε
π
Δ = rArr + =
Δprop minus minus = minus = minus
As a result of these equations the signal received by the PDH system located after the grating (ε2) is
directly proportional to the offset frequency (fCE) and then to the CEP Thus if we change the CEP of
the laser we should clearly see on the grating photodiode PDH signal ndash which measures the laser-FPC
detuning on the border of the spectrum (for an m index far from m0) ndash a linear relation between the error
signal and the CEP
This method well known for femtosecond lasers has been tested in our laboratory in the picosecond
regime Figure 65 shows our results (submitted to Applied Optics) on the locking of a 76MHz
TiSapph picosecond laser oscillator on a 30000 finesse cavity
Figure 65 Top experimental measurement of the CEP value of a TiSapph laser oscillator Bottom
measurement of the corresponding grating photodiode signal
In Fig 65 one sees that when the CEP is measured close enough to zero the grating photodiode
exhibits a linear relation with the CEP otherwise the photodiode is saturated (due to the PDH signal
saturation) In our experiment the ratio between the cavity reflected power and the incident beam
power was ~20 which leads to a value of 80 for the coupling of the incident beam power to the
cavity
These results demonstrate for the first time that a picosecond laser oscillator can be locked to a 30000
finesse cavity (implying a power enhancement factor of 10000) with a power coupling of 80
613 PDH Measurement technique
Having shown that we are able to measure independently both parameters of interest (the repetition rate
and the CEP) we will now describe the apparatus to transform these measurements into error signals by
lsquo
102
the PDH technique (see Fig 66) Side-bands to the laser optical harmonics are produced by an electro-
optic modulator (EOM) modulated by a generator coming from the digital system Side-bands and laser
harmonics are reflected by the FPC the first ones without acquiring any phase the second ones shifted
in relation with the discrepancy between the laser and the FPC frequencies All the harmonics and the
side-bands of the reflected laser are combined into the photodiodes and the front-end electronics giving
an error signal at the modulation frequency
Figure 66 Pound-Drever-Hall scheme with digital phase detection
The photodiode front-end (see Fig 67) amplifies these signals at a level compatible with the digital
feedback system (DFS)
Figure 67 Electronic scheme and picture of the PDH front-end
Inside the DFS the PDH signals are filtered by a band-pass filter to remove unwanted frequencies and
multiplied by the same modulating signal with a phase shift to allow for the phase detection A low-pass
filter at the end of the chain removes the image signal at twice the modulation frequency Figure 68
shows an example of the input and the output signals
lsquo
103
Figure 68 Modulated input (blue) error signal (yellow) FPC transmitted signal (pink)
614 Actuators
In the previous sections we described how the two parameters (the repetition rate and the CEP) of a
pulsed laser have to be controlled in order that all the laser harmonics match the FPC resonances and
we also detailed how to measure the variations of these two parameters We will now describe the
devices inside or around the laser on which we need to react to correct the variations of the laser
harmonics relative to the FPC resonant frequencies
Figure 69 Typical scheme of a laser with all its actuators
The actuators will produce an action on the two parameters to be controlled with different dynamic
ranges and different bandwidths (see chart in Fig 610)
Figure 610 Dynamic ranges and bandwidths of the laser actuators
lsquo
104
All of the actuators are located inside the laser except the AOM (see Fig 610) They are the key items
to achieve a good stabilization
All the actuators measurements have been made on the 1785MHz laser version used at KEK
- Mirror motor dynamic range precision and bandwidth
The motor is a step motor with a precision of 140 nm on the round trip variation (equivalent to 15 Hz on
Frep) Figure 611 shows the linearity and the maximum dynamic range available with the motor stage
while Fig 612 shows a step response which indicates the maximum bandwidth we can obtain
Figure 611 Mirror motor range Figure 612 Time response of Frep for
1k motor steps X-axis repetition rate
offset (Hz) Y-axis repetition rate
The repetition rate range is 178033 - 179244 MHz (gt 1MHz)
- Piezoelectric sensitivity dynamic range and bandwidth
The PZT sensitivity is approximately 3Hz V 28 nmV (length variation to be compared to the
length of a round trip of the FPC pulse)
The PZT can be operated up to 150V providing a total dynamic range of 42 microm (round trip length
change) Its first peak resonance has been measured and found above 40 kHz
lsquo
105
Figure 613 Repetition rate change produced by a Figure 614 PZT frequency response X-
PZT voltage variation from 0 to 28 V frequency range 01-5 kHz Y-axis
Attenuation (dB) ndash arbitrary offset
Figures 613 and 614 show the PZT sensitivity and its transfer function
- EOM sensitivity measurement
Measuring the sensitivity of the EOM is not direct thus we chose to apply a 10 V amplitude ndash 10 Hz
frequency modulation ndash to the EOM and to measure the power of the harmonics (see Fig 616)
Assuming a pure frequency modulation the power of the harmonics fis a Bessel series that depends on
the sensitivity Thus taking the ratio of two consecutive harmonics gives directly the sensitivity
Figure 615 EOM modulation spectrum used for sensitivity measurement X-axis frequency centered
on the repetition rate Y-axis relative optical power around the repetition rate due to the modulation
The table on the left gives the frequency and the power of each harmonic
0 5 10 15 20 25 300
10
20
30
40
50
60
70
80
90
100
PZT voltage (V)
Fre
p v
ariation (
Hz)
Frep variation vs PZT voltage
AJ0(z)
AJ1(z)
AJ2(z)
lsquo
106
We now check that the sensitivity calculated from the ratio of the first harmonic to the fundamental and
the one obtained from the ratio of the second harmonic to the fundamental are the same
∆
∆
~1
2
∆
10~ minus 2909 rarr ∆~07
∆
∆
~1
8
∆
10~ minus 6398 rarr ∆~07
Since the ratio is 07 Hz for an applied sinusoidal signal of 10 V the sensitivity is around 007 HzV
- Isochronic wedge
It is made of two glass prisms with different refraction indexes chosen to maintain a constant group
delay in order to avoid repetition rate variation
We measured that the maximum variation of the repetition rate for a full CEP range (several times 2π)
is about 300 Hz for a repetition rate of 1785 MHz (see Fig 616 which shows the measured repetition
rate shift plotted along the X-axis versus the date of the measurement plotted along the Y-axis) Thus
the influence of this actuator on the repetition rate is about 210-6
Figure 616 Measurement of the influence of wedge displacement on Frep
- Acousto-Optic Modulator (AOM)
Such an actuator has already been used with a laser which is not the same as the one used for the other
actuators namely a 76 MHz laser locked on a two-mirror FPC
In Fig 617 we present the closed loop measurement The lowest frequency part (lt10 kHz) is
attenuated by the loop gain If the loop were open this part of the transfer function would be quite flat
To estimate the AOM bandwidth limits we can use the frequencies where the gain starts to drop
significantly this bandwidth is about 300 kHz
300 Hz
lsquo
107
Figure 67 Frequency response of the AOM in closed loop
615 The digital feedback system
The feedback system will deal with several coupled inputs and outputs To ensure flexibility we have
chosen a digital system Since we also want to work with a large bandwidth in closed loop (gt 1MHz) a
low latency system (lt 100 ns) is needed A digital system featuring such performance does not exist on
the shelf Thus we decided to design and build such a system at LAL (see Fig 618)
Figure 618 Scheme of the DFS and picture of the DAC daughterboard
This system will perform several functions (see Fig 619)
103
104
105
106
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
Frequency (Hz)
Magnitude (
dB
)
Bode plot
lsquo
108
Figure 619 Organization of the digital feedback functions
- The first important function is to produce the error signal from the modulated signal coming from the
photodiodes It is a phase detection block (Digital PDH DPDH) the main problem being to reduce the
latency as much as possible This will be achieved by optimizing the low-pass filter and the modulation
frequency (see Fig 620 for a digital implementation)
Figure 620 Digital phase detection (DPDH) to produce the PDH error signals
- The second important function is to provide Infinite Impulse Response (IIR) filters to build
Proportional-Integrator (PI) filters in order to cancel the system drifts
lsquo
109
Figure 621 Digital IIR filter implementation with adders and multipliers
These filters can be programmed by the user to change the filter coefficients and it can be cascaded to
realize complex filters They have been cut in Second Order Sections (SOS) to minimize the
propagation of fixed precision errors due to quantized data and coefficients (see Fig 621)
- The last important function integrated in the digital system is performed by an identification block
which works like a network analyzer by (i) sending a small harmonic signal (ii) looking at the system
response in closed loop at this frequency and (iii) sweeping this small harmonic signal frequency to
cover all the range to be measured (see Fog 622)
Figure 622 Identification system
lsquo
110
This kind of identification system can provide information on (i) the feedback system itself (ii) the
system to be stabilized or (iii) the open and closed loop transfer functions by producing Bode or
BlackNichols diagrams (see Fig 623) These informations are very useful for system analysis and for
tuning or designing the feedback filters
Figure 623 Typical Bode (top-left magnitude bottom-left phase) and Black-Nichols (right) diagrams
produced by the identification system
62 High average power laser system and its coupling to a high gain FP cavity
The all-fibered laser system will draw from the system developed in the MightyLaser project It will
consist in an oscillator associated with three amplifying stages involving single and double clad Yb-
doped fibers (see Fig 625) At present the amplifiers are based on commercial components to facilitate
a potential replacement of damaged parts The architecture is designed to provide the mandatory
diffraction-limited beam This relatively simple and compact architecture will implement the lsquoChirp
Pulse Amplificationrsquo (CPA) scheme In order to limit non-linearity effects in the fiber the signal is
initially stretched in time thus reducing its intensity in the waveguide then amplified in the fiber and
finally recompressed to its minimal duration The difference between the MightyLaser and the ThomX
repetition frequencies (respectively 1785 MHz and ~357 MHz imposed by the different repetition rates
of the two accelerators) requires an adjustment of the laser design
Figure 625 Sketch of the all-fiber laser system
lsquo
111
621 The MightyLaser oscillator
This oscillator is an all-normal dispersion mode-locked fiber laser based on the nonlinear polarization
evolution technique equipped with a fibered output It uses an ytterbium doped fiber which allows an
emission centered around 1030 nm with a full width at half maximum of 10 nm (see Fig 626) It
delivers short pulses at a repetition rate of 1785 MHz with a low average output power (~10 mW) in
order to minimize the noise level (see Fig 628) The main advantage of the all-normal dispersion
regime resides in the significant peak power reduction provided by the pulse stretching during the
propagation along the cavity Non-linear effects are thus reduced allowing a high-energy extraction
The output pulses are naturally chirped with an estimated duration of 2 ps
Figure 626 Emission spectrum of the oscillator
The oscillator is passively mode-locked by taking advantage of the non-linear evolution of the
polarization This technique is based on the optical Kerr effect The polarization of a pulse propagating
in a fiber evolves non-linearly as a function of intensity Thus by combining polarization controllers
and a polarizer one may generate losses that depend on the intensity (see Fig 627) Beyond the
polarizer all the pulse components are linearly polarized The wave plates transform the linear
polarization into an elliptic one The latter polarization state then evolves in a non-linear way while it
propagates along the fiber This evolution results from the combined effect of the self-phase and the
cross-phase modulations induced on the two polarization components The polarization rotation of the
more intense central part of the pulse is larger than the polarization rotation of the pulse wings Since
the pulse center crosses the analyzer with a reduced loss compared to the wingsrsquo loss the pulse duration
is shortened
Figure 627 Principle of mode locking based on the non-linear polarization evolution
lsquo
112
Figure 628 Spectrum of the oscillator phase noise noise level (dBcHz) versus offset frequency (Hz)
622 Amplification stages
The amplification chain is divided in three stages in order to distribute the gain and to saturate the
second and the third amplifying stages The first and the second stages are based on the same
architecture It is an all-fibered amplifier whose central component is a polarization maintaining single
mode using an Ytterbium doped fiber with a core diameter of 6 microm and a clad diameter of 125 microm (see
Fig 629) A wavelength division multiplexer (WDM) is spliced on the doped fiber to transfer the pump
power into the gain medium and to couple the signal of the picosecond oscillator to the amplifier The
first stage is pumped by a single mode diode centered at 976 nm and delivering 200 mW The second
one is pumped at a higher average power namely600 mW The two amplifying stages are separated by
an optical isolator to avoid any parasitic feedback from one amplifier to the other
Figure 629 Schematic of the first and the second amplification stages
The average power level at the input of the first amplifying stage is about 1 mW A limitation is
imposed on the output of this stage which should remain below the damage threshold of the isolator
fixed at 100 mW Consequently we adjust the pump level in order to obtain 80 mW at the output of the
ytterbium-doped fiber The 3 dB losses of the isolator allow one to inject 40 mW into the second
amplifying stage The evolution of the average output power versus the pump level is shown in Fig
630 For the maximum pump level of 600 mW the second amplifying stage delivers 330 mW
lsquo
113
Figure 630 Second stage output power plotted versus the pump power
Beyond the second isolator the third amplifying stage (see Fig 632) is built by using as its central
component a polarization maintaining double-clad Ytterbium-doped fiber with a core diameter of 25
microm and a clad diameter of 250 microm The amplifier is clad-pumped through a special combiner 6+11
(see Fig 631) In practice this component has an input signal fiber 6125 microm directly spliced to the
previous amplification stages an output signal fiber 25250 microm spliced to the double-clad Ytterbium
doped fiber and six multimode fibers of 105 microm to couple the pump diodes Here only three pump
ports are spliced to multimode diodes emitting up to 55 W at 976 nm corresponding to a pump power of
at most 165 W The development of a mode field adapter was necessary to splice the pump combiner to
the double-clad Ytterbium-doped fiber because their numerical apertures are different Losses entailed
by this component are compensated by the last amplification stage
Figure 631 Pump combiner Figure 632 Schematic of the third amplification stage
Figure 633 is a plot of the average output power versus the pump level For a pump power level of
76W the last amplification stage delivers 52W This was the first objective The fiber laser power
stability was measured during 1 12 hour at this power level (see Fig 634) The residual pump power
the pump and signal feedbacks into the pump diodes are also plotted in this figure
lsquo
114
Figure 633 Third stage output power Figure 634 Laser stability at 52 W
versus the pump power
New measurements were made after optimizing the central wavelength of the multi-mode pump diodes
The efficiency was increased to reach 787 for an output power of 24 W (see Fig 635) The fiber
laser power has been tested at this level during two hours (see Fig 636) The observed power variation
is less than 01
Figure 635 Output power versus pump power Figure 636 Laser stability at 24 W
after wavelength optimization
The all-fibered amplifier setup has been integrated in a 19rsquorsquo rack as shown in Fig 637 An FCPC
connector placed in the front panel allows the connection of the fibered oscillator to the master
amplifier module Several 955 optical fiber couplers have been inserted in our setup at strategic
locations to monitor the power level in real time by using photodiodes they may also signal potential
problems The output fiber comes out of the module through its front panel The back-side is dedicated
to all the electronic controls (diode pump photodiodes and modulators for noise management described
elsewhere)
lsquo
115
Figure 637 Pictures of the 52 W all-fibered setup integrated in a 19rsquorsquo rack
623 Stretcher and compressor
A chirp volume Bragg grating (CVBG) is used in our setup to stretch and compress the pulse with a
very good efficiency of 94 The CVBG transmission bandwidth is around 2 nm (see Fig 638) leading
to a pulse duration ranging from 200 to 250 ps depending on the grating dispersion It is possible to
recompress the pulse after amplification by using a second CVBG Then the pulse length ranges
between 20 and 87 ps depending on the second CVBG dispersion (see Fig 639)
Figure 638 Transmission bandwidth of the CVBG
1‐2 couple 3‐2 couple 3‐1 couple
Figure 639 Time profile of the recompressed pulses for different grating pairs
624 ThomX development
Some changes must be made to the all-fibered lsquoMightylaserrsquo design in order to use it in the ThomX
project The oscillator repetition rate needs to be scaled down to match the frequency imposed by the
accelerator As far as the laser design is concerned the stretchercompressor couple must be modified to
limit non-linear effects The increase of the output power from 50 to 150 W imposes an upgrade of the
second amplification stage and eventually of the third one
lsquo
116
6241 Oscillator
The oscillator for ThomX will be based on the same principle as the one used for Mightylaser The only
difference is the repetition frequency imposed by the accelerator (357 MHz for ThomX) The low
average output power around 5 mW permits to limit the phase noise A particular attention will be paid
to the noise level of such a long cavity as no commercial products have been tested at this repetition
rate
6242 Amplification stages
For the ThomX project the lower repetition rate and the higher average output power require a change
of the second amplification stage architecture in order to limit non-linear effects and to saturate the last
amplification stage Consequently the second pre-amplifier will be based on a polarization-maintaining
double-clad Ytterbium doped fiber with a core diameter of 15 microm and a clad diameter of 130 microm (see
Fig 640) The amplifier will be clad-pumped through a special combiner 2+11 with an input signal
fiber 6125 microm directly spliced to the previous amplification stages an output signal fiber of 15130
microm spliced to the double-clad Ytterbium doped fiber and two multimode fibers of 105 microm to couple
the 9 W pump diodes
Figure 640 Schematic of the second amplification stage of the ThomX laser
Concerning the final amplification stage various fibers are available (see below) Micro-structured
fibers can be engineered and tailored but suffer from the lack of splicing techniques Hence their use in
an all-fibered setup will require additional developments dedicated to splicing (see Fig 641) and
integration If splicing is not possible we still have the possibility of proceeding with air propagation
and injection However this approach requires precise mechanics control and advanced engineering
The lsquocoherent beam combining techniquersquo will also be considered at this point In such a configuration
amplification would be supported by two parallel amplification stages dividing by two the energy level
in the gain medium Yet a first version will be developed with commercially spliceable flexible fibers
and preliminary tests of cavity locking with high power from these fibers will be performed to validate
or invalidate operating with a high level of non-linear effects
lsquo
117
Fig641 A micro-structured fiber
6243 Stretcher and compressor
Because of the high average output power required by ThomX we must modify the stretcher size Two
techniques can be considered First we may continue to use a CVBG which can stretch the pulse up to
450 ps with a maximum size of 25 cm (see Fig 642) It would be possible to stretch the pulse up to
900 ps with an efficiency of 88 through a double pass in a CVBG or a single pass in cascaded CVBG
The second way involves a chirp fiber brag grating (CFBG) as depicted in Fig 643 When associated
with an optical circulator it allows a pulse stretching with up to 800 ps in an all-fibered setup The
efficiency is around 70 (92 for the CFBG and 75 for the circulator) in this configuration This
solution is less efficient than the CVBG but the all-fibered configuration ensures the set-up robustness
and stability and the losses can be easily recovered by the amplifier chain However dispersion issues
have been reported for these devices and a detailed investigation is needed prior to a definitive
integration
Fig 642 Examples of CVBG Fig643 Example of CFBG
The high output average power forces us to use a CVBG to compress the pulse one or two units
depending on the stretcher configuration The final pulse duration can be varied between 1 ps and 50 ps
thus leaving the possibility to optimize the X-ray production by adjusting the pulse duration
We also performed simulations to evaluate the influence of the couple stretchingfiber core diameter on
the non-linearity with 3 different fiber cores 25 microm 30 microm and 40 microm Our results are summarized in
Table 61
lsquo
118
Fiber
dimensions
mode field
diameter stretching B-Integral
25250 20 microm 250 ps 47 rad
25250 20 microm 500 ps 263 rad
25250 20 microm 1 ns 118 rad
30150 22 microm 1 ns 09 rad
40200 30 microm 1 ns 0525 rad
Table 61 Pulse stretching as a function of the fiber core diameter (simulations)
A first configuration will be developed with a stretching of 500 ps If the non-linearity effects remain
too high to allow for an efficient locking on the Fabry-Perot cavity we will modify the setup to induce
a larger stretching factor or insert a fiber with a larger core diameter in the last amplification stage or
both
63 Mechanical design
Collisions between the laser pulse and the stored electrons will take place between two dipoles (see Fig
643) The constraints on the optical cavity mechanical design are as follows
- High stability level (vibrations)
- Good accessibility to the cavity mirrors
- Weak impact on the electron beam dynamics
- Ultra high vacuum (UHV) system with baking capability
- Motorized granite table for micrometric scan oof the electron beam
lsquo
119
Figure 643 Implementation of the cavity in the electron ring
The cavity mirror system is mounted on a massive granite table which can be driven to perform a
micrometric electron beam scan (see Fig 643)
lsquo
120
Figure 644 Scheme of the Fabry-Perot cavity mounted on the optical table
The Fabry-Perot cavity is composed of two cylindrical UHV vessels linked by a dipole chamber
Bellows are placed between each element to absorb the dilatation due to the backing (see Fig 644)
e‐
e‐
X‐Ray
1rsquorsquoMirrors
IP
Bellows
1rsquorsquoMirrors
Vessel
Vessel
Dipole
chamber
lsquo
121
Figure 645 On the left detail of the standard mirror assembly and on the right detail of the piezo
electric mirror assembly
The Fabry-Perot cavity is composed of two spherical and one flat mirror mounted in a standard
assembly and one flat mirror mounted on a special assembly with a piezo electric actuator for very
small longitudinal displacements (see Fig 645)
Figure 646 Detail of a truck system
Mirrors
Piezo
electric
actuator
Mirror
Gimbal
Actuators
lsquo
122
Figure 647 Cut of an UHV actuator
All the mirrors are mounted on a truck system that porvides three degrees of freedom (two rotations and
one translation) driven by three micrometric actuators These actuators are specially designed to be
compatible with UHV and backing constraints (see Fig 646 and Fig 647)
Figure 648 Cut of a vacuum vessel
Trucks (two trucks per vacuum vessel) and plugs with special cables composed of ceramic connectors
oxygen- free copper cables and ceramic pearls and spacers for electrical isolation are placed in a UHV
vacuum vessel One of the vacuum vessels has a beryllium window to extract the X-ray beam (see Fig 648)
Micrometric Actuator
Capsule
Trucks
Beryllium
window
lsquo
123
631 Optical cavity vacuum
A simulation was carried out in the most unfavorable case when the whole optical vacuum chamber is not
baked The area of one half of the optical cavity is about 2 m2 the conductance of the rectangular chamber
connecting the two cavity halves is C =70810-1
m3s
-1 (value for one half of that rectangular chamber) The
conductances of the slits connecting the optical cavity to the ring are respectively C = 45210-2
m3s
-1 (9 x 48)
and C = 16010-2
m3s
-1 (6 x 30) The outgassing rate of one optical cavity half without any baking (σ = 210
-8
Pams-1
) is Q = 410-8
Pam3s
-1 The outgassing rate of one rectangular chamber half (σ=10
-9 Pams
-1) is Q =
7610-10
Pam3s
-1 The outgassing rates of the two slits (σ=10
-9 Pams
-1) are respectively Q = 22810
-13
Pam3s
-1 (9x48) and Q=14410
-13 Pam
3s
-1 (6x30) The diode ion pump used for the optical cavity has a
pumping speed of 05 m3s
-1 DN 150 S = 0320 m
3s
-1 at ~10
-10 mbar The conductance of the pumping port
being C = 136 m3s
-1 the effective pumping speed is Seff= 0278 m
3s
-1 The diode ion pump used for one half
of the rectangular chamber has a pumping speed of 015 m3s
-1 DN 100 S = 0090 m
3s
-1 at ~10
-10 mbar The
conductance of the pumping port being C = 0046 m3s
-1 the effective pumping speed is Seff= 0075 m
3s
-1
The outgassing rate of the optical cavity without baking is very high 2 x 410-8
Pam3s
-1 (which is
approximately 40 times the outgassing rate of the whole ring) With such a rate one would need a pumping
speed of 8 m3s
-1 to reach the required pressure level of 10
-8 Pa (the present pumping speed is 07 m
3s
-1) The
conductance of the rectangular chamber that connects the two halves of the optical cavity (710-1
m3s
-1) and
the conductance of the two slits between the optical cavity and the ring (610-2
m3s
-1) are high These figures
do not allow a differential pumping between the cavity and the ring Simulations of the ring vacuum level
show that it is mandatory to bake the whole optical cavity
Figure 649 Location of the cavity pumps
The vacuum equipment needed for the optical cavity is as follows
‐ 2 ion pumps 150 l s-1
with their baking equipment and their power supplies
‐ 2 ion pumps 500 l s-1
with their baking equipment and their power supplies
‐ 2 gate valves manually actuated (pipe diameter DN100) and their baking equipment
‐ 4 Pirani gauges
‐ 4 cold cathode gauges
‐ 2 power supplies for the gauges
‐ 4 residual gas analyzers (RGA)
‐ 1 baking equipment for the vacuum chamber
‐ 4 angle valves manually actuated for pre-pumping (pipe diameter DN63)
lsquo
124
Chapter 7 X-RAY BEAM LINE AND EXPERIMENTS
Introduction
The equipment of the X-ray beam line should firstly ensure a continuous detection and monitoring of
the beam (mainly in the first optical hutch) and secondly allow a complete characterization of the
beam (in the experimental hutch) This characterization consists in measuring the beam absolute
intensity its total energy range its spectral and angular distributions and its degree of transverse
coherence Time analyses of the bunched X-ray beam will also be performed This line also aims at
confirming the ThomX assets in such fields as cultural heritage biomedical and materials sciences by
performing demonstrative experiments
71 Available extracted X-ray beam
The X-ray beam is extracted through a 50 mm diameter beryllium window as schematized in Fig 71
Because of limitations due to the dimensions and the location of the Fabry-Peacuterot cavity mirrors the X-
ray beam available for characterization and experimentation is a cone with a half-aperture of 16 mrad
Figure 71 The X-ray beam extracted from ThomX through a beryllium window The beam fills a cone
whose total aperture is 2 x16 mrad
In the case of a lsquoperfectrsquo beam (ie without taking into account the electron beam emittance) the X-ray
spectral and energy distributions are shown in Fig 72 as a function of the scattering angle for two
electron beam energies 50 and 70 MeV and a laser wavelength of 106 microm In Fig 73 the energy
bandwidth and the flux per unit solid angle are shown for on-axis Compton photons produced by either
a 50 MeV or a 70 MeV electron beam
72 Global view of the X-ray lne
As can be seen in Fig 74 the positioning of the machine in the Igloo imposes the splitting of the X-ray
line into two sections a first optical table (3x2 m2) located just after the beryllium exit window and a
second table (4x2 m2) located outside the Igloo in hall D1 Along its path and until the optical table 2
the beam is transported in the primary vacuum inside stainless steel pipes with the exception of a short
upstream section of a few tens of cm between the beryllium window and the optical table this
insulating section will prevent the transmission of harmful vibrations to the Fabry-Peacuterot cavity This
primary zone is kept oxygen free by a nitrogen flow that prevents ozone production
lsquo
125
The beam diameter at the entry of the first optical table is 45 cm After this first optical table the beam
reaches the second table via a 13 cm diameter beam pipe inserted in a hole which was drilled long ago
in the Igloo wall About 8 mrad beam angular divergence are preserved at the entry of the second
optical table where the beam diameter is 13 cm
Figure 72 X-ray spectral distributions (left panel) and X-ray energy (right panel) versus the scattering
angle for an e- beam of 50 MeV (black) and 70 MeV (red) The laser wavelength is 106 microm Grey
areas correspond to emission angles larger than 16 mrad
Figure 73 X-ray flux (left panel) and X-ray energy bandwidth (right panel) of on-axis photons per unit
solid angle versus the emission angle for an e- beam of 50 MeV (black) and 70 MeV (red) The laser
wavelength is 106 microm
lsquo
126
Figure 74 Global view of the X-ray line in ThomX
73 Chraracterization of the X-ray beam
731 Optical table 1
A global view of the optical table 1 design is presented in Fig 75 Just upstream of this optical table a
60 mm diameter beam shutter activated when one needs to take background data will allow stopping the
beam A few millimeter thick tungsten is sufficient to stop the full beam (tungsten attenuation mass
factor microen ~ 4043 cm-1
for 100 KeV photons) This shutter does not have to be cooled since the ThomX
radiated power will not exceed 100 mW A drawing of the shutter mechanical design is shown in Figure
76 (a)
Figure 75 Global view of the optical table 1 showing the location of the main devices placed on it
lsquo
127
Beam monitoring
The optical table 1 equipment (see Fig 75) is mainly devoted to the detection and the continuous
monitoring of the X-ray beam The first device monitoring the beam presence is a fluorescent screen
mounted on a two-position motor (up or down) as shown in Fig 76 (b) It is inserted in the beam path
and viewed by a CCD camera Its screen absorbs almost the totality of the beam and thus has to be put
in the up position when not in use Downstream of the fluorescent screen a device composed of two Si
PIN diodes measures the beam scattered by a thin Kapton foil inserted in the beam path This setup
shown in Fig 76 (c) will monitor the beam flux variations and beam position displacements The
Kapton foil is fixed at an inclination angle of about 60o with respect to the beam path The sum and the
difference of the two diode currents will provide a relative measurement of the beam flux and of the
beam position respectively This set-up which is nearly transparent to 20-90 keV radiations is a non-
intrusive monitor and can be used continuously Then a high precision double slit [71] intercepts the
X-ray beam and allows one to let through only the desired part of the Compton cone (see Fig 76 (d))
The minimum dimensions of the slit aperture are 10 microm in the horizontal and the vertical directions
Beyond the double slit a wire monitor [72] shown in Fig 76 (e) is inserted in the beam This device
consists in two tungsten wires stretched horizontally and spaced by a few tens of a millimeter This
monitor will be driven vertically through the beam The difference of the two wire signals allows the
measurement of the absolute beam position
Figure 76 Drawings of the X-ray line devices located on the optical table 1 (a) Beam shutter
(b) Motorized two-position fluorescent screen (c) Kapton foil and Si diodes (d) Double slit (e) Wire
profiler
lsquo
128
Focusing device
Downstream of the beam monitoring instruments a focalizing device will be installed (see Fig 75) to
make the X-ray beam parallel andor to focus it on a sample It can be either a device based on
compound refractive lenses [73] (a so-called lsquotransfocatorrsquo) or two bendable mirrors in the Kirkpatrick-
Baez configuration (KB Optics) or a capillary optical device These three techniques will be
investigated to evaluate the advantages and disadvantages (aperture efficiency focusing strength etc)
of their eventual implementation in the ThomX facility Such a focusing apparatus has to be located as
close as possible to the X-ray source because of the rapid growth of the beam transverse dimension
Finally at the end of the optical table 1 some space is kept free for any specific study which might
concern optics detectors or samples
732 Beam transport
As shown schematically in Fig 77 beyond the optical table 1 the X-ray beam crosses the Igloo wall
inside a beam pipe that runs through a 15 cm diameter hole and reaches the D1 hall where the second
optical table is installed in the lsquoexperimental hutchrsquo At the entrance of the wall hole a second beam
shutter in lead around 15 cm in diameter and 25cm thickness is installed This shutter is connected to
an interlock system that ensures radioprotection It absorbs radiations coming from the electron
machine which propagate through the Igloo cylindrical hole and allows X-ray users to work in the
experimental hutch without stopping the electron machine
Figure 77 Schematic of the X-ray beam transport across the Igloo wall from the optical table 1 located
in the Igloo until the optical table 2 located in the D1 hall
733 Optical table 2
Several kinds of instruments will be placed on the optical table 2 The goal is to study in detail the X-
ray beam produced by ThomX firstly to determine how the beam characteristics depend upon the
machine operation parameters and secondly to prepare the demonstration experiments A global view
of the optical and detection instruments placed on that table is provided by Fig 78
lsquo
129
Figure 78 Global view of the optical table 2 indicating the main instruments that will be used to
characterize the beam
Absolute and spectral flux measurement
The measurement of the X-ray beam absolute flux and of its energy spectrum will be performed by
using two detectors a calibrated Si diode [74] and a cooled high-purity Ge solid-state detector (SSD)
[75] The Si diode will be bought already calibrated [76] A lsquogoodrsquo Ge SSD has an energy resolution
(ΔEE) around 1 at 20 keV 06 at 45 keV and 045 at 90 keV [77] The energy calibration of the
Ge SSD will be carried out in the X-ray beam line by using the Kα fluorescence lines of reference
materials (see Table 71)
Element Ge Mo Sn Cs Ce
EKα1 (keV) 9886 17479 25271 30973 34719
EKα2 (keV) 9855 17374 25044 30625 34279
Element Eu Er W Pt Pb
EKα1 (keV) 41542 49128 59318 66832 74969
EKα2 (keV) 40902 48221 75981 65112 72804
Table 71 Kα fluorescence lines of chemical elements used to calibrate the Ge SSD in the 10-90 keV
energy range
The two detectors will be mounted alternately on a (X Y) translation stand to measure a large number
of points of the emission spectrum
- the mean emission energy E (Ge SSD)
- the energy spread ΔE (Ge SSD)
- the absolute flux (photon numbers) (Si diode)
These beam characteristics will be measured as a function of the size of the selected X-ray sample the
selection being carried out with a double slit device (see Fig 76 (d)) Our goal is to check
lsquo
130
experimentally the theoretical angularspectral distributions shown in Fig 12 of the Introduction by
measuring the number of Compton photons per second per mrad2 and per 01 of energy bandwidth
Time analysis of the bunch structure
The X-ray source has a time structure which involves two main scales a 50 ns period which is the
interval between two successive collisions between the laser pulse and the electron bunch and the 20
ms e- storage cycle To perform a pulse by pulse analysis we will use a high resolution time detector
(~1-5 ns) with a high repetition frequency (~10-100 MHz) eg a micro-channel plate (MPC) detector
[78] or an avalanche photo-detector (APD) Such time measurements have already been performed at a
similar facility by Sakaue et al [79] MCP Hamamatsu devices can reach a 07 ns (FWHM) time
resolution which is good enough not only to resolve each one of the collisions between the laser pulse
and the electron bunch but also to measure their relative intensity accurately from the beginning of the
e- storage cycle to the end These measurements will be performed on the X-ray beam axis They will
have to be synchronized with the ThomX clock
Transverse coherence of the beam
Some simple diffraction and interference measurements will be performed in order to analyze the
source spatial coherence The diffraction of the X-ray beam by a slit of a known aperture or the
interference effect produced by the insertion of a phase-shifting object in the beam allows one to
quantify the transverse coherence of the beam [710] In these experiments the on-axis photon intensity
distribution will be recorded with a 2D high resolution detector (such as for instance an XPAD CdTe
camera with 130 microm pixels) This measurement will be repeated at several positions along the beam
The pattern analysis will inform us about the degree of coherence of the source A monochromator will
be inserted upstream of the diffractive slit or the phase-shifting object to select a well defined energy
Later on Young double-slit experiments could also be carried out [711] to measure the beam
coherence length beam
74 Demonstration experiments
In parallel with the complete characterization of the ThomX beam described in the previous
sections some simple experiments will be performed in order to demonstrate the potentialities
of the ThomX source In this phase we will focus on medical and materials science
applications An important aim of the ThomX project is to perform medical and materials
imaging exploiting the high and tunable energy of its beam A second aim of the ThomX
project is to show its capability in the structural investigation of materials using X-ray
diffractionscatteringspectroscopy techniques
741 Experiments using the full X-ry beam
Three experiments issued from the expertise of the ESRF-INSERM teams in medical applications of
synchrotron radiation have been chosen to be carried out as demonstrators on the ThomX prototype
The goal of setting up such experiments is to demonstrate (i) the suitability of ThomX of performing
both imaging and therapy experiments (ii) the tenability of the source adjustable at energies
corresponding to the K edge of high Z elements routinely used in medicine (Gd I) or of those of new
experimental radiotherapy adjuvants (Au Gd nanoparticles CisPt) (iii) suitability of the source for
phase contrast imaging (iv) exploitation of the advantage of using a 2D beam with respect to the typical
laminar synchrotron X-ray beam Some imaging and therapy experiments in the biomedical field
require a wide 2D beam Using the natural divergence of the ThomX beam a sample of a few
lsquo
131
centimeters in diameter located at 10-20 meters from the ThomX interaction region can be fully
illuminated by the a conic beam with no need of optical elements Calculations have been performed in
order to show the potential of using the ThomX source in comparison with a hospital X-ray generator
(fluoroscopy unit type) as well as in comparison with the beam from the biomedical line ID17 at ESRF
Results of these comparisons are given below for each medical experiment we intend to perform with
ThomX
An imaging experiment
For imaging experiments the energy range of the beam has to around 60-90 keV According to Fig 73
the corresponding half-aperture will be 5 mrad leading to a photon flux of 61012
phs When the beam
reaches the sample stand placed on Table 2 at 20 m from the ring interaction region it will have a
diameter of 20 cm This conical beam will then have a cross section of ~300 cm2 and permit
tomographic acquisitions of a full human head using a 2D detector This wide 2D beam is a major
advantage of the ThomX source for biomedical imaging with respect to the laminar synchrotron
radiation beam The dose rate (DR) in Gys can be expressed as
(71) DR = 16 10-13
Ф[phscm2] E[keV] (microenρ)[cm
2g]
where Ф is the photon flux per cm2 E is the mean energy of the beam and (microenρ) is the mass
attenuation coefficient For an X-ray beam of 75 keV (ie the average of our 60-90 keV energy range)
the mass attenuation coefficient microenρ for soft tissues is 0028 cm2g leading to a dose rate of 67 mGys
delivered to a 300 cm2 area The corresponding expected lsquodose area productrsquo (DAP) of the ThomX
source for such imaging experiments is DAP = 20 Gyscm2 For comparison at the ESRF ID17 line a
dose rate of 800 mGys is delivered to an area of 150x2 mm2 corresponding to a DAP of 24 Gyscm
2
As far as conventional sources are concerned typical laboratory devices used in 2D fluoroscopy deliver
a DAP of 06 Gyscm2 [712]
A feasibility experiment will be performed by using a standard human head phantom (see Fig 79)
which will be mounted on a rotating stand that allows a 360deg rotation It will be possible to translate this
stand in three directions (along the X-ray beam axis and in the two other perpendicular directions) to
optimize the sample position in the beam Acquisition will be performed using the 60-90 keV pink
beam before and after the insertion of a high-Z element solution (typically iodine or gadolinium) in a
slot available in the phantom head Several images will be acquired using different concentrations of
high-Z element chosen in the range of concentrations that can be typically reached into a tumor (in
humans the high Z element is injected intravenously) The study will also aim at finding for each high-
Z element the lowest concentration that can be measured with the ThomX imaging system
Figure 79 Typical phantom featuring a human skull from X-ray absorption point of view
lsquo
132
The detector will be placed downstream of the sample A typical detector that may be used is a large
field of view 2D detector equipped with Si or CdTe sensors with ~100-200 microm pixels [713] Finally a
standard computed tomography (CT) algorithm will be used to reconstruct the tomography images and
to calculate the concentrations (in mgml) of the given high Z element in the skull The results will be
compared to previous work done at the ESRF [714]
A radiotherapy experiment
For this experiment the targets will be rats implanted in their right brain hemisphere with rodent brain
tumors (F98 9L) at an appropriate tumor stage (typically at D10 or D14 after implantation) For the
irradiation we need a pink beam with a 10 keV energy bandwidth and a median energy above the K-
edge of the high-Z element to be injected as a radiosensitizer (ie in the 45-80 keV range for both
iodine and gadolinium) The beam size will be a few centimeters in diameter Figure 710 shows the X-
ray beam energy and beam flux generated by a 69 MeV electron beam versus the photon back-
scattering angle X-rays of energy 80 keV within a 10 keV bandwidth are contained in a cone of half-
aperture 27 mrad and their flux is ~31012
phs At the sample location on optical Table 2 at 9 m from
the ring interaction region the beam has a diameter of 5 cm ie a cross section of ~20 cm2
The center of the ratrsquos right hemisphere will be placed on the rotation axis of the irradiation system and
slits will limit the beam transverse extension in order to define a precise field of irradiation Assuming
an X-ray flux (Ф) of 31012
20 phscm2 with an average energy (E) of 80 keV a mass attenuation
coefficient of 0025 cm2g (soft tissue 80 keV X-rays) Eq (71) leads to a dose rate DRe of 48 mGys
close to the tissue region where the X-ray beam penetrates Before reaching the tumor the radiation is
attenuated by the bone (~ 02 cm thickness (microρ)att = 022 cm2g ρ=192 gcm
3) and by the skull healthy
tissues (~ 15 cm thickness (microρ)att = 018 cm2g ρ = 10 gcm
3) Thus the dose rate DRt delivered at
the tumor is DRt = TDRe (where T = exp (-02219202) exp (-0181015) = 070) and is
expected to be 337 mGys The dose will be delivered while the rat undergoes a 360deg rotation The total
dose to be delivered to a tumor ranges between 10 to 20 Gy Therefore according to the expected rate
of 337 mGys the irradiation will take typically from 5 to 10 minutes with the big advantage of not
having to move the rat A dosimetry protocol (such as the one used at ID17 [715]) will be set to check
the homogeneity of the irradiation and to measure the absolute dose with a 2 to 5 uncertainty
Figure 710 X-ray energy (left panel) and X-ray flux (right panel) versus the photon back-scattering
angle for stored electrons of 69 MeV The laser wavelength is 106 microm
For humans and a tumor placed at 8 cm into the skull the transmission coefficient becomes T= 019
Therefore the expected dose rate DRt delivered by the ThomX source at the tumor is 91 mGys One
may compare with the ESRF ID17 beam-line where 800 mGys can be delivered for irradiation on a
lsquo
133
surface of 2x50 mm2 ie with T=019 a dose rate DRt of 015 Gys on 2 mm in height at the tumor
which is equivalent to a rate of 6 mGys to cover 5 cm of height with a scanning rate of 50 mms
ThomX could therefore deliver doses for human skull irradiation larger than those available at
synchrotron radiation facilities and also perform the irradiation without having to move the irradiated
sample Concerning the conventional hospital devices they deliver low doses deposited by X-rays
whose energy spectrum is broad and moreover they are not made to operate continuously for such long
time irradiation
Also the tunability of the ThomX beam will allow testing several high Z drugs containing iodine
gadolinium platinum or gold proven to be promising radiosensitizers [716]
Conformal radiotherapy
A final step in this sequence is performing conformal radiotherapy experiments ie doing both imaging
and therapy at the same time In fact one major issue of radiotherapy is to spare as much as possible
healthy tissues located around the tumor and at its border In this regard it is a key issue to adapt the
field of irradiation to tumor geometry size and positioning by performing first a tomography
Phase contrast imaging
The absorption medical imaging technique is based on the fact that different parts of a sample absorb X-
rays differently But the fact that X-ray absorption by biological material is weak constitutes also the
limitation of this technique To enhance or replace absorption-based imaging refraction diffraction and
interference of X-rays can also be used A better visibility at the edge of different regions of a specimen
can therefore be obtained by using the information contained in the phase of an X-ray wave For this a
spatially coherent X-ray source is required and was available only since the development of modern
synchrotron sources Experimentally the simplest way to visualize phase-contrast imaging is provided
by the propagation-based imaging (PBI) method In this method a spatially quasi-coherent radiation
illuminates an object which gives rise to a spatially varying phase shift in areas with spatial variations
of the refractive index As the radiation propagates beyond the sample parts of the wavefront which
have experienced different deflections on both sides of an edge of the sample produce a serie of fringes
due to the classical Fresnel-edge diffraction phenomena [717]) This gives rise to a characteristic phase
contrast pattern which is then recorded by a 2D detector placed at a proper distance downstream of the
object (see Fig 711 left panel) The actual phase-contrast signal is convolved with the detector point
spread function which has to be sufficiently narrow to avoid the smearing of the interference pattern
Additionally the phase contrast signal can be visualized if and only if the transverse coherence is
sufficient ie if Lcoh = λRs is larger than the details to be imaged (here R is the source-sample distance
and s is the source size) In contrast the requirements in terms of longitudinal coherence
(monochromaticity) are rather weak even a beam with a bandwidth of several keV does not lead to a
significant smearing of the phase contrast signal [717]
Assuming s = 70 microm R = 15 m a sample-detector distance of 5 m and a detector point spread function
of 50 microm the phase contrast signal expected when the ThomX beam illuminates a nylon wire of 300
microm diameter ranges from 10 to 3 when the X-ray energy increases from 20 to 80 keV (see Fig
711 right panel)
lsquo
134
Figure 711 Left panel The set-up for propagation-based phase contrast imaging Right panel The
simulated signal produced by a nylon wire of 300 microm diameter illuminated by the ThomX source
(simulation)
The experiment with ThomX will comprise two parts a planar radiography and a computed
tomography For the radiography three kinds of samples will be used (i) a custom-made phantom
composed of nylon wires of different diameters (ii) the ACR (American College of Radiology)
accredited phantom used in mammography quality (iii) a 4 cm thick formalin-fixed human breast tissue
obtained from a hospital bank The X-ray energy will be chosen in the 20-80 keV range For each
sample and for several energy values images will be acquired at two different sample-detector
distances in the absorption regime for comparison purposes (the detector will be in contact with the
sample) and at 5 m A silicium detector with 50 microm pixels will be used Images will be acquired for
different doses (measured with a standard PTW dosimeter) in order to obtain the signal-to-noise ratio as
a function of the dose These measurements will allow one to qualify the phase contrast imaging set-up
at ThomX
Computed tomography will be applied to (i) a meacutethyl polymeacutethacrylate (PMMA) custom-made
cylindrical phantom containing insets of different compositions and densities (water ethanol paraffin
etc) (ii) a formalin-fixed human breast tissue (5 cm in diameter) Images will be acquired using the
same parameter values as the ones chosen for the radiography Samples will be installed on a rotating
stand positioned on a double cradle that will permit an alignment with the beam and with the detector
Images will be taken during the sample rotation over 360 degrees
The final step will consist in applying standard algorithms to the images in order to calculate the phase
and to extract quantitative information about the samples This data will be compared with equivalent
images that will be acquired at the ESRF
742 Experiments using the central part of the beam
Most specimens from cultural heritage and functional materials are heterogeneous these lsquorealrsquo
materials often exhibit complex hierarchical structures involving various levels of order and
heterogeneity To understand the properties of such materials better one needs at least a description of
their structure at the atomic scale and at the macroscopicmesoscopic scale which takes into account
weak structural signals (relating to disorder minor constituents microstructure hierarchical orderhellip)
lsquo
135
This requires much probing as already demonstrated with synchrotron radiation sources A major goal
of materials science is to link global properties to local atomic features (orientation strain structural
distortion grain size) of some components in relation with neighboring domains and to deduce from
that the macroscopic properties As the functionalities of these materials often stem from the association
of different phases or constituents or of specific inter-relation between them (microstructure) highly
selective structural probes need to be used to characterize them Also in the particular domain of
cultural heritage many specimens require non-destructive methods and probes In order to understand
how they were made and to preserve them from degradation over time their physical and chemical
properties must be determined There is an increasing demand for material probing with complementary
techniques Fluorescence emission techniques excel in revealing the elemental composition of the top-
most layers of samples Diffraction is suited for studying quantitatively the phase composition the
structural details of the constituent phases and the characteristic distribution of sizes and orientations of
crystallites Tomography techniques excel in revealing the 3D mapping of the chemical andor the
density heterogeneities within a sample This is why the cultural heritage community is today strongly
involved in the development of new instruments that may characterize patrimonial artifacts by mapping
not only the chemical composition of a variety of samples but also their physical phases
Experiments described in section 741 require a large 2D beam and exploit the natural divergence of
the ThomX beam In this section diffraction scattering and spectroscopy experiments require a
focalized or a parallel beam and it should also be monochromatic or lsquopinkrsquo The optics will therefore
select the central part of the beam ie its higher energy part Let us give an order of magnitude
assuming a perfect 50 MeV electron beam a collimator of 2 mm diameter located at 2 meter from the
IP will cut the X-ray conic beam at 05 mrad leading to a flux of ~8 1010
phs with an energy of 45 keV
and an energy bandwidth ΔEE on the order of 10-3
(see Fig 73) In practice this bandwidth will be
broadened and will reach a few percent because of the actual e- beam divergence and energy spread For
experiments requiring a narrower energy bandwidth an additional selection will be performed with a
monochromator For instance selecting an energy bandwidth of 10-4
would lead to a flux on the order
of 109 phs
Since the goal of the first series of demonstration experiments is to qualify ThomX as an X-ray source
these experiments will be performed with samples previously characterized with other sources such as
synchrotron radiation sources
Diffraction and scattering
Diffraction and scattering experiments are major ways of performing materials structural and micro-
structural analyses This concerns highly ordered samples as well as powders or ill-ordered samples
Complete scattering analyses are based not only on Bragg diffraction but also on diffuse scattering in
order to look beyond the average structure and to investigate the local structure or short-range order
The crystalline quality of powders depends on the preparation methods Grain sizes vary from about 10
nm to several hundreds of microm In the case of nano-materials Bragg peaks are strongly broadened and
X-ray diffraction performed with standard laboratory sources does not provide suitable data for
structural investigations As illustrated by Fig 712 laboratory X-ray sources do not allow the
identification of minor phases in a poorly crystallized powder Figure 713 shows an example of a
diffraction pattern obtained with an excellent resolution The sample used for this analysis is an
Egyptian cosmetic powder dated to 2000-1200 BC The patterns were obtained with the ESRF
synchrotron source (bottom panel) and with a classical laboratory source (top panel)
lsquo
136
Figure 712 Powder X-ray diffraction patterns obtained by using a laboratory X-ray source Top panel
Poorly crystallized Prussian blue sample Bottom panel Prussian blue crystals From [718]
Figure 713 Powder X-ray diffraction patterns obtained by using a laboratory X-ray source (top panel)
and the ESRF synchrotron source (bottom panel) Arrays indicate minor phases From [719]
We intend to use the ThomX source to record diffraction patterns and scattering plots in order to
characterize short-range ordering and identify minor phases in such heterogeneous samples
For the first demonstration diffractionscattering experiments we will study ill-ordered Prussian blue
and white lead samples Pigments used for paintings or frescos have various compositions Since their
usage followed a definite chronology such pigments can sign an era Pigment characterization can also
help the undertaking of a restoration while a thorough knowledge of pigment synthesis through
centuries can help to apprehend the degradation mechanisms better
lsquo
137
For ordered samples diffraction will be performed with a single crystal by using a monochromatic
beam but also by using a lsquopink beamrsquo as done in the Laue method For ill-ordered samples the whole
scattering pattern will be recorded and analyzed with the Pair Distribution Function (PDF) method This
analysis technique (often called the ldquototal scattering methodrdquo) can be used alone or associated with
diffraction For experiments using the lsquopink beamrsquo Laue method the beam will be focused directly on
the sample (by using convergent refractive lenses (CRLs) or a capillary optic system as mentioned in
Section 731) To determine powder crystalline parameters diffraction analyses require the
monochromatization of the beam in order to reach ΔEE ~ 10-4
In the optic hutch (optical Table 1) the
diverging beam will first be made parallel by the CRLs or the capillary optic method and then
monochromatized in the experimental hutch (optical Table 2) by a monochromator The set-up for these
measurements needs a goniometer to align the sample and a low noise 2D-detector sensitive to high
energy photons
Fluorescent spectroscopy
X-ray fluorescence spectroscopy experiments allow one to determine the chemical composition of
materials and the mass concentrations of elements Indeed depending on the atomic number of a given
chemical element energies of the fluorescence lines emitted after an X-ray excitation are a signature of
the element An illustration of this approach is presented in Figure 714 where the fluorescence
tomographic images of a Globorotalia inflata specimen are shown and compared with a classical
radiography image
Figure 714 Fluorescence tomographic imaging (a) Set-up (b) Radiography (c) Fluorescence
spectrum measured at a given angleposition of the sample (d) 3D tomography reconstruction From
[720]
We plan to carry out fluorescence experiments since the energy of the ThomX beam can reach the
excitation energy of most chemical elements usually involved in material science for instance the
critical excitation energy of lead regarded as a heavy element is 88 keV) For this experiment the
excellent energy resolution of cooled Ge detectors will be an asset to detect the fluorescence light (the
detectors will be oriented at 90o with respect to the incident beam)
lsquo
138
K-edge and XANES spectroscopy
In K-edge experiments two images of a given element present in some sample are performed one at an X-ray
energy just below the K edge of the chosen element and the other at an energy just above Then they are
subtracted (see Fig 715 left panel) The K-edge imaging technique is used in particular in the medical and
the cultural heritage domains In XANES (X-ray Absorption Near Edge Structure) the absorption by a
material is measured over an energy range of about 100 eV on both sides of the absorption edge This
technique provides information about the local structure prevailing close to a particular chemical element (see
Fig 715 right panel)
Figure 715 Left panel K-edge subtraction imaging Right panel Principle of the XANES method
The first XANES spectroscopy experiments with ThomX could exploit the K-edge of heavy elements
such as W Pt or Pb By first selecting the central part of the ThomX beam with an energy spread of a
few 10-2
a monochromator could then provide a beam with an energy spread of 10-4
Samples will be
aligned by using a goniometer and the transmitted intensity will be collected by a low noise 2D-
detector
Combined diffractionabsorptionfluorescence tomography
A major limitation of diffraction methods comes from the fact that only average properties can be
obtained while most lsquorealrsquo materials are heterogeneous and complex Thus structural studies of such
materials are difficult to carry out A new tool for structure mapping of materials by means of X-ray
diffractionabsorptionfluorescence combined with tomography allows one to perform non-destructive
3D structure selective mappings of heterogeneous samples This way one may identify minor phases
and determine crystallite orientation and grain size Furthermore the selectivity of the method allows
one to separate and locate the origin of the diffuse or diffracted patterns to identify amorphous phases
In archaeology such complementary micro-structural analyses could shed light on the mechanical
history of artifacts
A recent experiment carried out with a micrometer beam at synchrotron radiation sources have
established the validity of such a local probe by analyzing a mixture of chalcedony and iron pigments
containing haematite [721] This experiment illustrated in Fig 716 consists in (i) performing a
structure tomography mapping directly from the diffraction data (top panel) (ii) obtaining selective
structure patterns of unknown minor phases from these maps (bottom panel) This study was realized by
the Neacuteel institute and the C2RMF and was one of the first diffraction tomography experiments
performed with a micro-beam
With the ThomX source we will use poorly crystallized samples and combine in the same experiment
and simultaneously tomography diffraction with fluorescence and absorption measurements Diffraction
lsquo
139
tomography is beam-time consuming it will be performed to demonstrate its feasibility at ThomX The
experimental set-up will be similar to the one used for multimodal analyses (cf the top panel of Fig
716) The sample will rotate and this rotation will be combined with a translation to provide a 2Θ scan
of 2D diffraction patterns obtained with a 2D detector sensitive to high energy photons The diffracted
2D fluorescence patterns will be recorded with our Ge detector while the sample absorption will be
recorded with our calibrated Si diode
Figure 716 Top panel Experimental set-up for multimodal spectroscopydiffractiontomography
analysis Insets Images of the chalcedonyndashFe mixed-pigment sample are reconstructed by combining
the various signals X-ray absorption (bottom) Fe X-ray fluorescence (top left) and chalcedony
diffraction (top right) Bottom left panel Reconstructed diffraction image based on the whole
diffraction diagram (black) compared with the calculated pattern for pure chalcedony (red) Bottom
right panel Reconstructed image that only uses the iron pigment scattering contribution (black)
compared with the calculated pattern for hematite and siderite grains (red) From [721]
75 Summary
As shown in this chapter two types of beams will be available at the ThomX compact X-ray source
(i) wide beams well adapted for imaging (conventional or phase contrast imaging) and therapy in the
biomedical field and (ii) almost monochromatic beams that can be either focused or made parallel for
diffraction and spectroscopy experiments in the field of materials science
The X-ray line is designed
- to ensure a continuous monitoring of the X-ray beam
- to measure the beam brightness and its transverse coherence
- to demonstrate the potential of this new type of compact source and identify the benefits it can
bring to the currently available X-ray sources
lsquo
140
Chapter 8 CONTROL SYSTEM
Introduction
The ThomX control system combines those hardware and software elements involved in the
management of all the facility components But more than a system dedicated to carrying out operating
processes (processes launched from the X-ray beam control room in order to undertake some specific
task such as a performing an imaging experiment or a material chemical analysis) it must also be an
advanced tool for various studies (data processing new equipment integration operating mode
evolution) and scientific experiments which will be performed with the X-ray beam line Hence the
control system has to be a versatile tool easily scalable by various users (scientists operators IT
technicians)
In the following a lsquoSub-systemrsquo is a physically connected set of hardware items sharing a global logical
function The ThomX sub-systems are as follows the linac (LINAC) the transfer line (TL) the ring
cell 1 (RI-C1) the ring celle 2 (RI-C2) the optical cavity (OC) the extraction line (EL) the section
of the X-ray line located in the Igloo (XLI) and the section of the X-ray line located in the X-ray hall
(XLH) A lsquoDomainrsquo is a set of hardware items related by a common purpose The ThomX domains are
as follows alignmentmetrology (AM) control-command (CC) diagnostics (DG) infrastruture (IF)
magnetic elements (ME)n optics (OP) pulsed elements (PE) power supplies (PS) radio-frequency
(RF) safety (SA) synchronization (SY) and vacuum (VA)
81 General principles
Some general principles drive the design of such a control system In broad terms the following criteria
will serve as guidelines
- Since monitoring is a major role of a control system comprehensive monitoring interfaces must be
provided to all users Moreover the monitoring system must take into account sub-systems specificities
dedicated GUIs must be designed for every need for instance in synoptic form to give a global system
status overview or as charts to show the time evolution of some specific parameters of a particular
device (eg pressure readings on TPG300)
- Operating processes management a control system also aims at the prioritization and sequencing
of processes It has to provide high-level functions in order to remotely launch complex operations (eg
machine startstop interlockshellip) Computing and data analysis tools which could launch such particular
procedures will be integrated or developed to maximize the automation of the machine
- Default management the facility control system must be able to manage operation defaults It will
be necessary to design graphical tools to quickly identify defaults when they occur Color display of
warnings and errors according to a criticality-based convention will be implemented in addition to the
recording of running process logs
- Safety for obvious reasons the ThomX radioprotection and safety system must standalone It should
not use any data generated by the control system Nevertheless each end-user must be able to make
sure that the ThomX facility does not present any danger whenever the accelerator is running This
information must be available in every control room through a continuous display of all the safety
equipment status (radioprotection sensors door openingshellip) A full historical record of safety-related
lsquo
141
events will be made available for analysis and a procedure of detailed acknowledgement (operatorrsquos
name action to close an incident) will be defined in order to guarantee safe operations
- Ease of use The ThomX control system is intended for various usages therefore it will concern
various users including operators who are not familiar with software development Hence it is crucial
to make it modular through relevant to sub-systems GUIs with various visualization interfaces control
buttons graphs sub-system overviews in synopsis or detailed display of subset status The definition of
new interfaces will be possible through visual development frameworks to fit new needs
82 Technical considerations
In order to guarantee a proper accelerator operation to make its use as easy as possible and to limit
costs some technical points have to be taken into account
- Distributed architecture an accelerator is a complex structure which results from various designs in
different fields The hardware part of the control system relies on dedicated IT devices (industrial PCs
programmable logic controllers) whose role is to drive sub-systems Each of these nodes will be fully
and remotely controlled by the system software from the control rooms (or from any authorized user
terminal in the ThomX network) independently of possible other issues All of these components must
communicate with each other when required by the operation needs Moreover the supervision part of
the control system must be available in different places (main control room X-ray line control room
perhaps others) The chosen system must meet these requirements and must run on several terminal
types
- Standardization of the hardware items and of the software codes is necessary in order to simplify
their integration in the control system (eg same brandmodel for all the stepper motors) Then it will be
possible to mutualize high-level controlling tools whether pre-installed or homemade Additional
benefits of this approach concern the overall maintenance the deployment of new releases and the
replacement of hardware parts
- Reliability The service continuity of the control system must be guaranteed A spare strategy must be
set up to make sure that the replacement of defective parts will be done on short notice In particular
storage memory redundancy will guarantee that none of the machine parameters or of the collected data
will be lost Periodic backup of main software systems (device databaseshellip) will be performed right
from the start Uninterruptible power supplies are foreseen to protect major elements of the control
system from electrical failures
- Upgradability Special attention will be paid to communication features when choosing hardware
devices Ethernet is now one of the major protocols and must be preferred when possible For the PLC
driven devices the fieldbus Profibus [81] is recommended since one single master interface can
manage more than 100 slave devices without data losses This product is also valuable as far as cabling
cost is concerned
- Safety Since several users may wish to perform various machine-related tasks (integration
maintenance measurementshellip) a policy must be established to control the access to the ThomX
network and its control system The management of these access permissions will be provided It will
take into account the role and the field of expertise of each project member
lsquo
142
83 The TANGO control system software
Tango [82] is the software chosen to be the backbone of the ThomX control system It is an open-
source software initially developed at the ESRF It is now used in several Institutes to control huge
accelerator facilities like the SOLEIL synchrotron In fact the SOLEIL staff is playing a major role in
maintaining and developing new Tango features
831 Software bus
An accelerator equipment is usually highly heterogeneous Thus its control system must enable various
devices to communicate with each other even if they use different protocols or communication
standards like Ethernet RS232 USB PCI DAQ Cardshellip
The first asset of Tango is the communication standardization that it provides between any type of
devices Tango implements a CORBA-based communication protocol so that a common network is
virtually created for the whole hardware If needed the parameters of a given device are thus available
to every other element of the control system This is achieved by adding an extra Tango-compliant
software layer above the hardware low-level libraries This layer is specific to a device brand and model
and is a simple executable file which can be instantiated as many times as required by the number of
devices This key element of the Tango system is called a lsquoDevice serverrsquo
832 A control system for accelerators
For basic reasons the Tango core and many of the tools developed around this software bus are open
source tools It is therefore an accessible system fully customizable for specific ThomX needs and a
system which can be maintained Since Tango is Linux and Windows compliant issues originating in
driver availability for the Linux OS can be avoided while Tango enables the use of scientific
applications which are often developed for Linux
Furthermore Tango is developed and maintained by a community which involves several accelerator
Institutes Even if Tango is not an accelerator-dedicated tool a number of developments have been
carried out or are being carried out that deal with devices currently used at such facilities Thus a large
Tango library of device servers is presently available to any new user
Finally more than a simple tool which provides a network access to a set of devices Tango is also a
framework that provides high level operational services such as configuration tool logging archiving
services and alarms permission management For developers specific tools are provided for the
generation of codes that fit in the common structure required by all device servers Other tools allow
easy GUI development As a result the integration of new devices or the upgrade of control and
supervision windows do not need in-depth coding As far as the compatibility with commercial software
is concerned dedicated bindings can be freely used eg applications developped for Matlab or
Labview can be used to perform a variety of tasks involving the ThomX devices
lsquo
143
84 Control system architecture
841 Equipment to control
On a complex machine like ThomX a wide range of devices and control signals must be taken into
account motors cameras scopes diodes or valves will deal with analog and digital inputsoutputs
complex signals transmitted through GPIB USB Ethernet or profibus interfaces
The inventory of devices to be controlled is presented in the following table Safety equipment is not
mentioned as it is a fully standalone sub-system
LINAC TL Ring OC XL
Power Supplies
Coils 2
Steerers 2 4 24 2
Dipoles 5 4 8 3
Dipole chicane 1
Quadrupoles 7 24 3
Sextupoles 12
Pulsed elements
Septum 1
Kickers 2
Synchronization
3GHz synthesizer 1
500 MHz synthesizer 1
Phase shifter 5
Programmable delays 2
Counter 1
Vacuum
Ion pump 8 4 17 4
Gauge 8 8 22 8 1
Valve 4 4 4 2
Primary pump 1
Diagnostics
BPM 1 4 12
Low frequency Signal
generator 2
ICT 1 2
Stepper motors 1 3
Camera CCD 2 3 1
RF
Klystron coil 1
Modulator 1
Amplifier 1
Canon 1
Feedback 2
Source laser
Photodiode 1
Camera CCD 1
lsquo
144
Motor 1
Mirror 2
Laser ndash FP cavity
Camera 4
CCTV Camera 10
Motor 16
Thermal control unit 2
Laser equipment 3
Scopes 3
FPGA 1
X‐ray Line characterisation
Xpad camera 1
Si PIN diode 3
Ge diode 1
Motor 13
Stepper motor 25
Others
Translation table 1 1
Slot 1
Table 81 Devices to control (TL=Transfer line OC = Optical cavity and XL = X-ray line)
842 Inputsoutputs
All devices listed above communicate with each other by digital inputsoutputs (DI DO) ie one bit
signals analog inputsoutputs (AI AO) or more complex signals as n-bit digital signals (n = 8 12 or
16) Several communication protocols mentioned in sect832 above will be used
They are detailed in Table 82
PLC InputsOutputs Bus Ethernet
connections
DAQ (ADCampDAC)
Vacuum 26 DI 13 DO 26 profibus
Diagnostics 2 gpib 30 3 x ADC 12 bits 100 kHz
Linac 2 DI 2 DO 5 AI 3 ADC 8 bits 1 GHz
Magnets 35 profibus
Feedbacks RF (x2) 10 DI 10 DO 10 AI
10 AO
USB
Ampli RF 64 DI 8 DO 32 AI 16
AO
RS232 2 4 channels ADC 12 bits
1MHz
Modulator RF 23AI 4 AO 69 DI 5 DI 5 DO
8 channels ADC 8 bits
Pulsed Elements (x3) 12 DI 3DO profibus ADC 12 bits
Laser ndash FP cavity RS232 22
Timing 2 gpib 14 15 channels DAC 16 bits
X Line 26 DI 26 DO 7
Table 82 Equipment IO to control (not complete at the time of writing)
lsquo
145
843 Front-end machine architecture PLCs and industrial PCs
The front-end architecture will comprise equipments individually dedicated to sub-systems These
machines are controlling nodes which will be located in sub-system server racks as close as possible to
the equipment to be controlled (when required)
Two kinds of front-end machines will be used
- PLCs programmable logic controllers are automation OS-based machines able to manage digital and
analog IO and to control processes carried out by devices connected to a field bus (eg Profibus) or
specialized devices They are reliable devices designed to operate in an industrial environment A
proprietary operating system runs on these machines which are powerful tools (eg automats) to
implement logic appropriate to experimentation Interconnection with the other Tango network
equipment is made through an Ethernet adapter interface
- Compact PCIs (industrial PCs) these crates can embed complex operating systems like a Linux
distribution or some Microsoft Windows Operating Systems hard drives Ethernet connections and
USB adapters Such crates are based on a PCI bus backplane to interconnect specific cards (DAC
ADC counters optoelectronics convertershellip) cPCI is a standard widely used in an industrial
environment it is compatible with numerous card suppliers These front-end machines will also allow
the running of Tango processes or specific computing tasks locally installed in a sub-system
The ThomX distributed control system will be build on these controlling nodes This structure will
allow the bandwidth optimization of the network main links and will provide cabling simplifications
The control equipment may be placed in the accelerator hall as well as in the technical rooms where
most of the electronics that must be controlled will be clustered in 19rsquorsquo standard server racks
‐ Some devices need not be controlled through these front-end machines They will be directly
connected to the control network (eg ethernet cameras or electronics for motor control) Three
ways to interconnect such devices are shown in Fig 81
‐
Figure 81 Control architecture for one sub-system
lsquo
146
These acquisition and controlling units can be Profibus PLC or a cPCI crate Each driven element will
be controlled through one of these acquisition and controlling unit
The overall control system will be composed of more than 25 front-end machines as described in Table
83
PLC cPCI Crate
Linac (wout vacuum diags modulator
magnets) 2 1
Vacuum 2
Diagnostics 2
Feedbacks 2 2
RF amplifiercavity 1 1
RF modulator 1 1
Pulsed elements 3 3
Fabry-Peacuterot cavity 1 1
X-ray line 1
Synchronization 1
Magnets 2
Total 13 14
Table 83 List of front-end machines (not complete at the time of writing)
85 IT infrastructure
- Network The ThomX local network will be organized around an IP backbone It will be based on the
Ethernet TCPIP standard in order to link user terminals front-end machines specialized servers etc
Because of the large number of devices and the requirements bearing on the bandwidth (eg for image
storage) 1 Gbitss compliant equipment will be chosen (LAN adapters switcheshellip) Depending on the
actual number of cameras an image-dedicated network could be added On the cabling side shielded
copper cables will be used as interconnection wires to limit electromagnetic interferences as much as
possible
- Data storage During the ThomX commissioning or for the operation monitoring or for the X-ray
experiments it will be crucial to store a large amount of data It is foreseen to implement the three
storage processes provided by Tango historical (long term storage at a slow rate) temporary (short
term storage at a fast rate) and snapshot (storage of parameters on operator demand or event triggered)
It will also be possible to meet the X-ray line needs for the storage of large size images during short
periods (at most for several days) This specific storage will not be necessarily on local servers it could
be convenient to move these data and store them on servers close to the final users possibly outside the
ThomX network
Two storage-dedicated servers will be installed for analyses replay of accelerator operation scenarios
and for some specific needs of the X-ray line These storage units will be specific machines which
prevent data outages or losses by redundant power supplies stored data redundancy (RAID) hard disk
hot swap etc The first of these two storage units will be a 2 To server equipped with a disk array
dedicated to the accelerator data management The second one will have a 10 To capacity it will be
available for the large size images of the X-ray line
lsquo
147
- Equipment of the control rooms Users will work in control rooms on system administration
accelerator control development and integration (device servers GUIs high level computing software
safety analysis tasks) Administration and end-user terminals should be provided in sufficient number
(see below)
The main control room will be equipped with
- 5 PCs 1 for the Tango core system 2 for operations 1 for development 1 for safety (dedicated to
visualization storage and analysis of safety related events)
- 1 server to run high-level applications and device servers
- 2 screens one displaying the ThomX status with synopses and the other one for the safety
supervision
- camera monitors for some direct controls
- 1 UPS (uninterruptible power supply)
The X-ray line control room equipment will consist of
- 2 PCs for the operation equipment and for macro development
- 1 server to run high-level applications and device servers
- 2 screens dedicated to the monitoring of the machine status to safety synopses and to the display of
safety related events
- Software needs
- Both the Linux and Windows OS must be installed in the control room PCs in order to run Tango and
many other operating processes (see the Introduction above) for development purposes and to use
configuration software Together with the Tango core and specific tools high-level software will be
installed to meet usersrsquo needs Labview and Matlab for science applications SPEC to control the
motors and for the analysis of the X-ray line data
- Data bases will run on the MySQL open-source database managing system
- An open-source http server Apache will allow the implementation of WEB interfaces for the control
system authorized users this way these users will be able to remotely visualize experiment data or
equipment status
- Finally all end-user stations will provide standard office tools
The following Figure gives an overview of the control room equipment
lsquo
148
Figure 82 Schematics of the equipment foreseen for the main control room and the X-ray line control
room
Overall the ThomX control system ndash designed to ensure harmonization and standardization and based
on open source tools ndash should be cost effective The investment cost is estimated around 200 keuro
including IT small equipment not described in this document cables a gateway to provide internet
access extra screens to display the ThomX status in the laboratory etc
lsquo
149
Chapter 9 SAFETY AND RADIOPROTECTION
91 Introduction
The ThomX facility will be sited in an existing building called lsquoIgloorsquo where a source of hard X-rays was
operated until 2003 by the Laboratoire pour lrsquoUtilisation du Rayonnement Synchrotron The ThomX linear
accelerator and storage ring will be built inside this building while the facility control room and the one of the
X-ray line will be located respectively in the D1 and D2 experimental halls The D4 hall will be devoted to
experimental tests
The ThomX operation will produce high-energy electrons and other particles such as X-rays and neutrons
which generate ionizing radiations through their interactions with matter An adequate protection against these
radiations must be ensured for people working at the facility The safety logic adopted is the so-called
lsquopositive safetyrsquo which means that any system dysfunction will immediately cause a complete stop of the
accelerator Furthermore redundant safety devices will be implemented allowing various actions that will
guarantee the staff safety such as
- Shining the beam into an area shall be submitted to prior people evacuation and to the interlocking of this
arearsquos doorways
- Access to a sensitive area shall be submitted to the prior stop of the upstream beam by using at least two
different means independent of each other
Complementary to these two procedures safety measures will be applied involving hard-wired logic safety
PLCs shielding beam dumps shutters personnel exclusion interlocks radiation monitoring of the
experimentation areas staff dosimeters and radiation hazard training of the personnel In case of dysfunction
radiological monitoring hard-wired logic or safety PLCs will act through interlocks that will break
connections to the mains voltage (Pre-Amplifier modulator or electron gun depending on the situation) in
order to stop the accelerator
92 PLC safety
The safety hard-wired logic works in association with an automaton PLC system via two completely separate
circuits A Safety Programmable Logic Controller (PLC) is a special purpose machine which is used to
provide a critical control and safety applications to automaton users This controller is an integral part of
lsquosafety instrumented systemsrsquo (SIS) which are used to detect situations involving potentially dangerous
processes When such a situation occurs the SIS is programmed to bring the process to a safe state
automatically
A safety PLC is specifically designed to accomplish two important tasks (ensured by a diversitary redundant
architrecture)
- lsquodo not fail but if this cannot be avoidedrsquo
- lsquofail only in a predictable safe wayrsquo
Hard-wired logic is a safety circuit fully independent of automatons It takes in charge in particular the
access door interlocking feedback contacts which provide a large number of checks bearing on the magnetic
field the high voltage the pre-amplifier status etc and the running control panel
The PLC safety level must be as high as the one of a positive safety hard-wired logic It manages access doors
emergency stop buttons safety rounds and reports issue authorizations to operate from a command-control
system
lsquo
150
93 Radiation monitoring
Concerning the radiation protection the ThomX facility design satisfies the following principles
- No one will receive a dose of radiation in excess of the regulatory dose limits as a result of normal
operation
- Professionnal occupation exposures will be kept As Low As Reasonably Achievable (ALARA)
- All practical steps will be taken to prevent accidents with radiological consequences and to minimize the
radiological consequences of any accident
Arrangements will be made for the monitoring of working areas such as the Igloo the bunker and the Optical
hutch For this purpose several monitoring modes will be available such as thermo-luminescent dosimeters
measurements made by the qualified person in charge of radiation protection (Competent Radiological
Protection Person) and a modular system for continuous monitoring A continuous monitoring of gamma rays
and neutrons will provide radiation dose rates and will manage alarms its measured values will be transmitted
to a central data acquisition unit Placed in the Compton room a special treatment unit will also be
implemented that will provide a local display of the dose rate signage and will manage alarms
94 Radiation protection zoning
The D1 D2 and D4 halls are not classified lsquosupervised arearsquo therefore their access is free to people who
satisfy access requirements In contrast the Igloo hall is classified lsquosupervised arearsquo
The D1 hall contains the X-ray line and technical equipment
- The X-ray line is not a classified area with the exception of the Optical hutch
- The Optical hutch is alternately classified lsquoprohibited arearsquo and lsquonot classifiedrsquo area depending on the
permission conditions issued by functional elements important to safety (shutter position permanent
radiological level)
The D2 hall contains the ThomX control room and a room which contains the laser equipment
The D4 hall is devoted to tests and mechanical assembly
The Igloo hall contains the electron accelerator located inside a bunker The Igloo is classified lsquosupervised
arearsquo therefore only workers subject to individual dosimetry are admitted However in exceptional
circumstances public access might be authorized as long as regulations and procedures are respected
The Igloo bunker on the one hand and on the other hand the basement area devoted to the facility are
alternately lsquoprohibited arearsquo during the accelerator operation and respectively lsquosupervised arearsquo and lsquonot
classified arearsquo when the facility operation has stopped
The accelerator energy is high enough to expect nuclear reactions of photons and neutrons with matter
leading to the activation of structural materials This is why it is particularly important to use appropriate low
activation structural materials or structural materials that are activated for short periods of time only
Furthermore one must ensure a nuclear waste management which will require a specific room to store and
monitor activated materials This room should be a lsquosupervised arearsquo located in the Igloo
95 Access control
951 General statement
It is a matter of absolute necessity to have appropriate procedures and devices in place to identify and screen
all persons entering the ThomX facility Consequently the LAL access control should be extended to the D1
D2 and D4 halls and to the Igloo
lsquo
151
952 Particular requirements concerning the lsquorestricted areasrsquo
The validation of door contacts as well as the validation of the position of protective shields and real-time
radiation measurement are prerequisites to access the restricted areas (ie Optical hutch and the accelerator
area) As it applies to all functional elements important to safety signage and door statements (openedclosed
lockedunlocked) from classified areas must be transmitted to the control room Local indications are
displayed alternately with the status messages The measured values of the dose rate and alarm symbols are
shown permanently
953 Access to the accelerator area and to the Igloo basement
The accelerator and the storage ring will be located in a concrete enclosure that will provide an efficient
shielding to protect people against ionizing radiations inside the igloo in line with regulations Prior to the
validation the floor load-bearing capacity must be checked by experts
The concrete containment structure will have a removable roof made of staggered concrete slabs Civil
engineering will be needed for the anchorage and supports to be used to adapt the bunker concrete blockds to
the igloo circular wall which will be part of the enclosure and to make sure that the roof of the whole
lsquoprohibited arearsquo provides a complete shielding Since doors will be made of standard steel the bunker access
will be indirect ie through (short) mazes to ensure protection against ionizing radiations
Access to the bunker and to the basement area which is part of the ThomX facility will be determined by the
equipment that guarantees peoplersquos safety in accordance with the facility operation As mentioned above
these devices ensure redundancy by using wired logic safety PLCs and also special safety door locks which
are simple but effective devices A mechanical system called lsquotrapped key lockrsquo will be implemented the key
is trapped in the lock when the door is open and it can be recovered only once the door is properly locked
Turning on the beam in lsquoprohibited areasrsquo is allowed only if the safety systems have been unlocked which
implies that the collection of access keys is complete as will be checked by the control board To meet the
safety requirements new access doors must be installed and some existing doors must be modified
954 Access to the X-ray line and to the Compton room
The D1 hall will not be a lsquoclassified arearsquo and will be open to authorized people The X-ray line sections will
be in open access during normal operation with the exception of the Optical hutch To meet the radiation
protection requirements the Optical hutch partition and its roof will be made of thick metal sheets A thick
lead shield will be installed at the end of the X-ray beam axis
The Optical hutch will be alternately classified lsquoprohibited arearsquo and lsquono classifiedrsquo area (under the worst-case
conditions it might become a lsquoregulated arearsquo) depending on permission conditions issued by safety functional
items External and internal light-signaling warning will indicate the zoning status Thus access to the Optical
hutch is firstly subordinated to the removable shutter position and secondly to the local permanent
radiological measurement values The shutter is dimensioned to protect people from high radiation rates
effectively A fail-safe closer device triggers the shutter closing automatically in case of electrical power
failure too (compressed air power supply) When a door is open the shutter command is prohibited
A local radiation detector located in the Compton room will check he radiation exposure and the dose levels
down to as low as reasonably achievable values Measured high dose rates will automatically shut down the
accelerator and activate the evacuation alarm The validation of these safety devices is mandatory to grant
access permission
lsquo
152
96 Beam dumps
The ThomX facility will comprise tow beam dumps made of concrete that will contain high energy particles
They are very important to ensure an adequate protection of people from the ionizing radiations that would be
produced if they were not used One will be aligned with the linac axis and the other one will equip the
extraction area These shields will be installed as close as possible to the radiation source to absorb ndash or at
least to moderate ndash as much as possible of the produced radiations Their size and thickness will be
sufficiently large to reduce efficiently all secondary radiation (X-rays and neutrons) produced by the stopping
of the e- beam They will be made of a lined lead shield designed to moderate back-scattered radiation placed
in a cavity made of concrete blocks or high-density polyethylene Ne peut-on pas ecirctre plus preacutecis donner des
nombres sur les dimensions et preacuteciser les mateacuteriaux utiliseacutes avec une figure
97 Interlocks
Depending on the dysfunction origin the safety system will operate and break command circuits such as the
pre-amplifier trigger the modulator high voltage the electron gun trigger or its high voltage Feedback
information from the control systems may be gradually requested according to the anomaly
98 Accelerator emergency shutdown
Accelerator Emergency Shutdown (AES) are signal and alarm devices located in specific areas A sufficient
number of AES should be installed in prohibited zones namely the accelerator area the bunker basement and
the Optical hutch For the users to spot them easily AES are devices equipped with flashing lights which are
turned on whenever the local zone is closed
If people are still present in a dangerous zone when a machine run is about to start (ie when the access is
going to be forbidden after a short while) they should signal their presence by pushing an AES This action
prevents the accelerator operation The system reactivation must follow a specific procedure
99 Control console
Placed in the control room the control console collects all access keys The accelerator may be turned on if
and only if all keys are inserted in that console
The safety circuit validation (PLC safety hard wired logic) allows one to launch the accelerator running
procedure and triggers audible warning devices for 30 seconds before operation becomes possible
910 Evacuation of dangerous areas
A specific procedure must be followed for the evacuation of the bunker and its basement Operators should be
trained in methodical evacuations with the validation of all checkpoints (roundsman function) Non
compliance with the procedure will lead to the non validation of the facility operation
A sufficient number of roundsman functions should be located in the accelerator area Also an exit path
should be marked leading to those areas that provide a possibility to regress
911 Audio system and horns
Loud-speakers and horns will be installed in the Igloo basement (in the so-called lsquorotundarsquo) and pits and
inside the bunker The sound system must be of good quality speeches and broadcast messages must be
intelligible Horns must be loud enough 80 dBA at least
lsquo
153
912 Safety notices
Local notices Posters should be put up in lsquosupervisedrsquo and lsquocontrolled areasrsquo to remind people of the
regulation requirements Such posts maintain the workerrsquos awareness whenever they are in a radiological
environment Access doors to zones with multiple classifications should be equipped with highly visible
luminous warning panels Also measured radiation levels and messages could be flashed regularly Regarding
the radiation measured level in the Optical hutch an alarm symbol must be permanently displayed that
informs of this level
Signal boards at zone entrances and within the zones will specify the real-time safety conditions display
warnings (such as presence of live voltage ongoing operation mode etc) and the access permissions (such as
open access prohibited zone etc)
913 Radiological monitoring
The control of the radiological environment will comply with the ALARA principle It will be performed by
- using bimonthly exposure dosimeters based on radio-photo-luminescence (RPL) and neutron sensitive
dosimeters
- PCR controls with live measurements
- a centralized modular system which will continuously monitor exposures feeding a data acquisition system
that can handle up to 10 sensors
This centralized system will perform dose rate and integrated dose measurements manage alarms test
measurement channels check settings and calibrations It will allow a special device placed in the Optical
hutch to display in real time local safety information
The ALARA principle is a commitment to keep radiation exposures and doses at a level as low as reasonably
achievable (social and economic factors being taken into account)
lsquo
154
Chapter 10 MANAGEMENT OF THE THOMX PROJECT
Introduction
The management of the ThomX project will have a simple structure that takes into account the three
project phases 1) The construction 2) The exploitation 3) The technology transfer Three boards will
govern the ThomX activity covering all project aspects the machine design the purchasing and the
assembly the commissioning the usersrsquo access the communication and the relationships with the
industries This management board will comprise
1) A steering committee that will supervise the project activity and define the collaboration scientific
policy
2) A scientific committee composed of the scientific coordinators of the collaboration partners It will
assess the usersrsquo projects and prioritize the beam-time allocation This committee must also define the
scientific policy of the machine exploitation and RampD of the ThomX project
3) A partnership and valorisation office This body will evaluate the cultural and the economic
potential of the ThomX innovations It will stimulate transfer initiatives and check their outcome It will
work in close contact with the steering and the scientific committees
The composition of these three bodies is given in Fig 101
Figure 101 The ThomX project committees
In parallel the communication group (see Table 101) will be in charge of all activity concerning
communication and reach-out in the ThomX project It will organise events realise the publications
relating to the project status and progressrsquo and coordinate the official announcements of the
collaboration
LAL Orsay
Achille Stocchi
CELIA Bordeaux
Philippe Balcou
UMPC
Philippe Walter
Institut Neel
Alain Schuhl
INSERM Grenoble
Franccedilois Esteve
Synchrotron SOLEIL
Amor Nadji
ESRF
Jean Susini
THALES
Yves Trinquet
Alessandro Variola
LAL Orsay
Alberto Bravin
ESRF
Geraldine Le Duc
ESRF
Jean-Louis Hazemann
Institut Neel
Philippe Walter
UMPC
Alexandre Loulergue
Synchrotron SOLEIL
Francois Esteve
INSERM Grenoble
Eric Cormier
CELIA Bordeaux
Alessandro Variola
LAL Orsay
Pierre Bordet
Institut Neel
Philippe Deblay
Synchrotron SOLEIL
Yves Trinquet
THALES
Annabelle Alves
CNRS DR4
Edward Mitchell
ESRF
Fabian Zomer
LAL Orsay
lsquo
155
The project management is organized according to a matrix structure The ThomX facility has been
divided in five sub-systems Linac Ring and transfer lines X-ray line Laser and Fabry-Peacuterot cavity
There is a coordinator responsible for the design installation and operation of each sub-system All
these sub-systems involve various technologies which have been grouped in expertise domains
Mechanics and integration RF systems Magnets Diagnostics Power supplies and Pulsed elements
Vacuum Synchronisation Controls Beam Physics Alignment Cabling Nomenclature and
Infrastructures There is a coordinator for each one of these domains These coordinators will be in
charge of the activity harmonization within each technical field and of the maintenance during the
facility exploitation (or ldquoscientific phaserdquo)
The project leader his deputy and a project engineer are responsible for the project overall
coordination Fig 102 describes the project structure
Figure 102 The project structure
Project and Quality Engineer Technical
coordinator
AVermegraves Francois Wicek
LAL Orsay
Deputy Project leader
A Loulergue
Synchrotron SOLEIL
Project Leader
A Variola
LAL Orsay
Systems
LINAC RRouxLAL Orsay
Ring and Transfer Lines A Loulergue Synchrotron SOLEIL
X line M Jacquet LAL Orsay
Laser E Cormier CELIA Bordeaux
Fabry-Perot Cavity F Zomer LAL Orsay
Groups
Mechanics and Integration MLacroix LAL Orsay
Beam Physics ALoulergue Synchrotron SOLEIL
Alignment ALestrade Synchrotron SOLEIL
RF systems P Marchand Synchrotron SOLEIL
Magnets ME Couprie Synchrotron SOLEIL
Diagnostics L Cassinari Synchrotron SOLEIL
Infrastructures BPieyre MLanglet LAL Orsay
Control PGauron LAL Orsay
Vacuum C Preacutevost LAL Orsay
Power Supplies - Pulsed Magnets P Lebasque Synchrotron SOLEIL
Synchronisation N Delerue LAL Orsay
Cabling JN Cayla LAL Orsay
Nomenclature PCornebise LAL Orsay
lsquo
156
In the following Table the support structures are presented
Support Structures
Administration Communication Quality
Administration Brigitte Renard LAL
Orsay
Procurements Anngaeumllle Mardam
Beck LAL Orsay
EDMS Support Agnes Vermes LAL
Orsay
Secretary Chloe SevestreLAL Orsay
DR04 Support Edouard Dorion
Dominique Bony LAL Orsay
Marie-Pauline Gacoin
Synchrotron SOLEIL
Claus Habfast ESRF
Sandrine Agnes Vermes
LAL Orsay
Table 101
The ThomX collaboration needs an important support from all administrative services For this an
administrative cell has been set up which is headed by the person responsible for the administration the
purchasing office the project database management the secretariat and the contact with the regional
CNRS delegation support Last but not least a quality engineer plays a major role in the project
101 Partnersrsquo complementarity
The ThomX project requires a multidisciplinary environment given the complexity of the facility and of
the set-up operation Each partner contributes its competences to reach the final goal The ThomX
collaboration was organized so as to meet the requirements of a multicultural scientific environment
where different communities are called upon to share their know-how and their knowledge to develop a
common scientific program This was already the case ndash and still is the case ndash for the MightyLaser
project In this project LAL and CELIA teams have developed a fully integrated set-up composed of a
fibre laser and a high gain optical resonator which is presently installed at ATF (KEK Japan) There the
complementarity between the two teams was crucial LAL has a solid experience in developing optical
resonators in an accelerator environment while CELIA is among the leading laboratories in fibre
amplification technology This program aims at a progressive increase of the light beam power stored in
an optical cavity in order to maximize the Compton back-scattering rate The final goal of this RampD is
to demonstrate the feasibility of a γ factory based on Compton backscattering It started as a
demonstrator for Compton-based polarized positron sources needed for the next generation linear
colliders The interest shown by other scientific communities in this technology made it clear that a low
energy Compton source would be very useful in various research fields This conclusion led to the
launch of the ThomX collaboration
The ThomX CDR has already been published and the present TDR is about to be widely circulated For
the TDR writing it was mandatory to develop close connections between the various communities
lsquo
157
participating in the ThomX project in order to merge technological developments and usersrsquo
requirements in a single document All partners have contributed to the scientific case elaboration the
study of the facility feasibility and the definition of its main components
- LAL and SOLEIL contributed the description of the Compton backscattering characteristics and
carried out the study of the accelerators and the Fabry-Peacuterot resonator
- CELIA Bordeaux Lyon provided their expertise for the laser dimensioning
- THALES provided its experience in industrial components and sub-systems
- UMPC and ESRF enriched the scientific case
This task sharing results from the multidisciplinary approach to the ThomX project where half a dozen
institutes play an important role
The writing of this TDR provided a unique opportunity of collaborating for laboratories that belong to
different fields LAL specialized in fundamental high energy physics SOLEIL and CELIA in advanced
technology in laser and accelerators LC2RMF in social sciences and compact detectors ESRF in
medical sciences In the future the diversity of this set of teams will still increase with the participation
of the Neel Institute and the GIN-INSERM in Grenoble
After a prototyping phase ThomX will operate as a mini-synchrotron whose beam line can be accessed
not only by the facility partners but also by other communities Moreover the aim of the
industrialization phase is to provide the high technology scientific and industrial with compact and
reliable sources that can be operated with little or no accelerator and laser expertise This should open
the way to a large community of CBS source users that would benefit from the complementarity
between techniques developed at synchrotron sources and Compton back-scattering facilities Imaging
techniques biological tests on cells and animals metallurgy fast chemistry lithography are examples
of future possible applications of these unique instruments
102 ThomX larger framework
ThomX is one of the EQUIPEX projects submitted for approval to the French government by the P2IO
Consortium (Physique des deux Infinis et des Origines) of the Paris-Saclay Campus This Consortium is
composed of eight laboratories or institutes (CSNSM IAS IMNC IPNO IRFU LAL LLR LPT)
some of which are among the largest French laboratories All together their staffs comprise 2000
scientists engineers and technicians working in particle physics nuclear physics accelerator science
astroparticle physics astrophysics and their interfaces P2IO represents between 20 and 90 of the
national potential in the various sub-domains Although obviously not all of the eight laboratories
participate in each EQUIPEX project each project ndash including ThomX ndash is fully supported by P2IO and
will contribute to the very ambitious common technological platform that P2IO wants to create in the
coming years in the framework of the Paris-Saclay Campus initiative Once equipped with these
EQUIPEX facilities this campus will represent a unique opportunity for French researchers and
students
lsquo
158
References
11 Geoffrey A Krafft Gerd Priebe Compton Sources of Electromagnetic RadiationlowastReviews of Accelerator
Science and Technology Vol 3 (2010) 147ndash163
12 VBesson et al NIMA 608 (2009) S75-S77
13 T Akagi et al 2012 JINST 7 P01021
14 CDR ThomX LAL RT 0928 SOLEILSOU-RA-2678 editors AVariola FZomer ALoulergue
15 PRussbueldt et al Optics Express Vol 17 Issue 15 pp12230-12245 (2009)
16 IPupeza et al OPTICS LETTERS Vol 35 No 12 June 15 2010
17 M Y Shverdin et al 2009 OSAIPNRANLOSL 2009
18 CVaccarezza et al IPAC Conference proceedings New Orleans USA2012 TUOBB01
19 AVariola IPAC Conference proceedings San Sebastian Spain 2011 WEOAA01 1903-1905
21 S Doumlbert ldquoIntegration of the PHIN RF Gun into the CLIC Test Facilityrdquo Proceedings of EPAC 2006
Edinburgh Scotland 2006 MOPLS129
22 R Roux S Cavalier M Bernard G Bienvenu MJoreacuteBLeblondBMercier BMoutonCPrevost A
Variola ldquoPHIL a Test Beamline at LALrdquo Proceedings of EPAC08 Genoa Italy 2008 WEPP078
31 J Payet Beta Code CEA SACLAY
32 JB Murphy et al Longitudinal Wakefield for an Electron Moving on a Circular Orbit Vol 57
Part Acc 1997
33 K Bane M Sand The Short-Range Resistive Wall Wakefields SLAC-PUB 95-7074
34 E Forest Phys Tech Part Photon Beam
35 J Zang et al lsquoDipole fringe field effects in the Thom-X ringrsquo Proc IPAC13
36 R Nagaoka et al Recent Studies of Geometric impedance At SOLEIL PAC2006
37 Z Huang RD Ruth Phys Rev Lett 80976 (1998)
38 S K Mtingwa African Physical Review 2 (2008) 001
39 P Chen G Horton-Smith T Ohgaki AW Weidemann K Yokoya
NuclInstrumMethA355107-1101995
310 K Ohmi Phys Rev E 55 7550 (1997)
311 S Haifets D Teytelman Phys Rev Spec Top Acc and beams 8 064402 (2005)
312 E Weihreter Compact synchrotron light sources Series On Synchrotron Radiation Techniques
And Applications - Vol 3 World Scientific
313 P Lebasque et al Synchrotron SOLEIL lsquoEddy current septum magnets for booster injection and
extraction and storage ring injection at Synchrotron SOLEILrsquo EPAC 2006 Edinburgh Scotland
314 P Lebasque et al Synchrotron SOLEIL lsquoOptimisation of the coating thickness on the ceramic vacuum
chambers of SOLEIL storage ringrsquo EPAC 2006 Edinburgh Scotland
315 P Lebasque et al Synchrotron SOLEIL lsquoFour matched kicker systems for the SOLEIL storage ring
injection a full solid-state solution of pulsed power supplies working at high currentrsquo EPAC 2006
Edinburgh Scotland
316 F Marhauser E Weihreter (BESSY) ldquoImpedance Measurements of the HOM-Damped 500 MHz
Metrology Light Source Cavityrdquo BESSY Gmbh Germany 08 January 2007
317 M Langlois P Sanchez M Cornelis F Perez (ALBA) ldquoMeasurements on the RF cavity for the ALBA
storage ringrdquo Proceedings of EPAC08 Genoa Italy
318 R A Rimmer (Lawrence Bekeley Lab) et al ldquoDevelopment of a high-power RF cavity for the PEP-II B
factoryrdquo DE-AC03-76SF00098 (LBL)
lsquo
159
319 P Fernandes R Massarino R Parodi ATarditi (INFN Italy) A Massarotti (Sincrotrone Trieste Italy)
rdquoThe Design of the RF Cavities for ELETTRArdquo IEEE 1989
320 M Svandrlik A Fabris C Pasotti (Sincrotrone Trieste) rdquoImprovements in curing coupled bunch
instabilities at ELETTRA by mode shifting after the installation of the adjustable higher order mode frequency
shifter (HOMFS)rdquo IEEE 1998
321 W Bruns web site httpwwwgdfidlde
322 P Marchand et al Development of high RF power solid state amplifiers at SOLEIL Proceedings of the
IPAC2011 San Sebastian Spain p 376
323 C Pardine et al Status of the 476 MHz 50 kW solid state amplifier Proceedings of the IPAC2010
Kyoto Japan p 3972
41 YAG(CE) data sheet Saint Gobain Crystals httpwwwdetectorssaint-gobaincomYAG28Ce29aspx
42 SPI Suppliesreg YAGYAP Comparison Table
httpwww2spicomcatalogscintillsintillator_properties_comparison_tablehtml
43 K Nakamura et al (Particle Data Group) J Phys G 37 075021 (2010)
44 Wartski et al Detection of optical transition radiation and its application to beam diagnostics IEEE
TransNuclSci20544-5481973
45]Saturation of YAG Beam Profile Monitors Measured at ATF A Murokh J Rosenzweig V Yakimenko X
J Wang
46 Radiation Detection by Cerenkov Emission in Optical Fibers at TTF E Janata M Koumlrfer Tesla-Report
2000-27
47 CERN-2009-005 p273
48 CERN-2009-005 p272
51 J Bonis et al 2012 JINST 7 P01017 doi1010881748-0221701P01017
61 J Bonis et al Non-planar four-mirror optical cavity for high intensity g ray flux production by pulsed laser
beam Compton scattering off GeV-electrons JINST SISSA Vol 7 January 2012
62 R V Pound lsquolsquoElectronic Frequency Stabilization of Microwave Oscillatorsrsquorsquo Rev Sci Instrum 17 490ndash
505 (1946)
71 httpwwwjjxraydkhome
72 RW Alkire et al Nucl Inst and Meth A 350 (1994) 13-16
73 B Lengeler et al J Synchrotron Rad 6 (1999) 1153-1167
74 RL Owen et al J Synchrotron Rad 16 (2009) 143-151
75 MS Nogueira et al Radiation Protection Dosimetry 111 (2004) 105-110
76 M Krumrey et al AIP Conf Proc 705 (2004) 861-865
M Gerlach et al Metrologia 45 (2008) 577-585
77 httpwwwcanberracompdfProductsUltra-LEGe-SS-M3843pdf
78 K Sakaue et al Rev Sci Instrum 80 (2009) 123304
79 httpsaleshamamatsucomassetspdfparts_FF9892-31_-32_TMCP1039E03pdf
710 O Chubar et al Proc DIPAC (2001) CT09
711 D Paterson et al Opt Com 195 (2001) 79-84
712 M Mahesh RadioGraphics 21 (2001) 1033-1045
713 K Medjoubi et al JINST 6 C01080 (2011)
httpwwwvariancomusxrayproductsdigital_radiographypaxscan_productshtml
714 H Elleaume et al Physics in Medicine and Biology 47 (2002) 3369-3385
lsquo
160
715 Y Prezado et al Medical Physics 38 (2011) 1709
716 I Martiacutenez-Rovira and Y Prezadoa Medical Physics 38 (2011) 4430
717 L Samain Thesis Liege university (ULG) 2012
718 P Martinetto Thesis Joseph Fourier university (UJF) 2000
719 P Bleuet et al Analytical Chemistry 29 (2010) 518-527
720 P Bleuet et al Nature Materials 7 (2008) 468-472
81 httpwwwprofibuscomncdownloadspecifications-standardsdownloadsprofibus-standard-dp-
specificationdisplay
82 httpwwwtango-controlsorgDocumentstango-kernel
lsquo
161
Table of content
INTRODUCTION 2
1 The ThomX project 2
2 Scheme and options 3
3 General layout of the machine 4
4 Machine parameters 6
5 Summary of the TDR main technical points 8
Chapter 1 THE THOMX INTERACTION REGION 9
11 Electron beam and laser pulse characteristics in the interaction region 9
12 Expected flux and its characteristics 10
13 Position stability 11
14 Timing stability 12
15 Energy stability 13
Chapter 2 INJECTOR 14
21 Injector specifications 14
22 Technical description of the linac components 16
221 Photo‐injector 16
222 Solenoids 17
223 Laser 18
23 Acceleration section 20
231 The RF source 21
232 RF network 22
24 Beam dynamics 23
241 Nominal operation 23
242 Alternative operation modes 24
Chapter 3 TRANSFER LINE and RING 28
31 Transfer line 28
32 Extraction line 31
33 Storage ring 31
331 Optics and lattice 31
332 Non‐linear dynamics 35
333 Collective effects feedbacks 37
334 CBS and IBS effects 42
335 Scattering on the residual gas 46
336 Ion effects 47
337 Electron beam injection and extraction 48
338 Orbit corrections 50
341 Dipole magnetic design 53
342 Quadrupole magnetic design 54
343 Sextupole magnetic design 56
344 Corrector magnetic design 58
345 Injection dipole 59
lsquo
162
35 Power supplies 60
351 Pulsed magnets 60
36 Storage ring RF system 65
361 HOM impedance thresholds as regard to beam instabilities 66
362 Cures to the HOM induced instabilities 66
363 RF power source 70
364 Low level RF system and longitudinal feedback 71
365 RF equipment status 73
366 RF system conclusion 73
37 Transverse feedback 74
371 Specifications 74
372 Electronics 75
373 Kicker 76
38 Vacuum 77
381 Transfer line vacuum 77
382 Ring vacuum 78
Chapter 4 INJECTOR TRANSFER LINE AND RING DIAGNOSTICS 80
41 Charge measurement 80
411 Current transformers 80
412 Faraday cups 81
42 Transverse profile measurements 81
421 Diagnostics stations 81
43 Emittance measurement 84
44 Beam position measurement 85
441 Beam Position Monitors (BPM) 86
442 Orbit feedback system 90
443 Tune measurement and feedback 90
444 Energy measurement 90
45 Bunch length measurement 90
451 Cerenkov radiation 90
452 Synchrotron radiation 92
453 Transport of the Cerenkov and synchrotron radiations to the diagnostics hutch 93
454 Streak camera for the bunch length measurement 93
46 Bunch imaging using synchrotron radiation 94
47 Beam loss measurement 94
48 Beam dumps 94
Chapter 5 SYNCHRONIZATION 95
51 Overall scheme 95
52 Specifications 96
Chapter 6 OPTICAL SYSTEM DESIGN 97
61 Feedback system and results on a Fabry‐Perot cavity locking 97
611 Electron and FPC pulse synchronization 98
612 Laser parameters to be stabilized 99
lsquo
163
613 PDH Measurement technique 101
614 Actuators 103
615 The digital feedback system 107
62 High average power laser system and its coupling to a high gain FP cavity 110
621 The MightyLaser oscillator 111
623 Stretcher and compressor 115
624 ThomX development 115
63 Mechanical design 118
631 Optical cavity vacuum 123
Chapter 7 X‐RAY BEAM LINE AND EXPERIMENTS 124
Introduction 124
71 Available extracted X‐ray beam 124
72 Global view of the X‐ray lne 124
73 Chraracterization of the X‐ray beaugrave 126
731 Optical table 1 126
732 Beam transport 128
733 Optical table 2 128
74 Demonstration experiments 130
741 Experiments using the full X‐ry beam 130
742 Experiments using the central part of the beam 134
75 Summary 139
Chapter 8 CONTROL SYSTEM 140
Introduction 140
81 General principles 140
82 Technical considerations 141
83 The TANGO control system software 142
831 Software bus 142
832 A control system for accelerators 142
84 Control system architecture 143
841 Equipment to control 143
842 Inputsoutputs 144
843 Front‐end machine architecture PLCs and industrial PCs 145
85 IT infrastructure 146
Chapter 9 SAFETY AND RADIOPROTECTION 149
91 Introduction 149
92 PLC safety 149
93 Radiation monitoring 150
94 Radiation protection zoning 150
95 Access control 150
951 General statement 150
952 Particular requirements concerning the lsquorestricted areasrsquo 151
953 Access to the accelerator area and to the Igloo basement 151
954 Access to the X‐ray line and to the Compton room 151
lsquo
164
96 Beam dumps 152
97 Interlocks 152
98 Accelerator emergency shutdown 152
99 Control console 152
910 Evacuation of dangerous areas 152
911 Audio system and horns 152
912 Safety notices 153
913 Radiological monitoring 153
Chapter 10 MANAGEMENT OF THE THOMX PROJECT 154
Introduction 154
101 Partnersrsquo complementarity 156
102 ThomX larger framework 157